Effects of climate change

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The primary causes and the wide-ranging effects of global warming and resulting climate change. Some effects constitute feedback mechanisms that intensify climate change and move it toward climate tipping points.

The effects of climate change span the physical environment, ecosystems and human societies. It also includes the economic and social changes which stem from living in a warmer world. Human-caused climate change is one of the threats to sustainability.

Many physical impacts of climate change are already visible, including extreme weather events, glacier retreat, changes in the timing of seasonal events (e.g., earlier flowering of plants), sea level rise, and declines in Arctic sea ice extent. The ocean has taken up between 20 and 30% of human-induced atmospheric carbon dioxide since the 1980s, leading to ocean acidification. The ocean is also warming and since 1970 has absorbed more than 90% of the excess heat in the climate system.

Climate change has already impacted ecosystems and humans. In combination with climate variability, it makes food insecurity worse in many places and puts pressure on fresh water supply. This, in combination with extreme weather events, leads to negative effects on human health. Climate change has also contributed to desertification and land degradation in many regions of the world. This has implications for livelihoods as many people are dependent on land for food, feed, fibre, timber and energy. Rising temperatures, changing precipitation patterns and the increase in extreme events threaten development because of negative effects on economic growth in developing countries. Climate change already contributes to migration in different parts of the world.

The future impact of climate change depends on the extent to which nations implement prevention efforts, reduce greenhouse gas emissions, and adapt to unavoidable climate change effects. Much of the policy debate concerning climate change mitigation has been framed by projections for the twenty-first century. The focus on a limited time window obscures some of the problems associated with climate change. Policy decisions made in the next few decades will have profound impacts on the global climate, ecosystems and human societies, not just for this century, but for the next millennia, as near-term climate change policies significantly affect long-term climate change impacts.

Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels. Without mitigation, increased energy demand and the extensive use of fossil fuels may lead to global warming of around 4 °C. With higher magnitudes of global warming, societies and ecosystems will likely encounter limits to how much they can adapt.

Observed and future warming

Global surface temperature reconstruction over the last millennia using proxy data from tree rings, corals, and ice cores in blue. Observational data is from 1880 to 2019.

Global warming refers to the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of climate change, and has been demonstrated by the instrumental temperature record which shows global warming of around 1 °C since the pre-industrial period, although the bulk of this (0.9 °C) has occurred since 1970. A wide variety of temperature proxies together prove that the 20th century was the hottest recorded in the last 2,000 years. Compared to climate variability in the past, current warming is also more globally coherent, affecting 98% of the planet. The impact on the environment, ecosystems, the animal kingdom, society and humanity depends on how much more the Earth warms.

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." This has been brought about primarily through the burning of fossil fuels which has led to a significant increase in the concentration of GHGs in the atmosphere.

Emission scenarios

Individual consumers, corporate decision makers, the fossil fuel industries, government responses and the extent to which different countries agree to cooperate all have a profound impact on how much greenhouse gases the worlds emits. As the crisis and modelling techniques have evolved, the IPCC and other climate scientists have tried a number of different tools to estimate likely greenhouse gas emissions in the future.

Representative Concentration Pathways (RCPs) were based on possible differences in radiative forcing occurring in the next 100 years but do not include socioeconomic "narratives" to go alongside them. Another group of climate scientists, economists and energy system modellers took a different approach known as Shared Socioeconomic Pathways (SSPs); this is based on how socioeconomic factors such as population, economic growth, education, urbanisation and the rate of technological development might change over the next century. The SSPs describe five different trajectories which describe future climactic developments in the absence of new environmental policies beyond those in place today. They also explore the implications of different climate change mitigation scenarios.

Warming projections

CMIP5 average of climate model projections for 2081–2100 relative to 1986–2005, under low and high emission scenarios

The range in temperature projections partly reflects the choice of emissions scenario, and the degree of "climate sensitivity". The projected magnitude of warming by 2100 is closely related to the level of cumulative emissions over the 21st century (i.e. total emissions between 2000 and 2100). The higher the cumulative emissions over this time period, the greater the level of warming is projected to occur. Climate sensitivity reflects uncertainty in the response of the climate system to past and future GHG emissions. Higher estimates of climate sensitivity lead to greater projected warming, while lower estimates lead to less projected warming.

The IPCC's Fifth Report, states that relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is likely to exceed 1.5 °C and may well exceed 2 °C for all RCP scenarios except RCP2.6. It is likely to exceed 2 °C for RCP6.0 and RCP8.5, and more likely than not to exceed 2 °C for RCP4.5. The pathway with the highest greenhouse gas emissions, RCP8.5, will lead to a temperature increase of about 4.3˚C by 2100. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Even if emissions were drastically reduced overnight, the warming process is irreversible because CO
2 takes hundreds of years to break down, and global temperatures will remain close to their highest level for at least the next 1,000 years.

Mitigation policies currently in place will result in about 3.0 °C warming above pre-industrial levels. However, if current plans are not actually implemented, global warming is expected to reach 4.1 °C to 4.8 °C by 2100. There is a substantial gap between national plans and commitments and actual actions so far taken by governments around the world.

Warming in context of Earth's past

One of the methods scientists use to predict the effects of human-caused climate change, is to investigate past natural changes in climate. Scientists have used various "proxy" data to assess changes in Earth's past climate or paleoclimate. Sources of proxy data include historical records such as tree rings, ice cores, corals, and ocean and lake sediments. The data shows that recent warming has surpassed anything in the last 2,000 years.

By the end of the 21st century, temperatures may increase to a level not experienced since the mid-Pliocene, around 3 million years ago. At that time, mean global temperatures were about 2–4 °C warmer than pre-industrial temperatures, and the global mean sea level was up to 25 meters higher than it is today.

Physical impacts

Changes in climate indicators over several decades. Each of the different colored lines in each panel represents an independently analyzed set of data. The data come from many different technologies including weather stations, satellites, weather balloons, ships and buoys.

A broad range of evidence shows that the climate system has warmed. Evidence of global warming is shown in the graphs (below right) from the US National Oceanic and Atmospheric Administration (NOAA). Some of the graphs show a positive trend, e.g., increasing temperature over land and the ocean, and sea level rise. Other graphs show a negative trend, such as decreased snow cover in the Northern Hemisphere, and declining Arctic sea ice, both of which are indicative of global warming. Evidence of warming is also apparent in living (biological) systems such as changes in distribution of flora and fauna towards the poles.

Human-induced warming could lead to large-scale, abrupt and/or irreversible changes in physical systems. An example of this is the melting of ice sheets, which contributes to sea level rise and will continue for thousands of years. The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change.

Effects on weather

Global warming leads to an increase in extreme weather events such as heat waves, droughts, cyclones, blizzards and rainstorms. Such events will continue to occur more often and with greater intensity. Scientists have not only determined that climate change is responsible for trends in weather patterns, some individual extreme weather events have also directly be attributed to climate change.

Precipitation

Higher temperatures lead to increased evaporation and surface drying. As the air warms, its water-holding capacity also increases, particularly over the oceans. In general the air can hold about 7% more moisture for every 1 °C of temperature rise. In the tropics, there's more than a 10% increase in precipitation for a 1 °C increase in temperature. Changes have already been observed in the amount, intensity, frequency, and type of precipitation. Widespread increases in heavy precipitation have occurred even in places where total rain amounts have decreased.

Projections of future changes in precipitation show overall increases in the global average, but with substantial shifts in where and how precipitation falls. Projections suggest a reduction in rainfall in the subtropics, and an increase in precipitation in subpolar latitudes and some equatorial regions. In other words, regions which are dry at present will in general become even drier, while regions that are currently wet will in general become even wetter. Although increased rainfall will not occur everywhere, models suggest most of the world will have a 16–24% increase in heavy precipitation intensity by 2100.

Temperatures

As described in the first section, global temperatures have risen by 1 °C and are expected to rise further in the future. Over most land areas since the 1950s, it is very likely that at all times of year both days and nights have become warmer due to human activities. Night-time temperatures have increased a faster rate than daytime temperatures. In the U.S. since 1999, two warm weather records have been set or broken for every cold one.

Future climate change will include more very hot days and fewer very cold days. The frequency, length and intensity of heat waves will very likely increase over most land areas. Higher growth in anthropogenic GHG emissions would cause more frequent and severe temperature extremes.

Heat waves

Global warming boosts the probability of extreme weather events such as heat waves where the daily maximum temperature exceeds the average maximum temperature by 5 °C (9 °F) for more than five consecutive days.

In the last 30–40 years, heat waves with high humidity have become more frequent and severe. Extremely hot nights have doubled in frequency. The area in which extremely hot summers are observed has increased 50–100 fold. These changes are not explained by natural variability, and are attributed by climate scientists to the influence of anthropogenic climate change. Heat waves with high humidity pose a big risk to human health while heat waves with low humidity lead to dry conditions that increase wildfires. The mortality from extreme heat is larger than the mortality from hurricanes, lightning, tornadoes, floods, and earthquakes together.

Tropical cyclones

Global warming not only causes changes in tropical cyclones, it may also make some impacts from them worse via sea level rise. The intensity of tropical cyclones (hurricanes, typhoons, etc.) is projected to increase globally, with the proportion of Category 4 and 5 tropical cyclones increasing. Furthermore, the rate of rainfall is projected to increase, but trends in the future frequency on a global scale are not yet clear. Changes in tropical cyclones will probably vary by region.

On land

In the year 2019 the Intergovernmental Panel on Climate Change issued a Special Report on Climate Change and Land. The main statements of the report include:

• Humans affect 70% of the ice free land, that play a key role in supplying the needs of humans and in the climate system.

• The global food supply have raised what increased GHG emission, but 25% - 30% of the food is lost, 2 billion adults suffer from overweight while 821 million people suffer from hunger.

• The rate of soil erosion is 10 - 20 times higher than the rate of soil accumulation in agricultural areas that use no-till farming. In areas with tilling it is 100 times higher. Climate Change increases land degradation and desertification.

• In the years 1960 - 2013 the area of drylands in drought, increased by 1% per year.

• In the year 2015 around 500 million people lived in areas that was impacted by desertification in the years 1980s - 2000s.

• People who live in the areas affected by land degradation and desertification are "increasingly negatively affected by climate change".

Climate change will also cause soils to warm. In turn, this could cause the soil microbe population size to dramatically increase 40–150%. Warmer conditions would favor growth of certain bacteria species, shifting the bacterial community composition. Elevated carbon dioxide would increase the growth rates of plants and soil microbes, slowing the soil carbon cycle and favoring oligotrophs, which are slower-growing and more resource efficient than copiotrophs.

Flooding

High tides flooding is increasing due to sea level rise, land subsidence, and the loss of natural barriers.

Warmer air holds more water vapor. When this turns to rain, it tends to come in heavy downpours potentially leading to more floods. A 2017 study found that peak precipitation is increasing between 5 and 10% for every one degree Celsius increase. In the United States and many other parts of the world there has been a marked increase in intense rainfall events which have resulted in more severe flooding. Estimates of the number of people at risk of coastal flooding from climate-driven sea-level rise varies from 190 million, to 300 million or even 640 million in a worst-case scenario related to the instability of the Antarctic ice sheet. the Greenland ice sheet is estimated to have reached a point of no return, continuing to melt even if warming stopped. Over time that would submerge many of the world's coastal cities including low-lying islands, especially combined with storm surges and high tides.

Drought

Climate change affects multiple factors associated with droughts, such as how much rain falls and how fast the rain evaporates again. It is set to increase the severity and frequency of droughts around much of the world. Due to limitations on how much data is available about drought in the past, it is often impossible to confidently attribute droughts to human-induced climate change. Some areas however, such as the Mediterranean and California, already show a clear human signature. Their impacts are aggravated because of increased water demand, population growth, urban expansion, and environmental protection efforts in many areas.

Wildfires

Warm and dry temperatures driven by climate change increase the chance of wildfires.

Prolonged periods of warmer temperatures typically cause soil and underbrush to be drier for longer periods, increasing the risk of wildfires. Hot, dry conditions increase the likelihood that wildfires will be more intense and burn for longer once they start. In California, summer air temperature have increased by over 3.5 °F such that the fire season has lengthened by 75 days over previous decades. As a result, since the 1980s, both the size and ferocity of fires in California have increased. Since the 1970s, the size of the area burned has increased fivefold.

In Australia, the annual number of hot days (above 35 °C) and very hot days (above 40 °C) has increased significantly in many areas of the country since 1950. The country has always had bushfires but in 2019, the extent and ferocity of these fires increased dramatically. For the first time catastrophic bushfire conditions were declared for Greater Sydney. New South Wales and Queensland declared a state of emergency but fires were also burning in South Australia and Western Australia.

Cryosphere

Earth lost 28 trillion tonnes of ice between 1994 and 2017, with melting grounded ice (ice sheets and glaciers) raising the global sea level by 34.6 ±3.1 mm. The rate of ice loss has risen by 57% since the 1990s−from 0.8 to 1.2 trillion tonnes per year.

 

A map that shows ice concentration on 16 September 2012, along with the extent of the previous record low (yellow line) and the mid-September median extent (black line) setting a new record low that was 18 percent smaller than the previous record and nearly 50 percent smaller than the long-term (1979–2000) average.

The cryosphere is made up of those parts of the planet which are so cold, they are frozen and covered by snow or ice. This includes ice and snow on land such as the continental ice sheets in Greenland and Antarctica, as well as glaciers and areas of snow and permafrost; and ice found on water including frozen parts of the ocean, such as the waters surrounding Antarctica and the Arctic. The cryosphere, especially the polar regions, is extremely sensitive to changes in global climate.

The Intergovernmental Panel on Climate Change issued a Special Report on the Ocean and Cryosphere in a Changing Climate. According to the report climate change caused a massive melting of glaciers, ice sheets, snow and permafrost with generally negative effects on ecosystems and humans. Indigenous knowledge helped to adapt to those effects.

Arctic sea ice began to decline at the beginning of the twentieth century but the rate is accelerating. Since 1979, satellite records indicate the decline in summer sea ice coverage has been about 13% per decade. The thickness of sea ice has also decreased by 66% or 2.0 m over the last six decades with a shift from permanent ice to largely seasonal ice cover. While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur at least once every decade at a warming level of 2.0 °C.

Since the beginning of the twentieth century, there has also been a widespread retreat of alpine glaciers, and snow cover in the Northern Hemisphere. During the 21st century, glaciers and snow cover are projected to continue their retreat in almost all regions. The melting of the Greenland and West Antarctic ice sheets will continue to contribute to sea level rise over long time-scales.

Oceans

Global ocean heat content from 1955 to 2019

Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification. Other possible effects include large-scale changes in ocean circulation. The oceans also serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO
₂ have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO
₂. The oceans have also acted as a sink in absorbing extra heat from the atmosphere.

According to a Special Report on the Ocean and Cryosphere in a Changing Climate published by the Intergovernmental Panel on Climate Change, climate change has different impacts on the oceans, including an increase in marine heatwaves, shift in species distribution, ocean deoxygenation.

The decline in mixing of the ocean layers piles up warm water near the surface while reducing cold, deep water circulation. The reduced up and down mixing enhanced global warming. Furthermore, energy available for tropical cyclones and other storms is expected to increase, nutrients for fish in the upper ocean layers are set to decrease, as well as the capacity of the oceans to store carbon.

Sea Ice

Sea ice reflects 50% to 70% of the incoming solar radiation, while 6% of the incoming solar engery is reflected by the ocean. With less solar energy, the sea ice absorbs and holds the surface colder, which can be a positive feedback toward climate change.

Oxygen depletion

Warmer water cannot contain as much oxygen as cold water, so heating is expected to lead to less oxygen in the ocean. Other processes also play a role: stratification may lead to increases in respiration rates of organic matter, further decreasing oxygen content. The ocean has already lost oxygen, throughout the entire water column and oxygen minimum zones are expanding worldwide. This has adverse consequences for ocean life.

Ocean heat uptake

Oceans have taken up over 90% of the excess heat accumulated on Earth due to global warming. The warming rate varies with depth: at a depth of a thousand metres the warming occurs at a rate of almost 0.4 °C per century (data from 1981 to 2019), whereas the warming rate at two kilometres depth is only half. The increase in ocean heat content is much larger than any other store of energy in the Earth's heat balance and accounts for more than 90% of the increase in heat content of the Earth system, and has accelerated in the 1993–2017 period compared to 1969–1993. In 2019 a paper published in the journal Science found the oceans are heating 40% faster than the IPCC predicted just five years before.

As well as having effects on ecosystems (e.g. by melting sea ice affecting algae that grow on its underside), warming reduces the ocean's ability to absorb CO
₂. It is likely that the oceans warmed faster between 1993 and 2017 compared to the period starting in 1969.

Sea level rise

Historical sea level reconstruction and projections up to 2100 published in January 2017 by the U.S. Global Change Research Program

The IPCC's Special Report on the Ocean and Cryosphere concluded that global mean sea level rose by 0.16 metres between 1901 and 2016. The rate of sea level rise since the industrial revolution in the 19th century has been larger than the rate during the previous two thousand years.

Global sea level rise is accelerating, rising 2.5 times faster between 2006 and 2016 than it did during the 20th century. Two main factors contribute to the rise. The first is thermal expansion: as ocean water warms, it expands. The second is from the melting of land-based ice in glaciers and ice sheets due to global warming. Prior to 2007, thermal expansion was the largest component in these projections, contributing 70–75% of sea level rise. As the impact of global warming has accelerated, melting from glaciers and ice sheets has become the main contributor.

Even if emission of greenhouse gases stops overnight, sea level rise will continue for centuries to come. In 2015, a study by Professor James Hansen of Columbia University and 16 other climate scientists said a sea level rise of three metres could be a reality by the end of the century. Another study by scientists at the Royal Netherlands Meteorological Institute in 2017 using updated projections of Antarctic mass loss and a revised statistical method also concluded that, although it was a low probability, a three-metre rise was possible. Rising sea levels will put hundreds of millions of people at risk in low-lying coastal areas in countries such as China, Bangladesh, India and Vietnam.

Wildlife and nature

A vast array of physical and biological systems across the Earth are being affected by human-induced global warming.

Recent warming has strongly affected natural biological systems. Species worldwide are moving poleward to colder areas. On land, species move to higher elevations, whereas marine species find colder water at greater depths. Of the drivers with the biggest global impact on nature, climate change ranks third over the five decades before 2020, with only change in land use and sea use, and direct exploitation of organisms having a greater impact.

The impacts of climate change in nature and nature's contributions to humans are projected to become more pronounced in the next few decades. Examples of climatic disruptions include fire, drought, pest infestation, invasion of species, storms, and coral bleaching events. The stresses caused by climate change, added to other stresses on ecological systems (e.g. land conversion, land degradation, harvesting, and pollution), threaten substantial damage to or complete loss of some unique ecosystems, and extinction of some critically endangered species. Key interactions between species within ecosystems are often disrupted because species from one location do not move to colder habitats at the same rate, giving rise to rapid changes in the functioning of the ecosystem.

The Arctic is heating up twice as fast as the global mean. Seas are on track to rise one to four feet higher by 2100, threatening coastal habitats.

Terrestrial and wetland systems

Climate change has been estimated to be a major driver of biodiversity loss in cool conifer forests, savannas, mediterranean-climate systems, tropical forests, and the Arctic tundra. In other ecosystems, land-use change may be a stronger driver of biodiversity loss, at least in the near-term. Beyond the year 2050, climate change may be the major driver for biodiversity loss globally. Climate change interacts with other pressures such as habitat modification, pollution and invasive species. Interacting with these pressures, climate change increases extinction risk for a large fraction of terrestrial and freshwater species. Between 1% and 50% of species in different groups were assessed to be at substantially higher risk of extinction due to climate change.

Ocean ecosystems

A part of the Great Barrier Reef in Australia in 2016 after a coral bleaching event

Warm water coral reefs are very sensitive to global warming and ocean acidification. Coral reefs provide a habitat for thousands of species and ecosystem services such as coastal protection and food. The resilience of reefs can be improved by curbing local pollution and overfishing, but most warm water coral reefs will disappear even if warming is kept to 1.5 °C. Coral reefs are not the only framework organisms, organisms that build physical structures that form habitats for other sea creatures, affected by climate change: mangroves and seagrass are considered to be at moderate risk for lower levels of global warming according to a literature assessment in the Special Report on the Ocean and Cryosphere in a Changing Climate. Marine heatwaves have seen an increased frequency and have widespread impacts on life in the oceans, such as mass dying events. Harmful algae blooms have increased in response to warming waters, ocean deoxygenation and eutrophication. Between one-quarter and one-third of our fossil fuel emissions are consumed by the earth's oceans and are now 30 percent more acidic than they were in pre-industrial times. This acidification poses a serious threat to aquatic life, particularly creatures such as oysters, clams, and coral with calcified shells or skeletons.

Regional effects

Average global temperatures from 2010 to 2019 compared to a baseline average from 1951 to 1978. Source: NASA.

Regional effects of global warming vary in nature. Some are the result of a generalised global change, such as rising temperature, resulting in local effects, such as melting ice. In other cases, a change may be related to a change in a particular ocean current or weather system. In such cases, the regional effect may be disproportionate and will not necessarily follow the global trend.

There are three major ways in which global warming will make changes to regional climate: melting or forming ice, changing the hydrological cycle (of evaporation and precipitation) and changing currents in the oceans and air flows in the atmosphere. The coast can also be considered a region, and will suffer severe impacts from sea level rise.

The Arctic, Africa, small islands, Asian megadeltas and the Middle East are regions that are likely to be especially affected by climate change. Low-latitude, less-developed regions are at most risk of experiencing negative impacts due to climate change. Developed countries are also vulnerable to climate change. For example, developed countries will be negatively affected by increases in the severity and frequency of some extreme weather events, such as heat waves.

Projections of climate changes at the regional scale do not hold as high a level of scientific confidence as projections made at the global scale. It is, however, expected that future warming will follow a similar geographical pattern to that seen already, with the greatest warming over land and high northern latitudes, and least over the Southern Ocean and parts of the North Atlantic Ocean. Land areas warm faster than ocean, and this feature is even stronger for extreme temperatures. For hot extremes, regions with the most warming include Central and Southern Europe and Western and Central Asia.

On humans

The effects of climate change, in combination with the sustained increases in greenhouse gas emissions, have led scientists to characterize it as a climate emergency. Some climate researchers and activists have called it an existential threat to civilization. Some areas may become too hot for humans to live in while people in some areas may experience displacement triggered by flooding and other climate change related disasters.

The vulnerability and exposure of humans to climate change varies from one economic sector to another and will have different impacts in different countries. Wealthy industrialised countries, which have emitted the most CO₂, have more resources and so are the least vulnerable to global warming. Economic sectors that are likely to be affected include agriculture, human health, fisheries, forestry, energy, insurance, financial services, tourism, and recreation. The quality and quantity of freshwater will likely be affected almost everywhere. Some people may be particularly at risk from climate change, such as the poor, young children and the elderly. According to the World Health Organization, between 2030 and 2050, "climate change is expected to cause about 250,000 additional deaths per year." As global temperatures increase, so does the number of heat stress, heatstroke, and cardiovascular and kidney disease deaths and illnesses. Air pollution generated by fossil fuel combustion is both a major driver of global warming and – in parallel and for comparison – the cause of a large number of annual deaths with some estimates as high as 8.7 million excess deaths during 2018. It may be difficult to predict or attribute deaths to anthropogenic global warming or its particular drivers as many effects – such as possibly contributing to human conflict and socioeconomic disruptions – and their mortality impacts could be highly indirect or hard to evaluate.

Food security

Climate change will impact agriculture and food production around the world due to the effects of elevated CO₂ in the atmosphere; higher temperatures; altered precipitation and transpiration regimes; increased frequency of extreme events; and modified weed, pest, and pathogen pressure. Climate change is projected to negatively affect all four pillars of food security: not only how much food is available, but also how easy food is to access (prices), food quality and how stable the food system is.

Food availability

2011 projected changes in crop yields at different latitudes with global warming. This graph is based on several studies.

 

2011 projected changes in yields of selected crops with global warming. This graph is based on several studies.

As of 2019, negative impacts have been observed for some crops in low-latitudes (maize and wheat), while positive impacts of climate change have been observed in some crops in high-latitudes (maize, wheat, and sugar beets). Using different methods to project future crop yields, a consistent picture emerges of global decreases in yield. Maize and soybean decrease with any warming, whereas rice and wheat production might peak at 3 °C of warming.

In many areas, fisheries have already seen their catch decrease because of global warming and changes in biochemical cycles. In combination with overfishing, warming waters decrease the maximum catch potential. Global catch potential is projected to reduce further in 2050 by less than 4% if emissions are reduced strongly, and by about 8% for very high future emissions, with growth in the Arctic Ocean.

Other aspects of food security

Climate change impacts depend strongly on projected future social and economic development. As of 2019, an estimated 831 million people are undernourished. Under a high emission scenario (RCP6.0), cereals are projected to become 1-29% more expensive in 2050 depending on the socioeconomic pathway, particularly affecting low-income consumers. Compared to a no climate change scenario, this would put between 1-181 million extra people at risk of hunger.

While CO
2 is expected to be good for crop productivity at lower temperatures, it does reduce the nutritional values of crops, with for instance wheat having less protein and less of some minerals. It is difficult to project the impact of climate change on utilization (protecting food against spoilage, being healthy enough to absorb nutrients, etc.) and on volatility of food prices. Most models projecting the future do indicate that prices will become more volatile.

Droughts result in crop failures and the loss of pasture for livestock.

Water security

A number of climate-related trends have been observed that affect water resources. These include changes in precipitation, the cryosphere and surface waters (e.g., changes in river flows). Observed and projected impacts of climate change on freshwater systems and their management are mainly due to changes in temperature, sea level and precipitation variability. Changes in temperature are correlated with variability in precipitation because the water cycle is reactive to temperature. Temperature increases change precipitation patterns. Excessive precipitation leads to excessive sediment deposition, nutrient pollution, and concentration of minerals in aquifers.

The rising global temperature will cause sea level rise and will extend areas of salinization of groundwater and estuaries, resulting in a decrease in freshwater availability for humans and ecosystems in coastal areas. The rising sea level will push the salt gradient into freshwater deposits and will eventually pollute freshwater sources. The 2014 fifth IPCC assessment report concluded that:

• Water resources are projected to decrease in most dry subtropical regions and mid-latitudes, but increase in high latitudes. As streamflow becomes more variable, even regions with increased water resources can experience additional short-term shortages.
• Per degree warming, a model average of 7% of the world population is expected to have at least 20% less renewable water resource.
• Climate change is projected to reduce water quality before treatment. Even after conventional treatments, risks remain. The quality reduction is a consequence of higher temperatures, more intense rainfall, droughts and disruption of treatment facilities during floods.
• Droughts that stress water supply are expected to increase in southern Europe and the Mediterranean region, central Europe, central and southern North America, Central America, northeast Brazil, and southern Africa.

Health

Humans are exposed to climate change through changing weather patterns (temperature, precipitation, sea-level rise and more frequent extreme events) and indirectly through changes in water, air and food quality and changes in ecosystems, agriculture, industry and settlements and the economy. Air pollution, wildfires, and heat waves caused by global warming have significantly affected human health, and in 2007, the World Health Organization estimated 150,000 people were being killed by climate-change-related issues every year.

A study by the World Health Organization concluded that climate change was responsible for 3% of diarrhoea, 3% of malaria, and 3.8% of dengue fever deaths worldwide in 2004. Total attributable mortality was about 0.2% of deaths in 2004; of these, 85% were child deaths. The effects of more frequent and extreme storms were excluded from this study.

The human impacts include both the direct effects of extreme weather, leading to injury and loss of life, as well as indirect effects, such as undernutrition brought on by crop failures. Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever, which affects children most severely, and malaria. Young children are the most vulnerable to food shortages, and together with older people, to extreme heat.

According to a report from the United Nations Environment Programme and International Livestock Research Institute, climate change can facilitate outbreaks of Zoonosis, e.g. diseases that pass from animals to humans. One example of such outbreaks is the COVID-19 pandemic.

A minor further effect are increases of pollen season lengths and concentrations in some regions of the world.

Projections

A 2014 study by the World Health Organization estimated the effect of climate change on human health, but not all of the effects of climate change were included in their estimates. For example, the effects of more frequent and extreme storms were excluded. The report further assumed continued progress in health and growth. Even so, climate change was projected to cause an additional 250,000 deaths per year between 2030 and 2050.

The authors of the IPCC AR4 Synthesis report projected with high confidence that climate change will bring some benefits in temperate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission potential of malaria in Africa. Benefits were projected to be outweighed by negative health effects of rising temperatures, especially in developing countries.

Economic development is an important component of possible adaptation to climate change. Economic growth on its own, however, is not sufficient to insulate the world's population from disease and injury due to climate change. Future vulnerability to climate change will depend not only on the extent of social and economic change, but also on how the benefits and costs of change are distributed in society. For example, in the 19th century, rapid urbanization in western Europe led to health plummeting. Other factors important in determining the health of populations include education, the availability of health services, and public-health infrastructure.

On mental health

In 2018, the American Psychological Association issued a report about the impact of climate change on mental health. It said that "gradual, long-term changes in climate can also surface a number of different emotions, including fear, anger, feelings of powerlessness, or exhaustion". Generally this is likely to have the greatest impact on young people. California social scientist, Renee Lertzman, likens the climate-related stress now affecting teenagers and those in their 20s to Cold War fears that gripped young baby boomers who came of age under the threat of nuclear annihilation. Research has found that although there are heightened emotional experiences linked with acknowledgement and anticipation of climate change and its impact on society, these are inherently adaptive. Furthermore, engaging with these emotional experiences leads to increased resilience, agency, reflective functioning and collective action. Individuals are encouraged to find collective ways of processing their climate related emotional experiences in order to support mental health and well being. A 2018 study found that unusually hot days have profound effects on mental health and that global warming could contribute to approximately 26,000 more suicides in the U.S. by 2050. A study published in April 2020 found that by the end of the 21st century people could be exposed to avoidable indoor CO2 levels of up to 1400 ppm, which would be triple the amount commonly experienced outdoors today and, according to the authors, may cut humans' basic decision-making ability indoors by ~25% and complex strategic thinking by ~50%.

Migration

Gradual but pervasive environmental change and sudden natural disasters both influence the nature and extent of human migration but in different ways.

Slow onset

Slow-onset disasters and gradual environmental erosion such as desertification, reduction of soil fertility, coastal erosion and sea-level rise are likely to induce long term migration. Migration related to desertification and reduced soil fertility is likely to be predominantly from rural areas in developing countries to towns and cities.

Displacement and migration related to sea level rise will mostly affect those who live in cities near the coast. More than 90 US coastal cities are already experiencing chronic flooding and that number is expected to double by 2030. Numerous cities in Europe will be affected by rising sea levels; especially in the Netherlands, Spain and Italy. Coastal cities in Africa are also under threat due to rapid urbanization and the growth of informal settlements along the coast. Low lying Pacific island nations including Fiji, Kiribati, Nauru, Micronesia, the Marshall Islands, the Solomon Islands, Vanuatu, Timor Leste and Tonga are especially vulnerable to rising seas. In July 2019, they issued a declaration "affirming that climate change poses the single greatest threat to the human rights and security of present and future generations of Pacific Island peoples" and stated their lands could become uninhabitable as early as 2030.

The United Nations says there are already 64 million human migrants in the world fleeing wars, hunger, persecution and the effects of global warming. In 2018, the World Bank estimated that climate change will cause internal migration of between 31 and 143 million people as they escape crop failures, water scarcity, and sea level rise. The study only included Sub-Saharan Africa, South Asia, and Latin America.

A 2020 study projects that regions inhabited by a third of the human population could become as hot as the hottest parts of the Sahara within 50 years without a change in patterns of population growth and without migration, unless greenhouse gas emissions are reduced. The projected annual average temperature of above 29 °C for these regions would be outside the "human temperature niche" – a suggested range for climate biologically suitable for humans based on historical data of mean annual temperatures (MAT) – and the most affected regions have little adaptive capacity as of 2020. The following matrix shows their projections for population-sizes outside the "human temperature niche" – and therefore potential emigrants of their regions – in different climate change scenarios and projections of population growth for 2070:

Sudden onset

Sudden-onset natural disasters tend to create mass displacement, which may only be short term. However, Hurricane Katrina demonstrated that displacement can last a long time. Estimates suggest that a quarter of the one million people displaced in the Gulf Coast region by Hurricane Katrina had not returned to their homes five years after the disaster. Mizutori, the U.N. secretary-general's special representative on disaster risk reduction, says millions of people are also displaced from their homes every year as result of sudden-onset disasters such as intense heatwaves, storms and flooding. She says 'climate crisis disasters' are happening at the rate of one a week.

Conflict

A 2013 study found that significant climatic changes were associated with a higher risk of conflict worldwide, and predicted that "amplified rates of human conflict could represent a large and critical social impact of anthropogenic climate change in both low- and high-income countries." Similarly, a 2014 study found that higher temperatures were associated with a greater likelihood of violent crime, and predicted that global warming would cause millions of such crimes in the United States alone during the 21st century. Climate change can worsen conflicts by exacerbating tensions over limited resources like drinking water. Climate change has the potential to cause large population dislocations, which can also lead to conflict.

However, a 2018 study in the journal Nature Climate Change found that previous studies on the relationship between climate change and conflict suffered from sampling bias and other methodological problems. Factors other than climate change are judged to be substantially more important in affecting conflict (based on expert elicitation). These factors include intergroup inequality and low socio-economic development.

Despite these issues, military planners are concerned that global warming is a "threat multiplier". "Whether it is poverty, food and water scarcity, diseases, economic instability, or threat of natural disasters, the broad range of changing climatic conditions may be far reaching. These challenges may threaten stability in much of the world". For example, the onset of the Arab Spring in 2010 was partly the result of a spike in wheat prices following crop losses from the 2010 Russian heat wave.

Economic impact

Economic forecasts of the impact of global warming vary considerably. Researchers have warned that current economic modelling may seriously underestimate the impact of potentially catastrophic climate change, and point to the need for new models that give a more accurate picture of potential damages. Nevertheless, one recent study has found that potential global economic gains if countries implement mitigation strategies to comply with the 2 °C target set at the Paris Agreement are in the vicinity of US$17 trillion per year up to 2100 compared to a very high emission scenario.

Global losses reveal rapidly rising costs due to extreme weather events since the 1970s. Socio-economic factors have contributed to the observed trend of global losses, such as population growth and increased wealth. Part of the growth is also related to regional climatic factors, e.g., changes in precipitation and flooding events. It is difficult to quantify the relative impact of socio-economic factors and climate change on the observed trend. The trend does, however, suggest increasing vulnerability of social systems to climate change.

A 2019 modelling study found that climate change had contributed towards global economic inequality. Wealthy countries in colder regions had either felt little overall economic impact from climate change, or possibly benefited, whereas poor hotter countries very likely grew less than if global warming had not occurred.

The total economic impacts from climate change are difficult to estimate, but increase for higher temperature changes. For instance, total damages are estimated to be 90% less if global warming is limited to 1.5 °C compared to 3.66 °C, a warming level chosen to represent no mitigation. One study found a 3.5% reduction in global GDP by the end of the century if warming is limited to 3 °C, excluding the potential effect of tipping points. Another study noted that global economic impact is underestimated by a factor of two to eight when tipping points are excluded from consideration. In the Oxford Economics high emission scenario, a temperature rise of 2 degrees by the year 2050 would reduce global GDP by 2.5% - 7.5%. By the year 2100 in this case, the temperature would rise by 4 degrees, which could reduce the global GDP by 30% in the worst case.

Abrupt or irreversible changes

Self-reinforcing feedbacks amplify and accelerate climate change. The climate system exhibits threshold behaviour or tipping points when these feedbacks lead parts of the Earth system into a new state, such as the runaway loss of ice sheets or the destruction of too many forests. Tipping points are studied using data from Earth's distant past and by physical modelling. There is already moderate risk of global tipping points at 1 °C above pre-industrial temperatures, and that risk becomes high at 2.5 °C.

Tipping points are "perhaps the most ‘dangerous’ aspect of future climate changes", leading to irreversible impacts on society. Many tipping points are interlinked, so that triggering one may lead to a cascade of effects. A 2018 study states that 45% of environmental problems, including those caused by climate change are interconnected and make the risk of a domino effect bigger.

Amazon rain forest

Rainfall that falls on the Amazon rainforest is recycled when it evaporates back into the atmosphere instead of running off away from the rainforest. This water is essential for sustaining the rainforest. Due to deforestation the rainforest is losing this ability, exacerbated by climate change which brings more frequent droughts to the area. The higher frequency of droughts seen in the first two decades of the 21st century signal that a tipping point from rainforest to savanna might be close.

Greenland and West Antarctic Ice sheets

Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse. Part of the ice sheet is grounded on bedrock below sea level, making it possibly vulnerable to the self-enhancing process of marine ice sheet instability. A further hypothesis is that marine ice cliff instability would also contribute to a partial collapse, but limited evidence is available for its importance. A partial collapse of the ice sheet would lead to rapid sea level rise and a local decrease in ocean salinity. It would be irreversible on a timescale between decades and millennia.

In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to be taking place more gradually over millennia. Sustained warming between 1 °C (low confidence) and 4 °C (medium confidence) would lead to a complete loss of the ice sheet, contributing 7 m to sea levels globally. The ice loss could become irreversible due to a further self-enhancing feedback: the elevation-surface mass balance feedback. When ice melts on top of the ice sheet, the elevation drops. As air temperature is higher at lower altitude, this promotes further melt.

Atlantic Meridional Overturning Circulation

This map shows the general location and direction of the warm surface (red) and cold deep water (blue) currents of the thermohaline circulation. Salinity is represented by color in units of the Practical Salinity Scale. Low values (blue) are less saline, while high values (orange) are more saline.

The Atlantic Meridional Overturning Circulation (AMOC), an important component of the Earth's climate system, is a northward flow of warm, salty water in the upper layers of the Atlantic and a southward flow of colder water in the deep Atlantic. Potential impacts associated with AMOC changes include reduced warming or (in the case of abrupt change) absolute cooling of northern high-latitude areas near Greenland and north-western Europe, an increased warming of Southern Hemisphere high-latitudes, tropical drying, as well as changes to marine ecosystems, terrestrial vegetation, oceanic CO
₂ uptake, oceanic oxygen concentrations, and shifts in fisheries.

According to a 2019 assessment in the IPCC's Special Report on the Ocean and Cryosphere in a Changing Climate it is very likely (greater than 90% probability, based on expert judgement) that the strength of the AMOC will decrease further over the course of the 21st century. Warming is still expected to occur over most of the European region downstream of the North Atlantic Current in response to increasing GHGs, as well as over North America. With medium confidence, the IPCC stated that it is very unlikely (less than 10% probability) that the AMOC will collapse in the 21st century. The potential consequences of such a collapse could be severe.

Irreversible change

Warming commitment to CO
2 concentrations.

If emissions of CO
2 were to be abruptly stopped and no negative emission technologies deployed, the Earth's climate would not start moving back to its pre-industrial state. Instead, temperatures would stay elevated at the same level for several centuries. After about a thousand years, 20% to 30% of human-emitted CO
2 will remain in the atmosphere, not taken up by the ocean or the land, committing the climate to warming long after emissions have stopped. Pathways that keep global warming under 1.5 °C often rely on large-scale removal of CO
2, which feasibility is uncertain and has clear risks.

Irreversible impacts

There are a number of examples of climate change impacts that may be irreversible, at least over the timescale of many human generations. These include the large-scale singularities such as the melting of the Greenland and West Antarctic ice sheets, and changes to the AMOC. In biological systems, the extinction of species would be an irreversible impact. In social systems, unique cultures may be lost due to climate change. For example, humans living on atoll islands face risks due to sea level rise, sea surface warming, and increased frequency and intensity of extreme weather events.

Metallic hydrogen

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Metallic_hydrogen 

Metallic hydrogen is a phase of hydrogen in which it behaves like an electrical conductor. This phase was predicted in 1935 on theoretical grounds by Eugene Wigner and Hillard Bell Huntington.

At high pressure and temperatures, metallic hydrogen can exist as a liquid rather than a solid, and researchers think it might be present in large quantities in the hot and gravitationally compressed interiors of Jupiter and Saturn and in some exoplanets.

Theoretical predictions

A diagram of Jupiter showing a model of the planet's interior, with a rocky core overlaid by a deep layer of liquid metallic hydrogen (shown as magenta) and an outer layer predominantly of molecular hydrogen. Jupiter's true interior composition is uncertain. For instance, the core may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. Furthermore, there is no clear physical boundary between the hydrogen layers—with increasing depth the gas increases smoothly in temperature and density, ultimately becoming liquid. Features are shown to scale except for the aurorae and the orbits of the Galilean moons.

Hydrogen under pressure

Though often placed at the top of the alkali metal column in the periodic table, hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms diatomic H
2 molecules, analogous to halogens and some nonmetals in the second row of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 degrees and 14 degrees above absolute zero, respectively). Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around 25 GPa (250,000 atm; 3,600,000 psi), hydrogen would display metallic properties: instead of discrete H
2 molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons delocalized throughout. Since then, producing metallic hydrogen in the laboratory has been described as "...the holy grail of high-pressure physics."

The initial prediction about the amount of pressure needed was eventually shown to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations point towards higher but nonetheless potentially achievable metallization pressures of around 400 GPa (3,900,000 atm; 58,000,000 psi).

Liquid metallic hydrogen

Helium-4 is a liquid at normal pressure near absolute zero, a consequence of its high zero-point energy (ZPE). The ZPE of protons in a dense state is also high, and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by Neil Ashcroft and others that there is a melting point maximum in compressed hydrogen, but also that there might be a range of densities, at pressures around 400 GPa, where hydrogen would be a liquid metal, even at low temperatures.

Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of 200–250 K (−73 – −23 °C) at pressures of 500–1,500 GPa (4,900,000–14,800,000 atm; 73,000,000–218,000,000 psi).

Within this flat region there might be an elemental mesophase intermediate between the liquid and solid state, which could be metastably stabilized down to low temperature and enter a supersolid state.

Superconductivity

In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor, up to room temperature (290 K or 17 °C). This hypothesis is based on an expected strong coupling between conduction electrons and lattice vibrations.

As a rocket propellant

Metastable metallic hydrogen may have potential as a highly efficient rocket propellant, with a theoretical specific impulse of up to 1700 seconds, although a metastable form suitable for mass-production and conventional high-volume storage may not exist.

Possibility of novel types of quantum fluid

Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they might have quantum ordered states that cannot be classified as superconducting or superfluid in the usual sense. Instead, they might represent two possible novel types of quantum fluids: superconducting superfluids and metallic superfluids. Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of a magnetic field, hydrogen might exhibit phase transitions from superconductivity to superfluidity and vice versa.

Lithium alloying reduces requisite pressure

In 2009, Zurek et al. predicted that the alloy LiH
6 would be a stable metal at only one quarter of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiH_n and possibly "other alkali high-hydride systems", i.e. alloys of type XH_n where X is an alkali metal. This was later verified in AcH₈ and LaH₁₀ with Tc approaching 270K leading to speculation that other compounds may even be stable at mere MPa pressures with room temperature superconductivity.

Experimental pursuit

Shock-wave compression, 1996

In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced the first identifiably metallic hydrogen for about a microsecond at temperatures of thousands of kelvins, pressures of over 100 GPa (1,000,000 atm; 15,000,000 psi), and densities of approximately 0.6 g/cm³. The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen, thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 250 GPa (2,500,000 atm; 37,000,000 psi), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes they expected. The researchers used a 1960s-era light-gas gun, originally employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 140 GPa (1,400,000 atm; 21,000,000 psi), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about 15 eV, making it an insulator but, as the pressure increases significantly, the band-gap gradually fell to 0.3 eV. Because the thermal energy of the fluid (the temperature became about 3,000 K or 2,730 °C due to compression of the sample) was above 0.3 eV, the hydrogen might be considered metallic.

Other experimental research, 1996–2004

Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998, and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (320–340 GPa or 3,200,000–3,400,000 atm) and temperatures of 100–300 K (−173–27 °C), hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also ongoing on deuterium. Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (Rydberg matter) are effective promoters to metallic hydrogen.

Pulsed laser heating experiment, 2008

The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. Hydrogen-rich molecular silane (SiH
4) was claimed to be metallized and become superconducting by M.I. Eremets et al. This claim is disputed, and their results have not been repeated.

Observation of liquid metallic hydrogen, 2011

In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 260–300 GPa (2,600,000–3,000,000 atm). This claim was questioned by other researchers in 2012.

Z machine, 2015

In 2015, scientists at the Z Pulsed Power Facility announced the creation of metallic deuterium using dense liquid deuterium, an electrical insulator-to-conductor transition associated with an increase in optical reflectivity.

Claimed observation of solid metallic hydrogen, 2016

On 5 October 2016, Ranga Dias and Isaac F. Silvera of Harvard University released claims of experimental evidence that solid metallic hydrogen had been synthesised in the laboratory at a pressure of around 495 gigapascals (4,890,000 atm; 71,800,000 psi) using a diamond anvil cell. This manuscript was available in October 2016, and a revised version was subsequently published in the journal Science in January 2017.

In the preprint version of the paper, Dias and Silvera write:

With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7×10²³ particles/cm³, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.

— Dias & Silvera (2016)

Silvera stated that they did not repeat their experiment, since more tests could damage or destroy their existing sample, but assured the scientific community that more tests are coming. He also stated that the pressure would eventually be released, in order to find out whether the sample was metastable (i.e., whether it would persist in its metallic state even after the pressure was released).

Shortly after the claim was published in Science, Nature's news division published an article stating that some other physicists regarded the result with skepticism. Recently, prominent members of the high pressure research community have criticised the claimed results, questioning the claimed pressures or the presence of metallic hydrogen at the pressures claimed.

In February 2017, it was reported that the sample of claimed metallic hydrogen was lost, after the diamond anvils it was contained between broke.

In August 2017, Silvera and Dias issued an erratum to the Science article, regarding corrected reflectance values due to variations between the optical density of stressed natural diamonds and the synthetic diamonds used in their pre-compression diamond anvil cell.

In June 2019 a team at the Commissariat à l'énergie atomique et aux énergies alternatives (French Alternative Energies & Atomic Energy Commission) claimed to have created metallic hydrogen at around 425GPa using a toroidal profile diamond anvil cell produced using electron beam machining

Experiments on fluid deuterium at the National Ignition Facility, 2018

In August 2018, scientists announced new observations regarding the rapid transformation of fluid deuterium from an insulating to a metallic form below 2000 K. Remarkable agreement is found between the experimental data and the predictions based on Quantum Monte Carlo simulations, which is expected to be the most accurate method to date. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.

Degenerate matter

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Degenerate_matter 

Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant pressure in addition to, or in lieu of thermal pressure. The description applies to matter composed of electrons, protons, neutrons or other fermions. The term is mainly used in astrophysics to refer to dense stellar objects where gravitational pressure is so extreme that quantum mechanical effects are significant. This type of matter is naturally found in stars in their final evolutionary states, such as white dwarfs and neutron stars, where thermal pressure alone is not enough to avoid gravitational collapse.

Degenerate matter is usually modelled as an ideal Fermi gas, an ensemble of non-interacting fermions. In a quantum mechanical description, particles limited to a finite volume may take only a discrete set of energies, called quantum states. The Pauli exclusion principle prevents identical fermions from occupying the same quantum state. At lowest total energy (when the thermal energy of the particles is negligible), all the lowest energy quantum states are filled. This state is referred to as full degeneracy. This degeneracy pressure remains non-zero even at absolute zero temperature. Adding particles or reducing the volume forces the particles into higher-energy quantum states. In this situation, a compression force is required, and is made manifest as a resisting pressure. The key feature is that this degeneracy pressure does not depend on the temperature but only on the density of the fermions. Degeneracy pressure keeps dense stars in equilibrium, independent of the thermal structure of the star.

A degenerate mass whose fermions have velocities close to the speed of light (particle energy larger than its rest mass energy) is called relativistic degenerate matter.

The concept of degenerate stars, stellar objects composed of degenerate matter, was originally developed in a joint effort between Arthur Eddington, Ralph Fowler and Arthur Milne. Eddington had suggested that the atoms in Sirius B were almost completely ionised and closely packed. Fowler described white dwarfs as composed of a gas of particles that became degenerate at low temperature. Milne proposed that degenerate matter is found in most of the nuclei of stars, not only in compact stars.

Concept

If a plasma is cooled and under increasing pressure, it will eventually not be possible to compress the plasma any further. This constraint is due to the Pauli exclusion principle, which states that two fermions cannot share the same quantum state. When in this highly compressed state, since there is no extra space for any particles, a particle's location is extremely defined. Since the locations of the particles of a highly compressed plasma have very low uncertainty, their momentum is extremely uncertain. The Heisenberg uncertainty principle states

${\displaystyle \Delta p\Delta x\geq {\frac {\hbar }{2}}}$,

where Δp is the uncertainty in the particle's momentum and Δx is the uncertainty in position (and ħ is the reduced Planck constant). Therefore, even though the plasma is cold, such particles must on average be moving very fast. Large kinetic energies lead to the conclusion that, in order to compress an object into a very small space, tremendous force is required to control its particles' momentum.

Unlike a classical ideal gas, whose pressure is proportional to its temperature

${\displaystyle P=k_{\rm {B}}{\frac {NT}{V}}}$,

where P is pressure, k_B is Boltzmann's constant, N is the number of particles—typically atoms or molecules—, T is temperature, and V is the volume, the pressure exerted by degenerate matter depends only weakly on its temperature. In particular, the pressure remains nonzero even at absolute zero temperature. At relatively low densities, the pressure of a fully degenerate gas can be derived by treating the system as an ideal Fermi gas, in this way

${\displaystyle P={\frac {(3\pi ^{2})^{2/3}\hbar ^{2}}{5m}}\left({\frac {N}{V}}\right)^{5/3}}$,

where m is the mass of the individual particles making up the gas. At very high densities, where most of the particles are forced into quantum states with relativistic energies, the pressure is given by

${\displaystyle P=K\left({\frac {N}{V}}\right)^{4/3}}$,

where K is another proportionality constant depending on the properties of the particles making up the gas.

Pressure vs temperature curves of classical and quantum ideal gases (Fermi gas, Bose gas) in three dimensions.

All matter experiences both normal thermal pressure and degeneracy pressure, but in commonly encountered gases, thermal pressure dominates so much that degeneracy pressure can be ignored. Likewise, degenerate matter still has normal thermal pressure, the degeneracy pressure dominates to the point that temperature has a negligible effect on the total pressure. The adjacent figure shows how the pressure of a Fermi gas saturates as it is cooled down, relative to a classical ideal gas.

While degeneracy pressure usually dominates at extremely high densities, it is the ratio between degenerate pressure and thermal pressure which determines degeneracy. Given a sufficiently drastic increase in temperature (such as during a red giant star's helium flash), matter can become non-degenerate without reducing its density.

Degeneracy pressure contributes to the pressure of conventional solids, but these are not usually considered to be degenerate matter because a significant contribution to their pressure is provided by electrical repulsion of atomic nuclei and the screening of nuclei from each other by electrons. The free electron model of metals derives their physical properties by considering the conduction electrons alone as a degenerate gas, while the majority of the electrons are regarded as occupying bound quantum states. This solid state contrasts with degenerate matter that forms the body of a white dwarf, where most of the electrons would be treated as occupying free particle momentum states.

Exotic examples of degenerate matter include neutron degenerate matter, strange matter, metallic hydrogen and white dwarf matter.

Degenerate gases

Degenerate gases are gases composed of fermions such as electrons, protons, and neutrons rather than molecules of ordinary matter. The electron gas in ordinary metals and in the interior of white dwarfs are two examples. Following the Pauli exclusion principle, there can be only one fermion occupying each quantum state. In a degenerate gas, all quantum states are filled up to the Fermi energy. Most stars are supported against their own gravitation by normal thermal gas pressure, while in white dwarf stars the supporting force comes from the degeneracy pressure of the electron gas in their interior. In neutron stars, the degenerate particles are neutrons.

A fermion gas in which all quantum states below a given energy level are filled is called a fully degenerate fermion gas. The difference between this energy level and the lowest energy level is known as the Fermi energy.

Electron degeneracy

In an ordinary fermion gas in which thermal effects dominate, most of the available electron energy levels are unfilled and the electrons are free to move to these states. As particle density is increased, electrons progressively fill the lower energy states and additional electrons are forced to occupy states of higher energy even at low temperatures. Degenerate gases strongly resist further compression because the electrons cannot move to already filled lower energy levels due to the Pauli exclusion principle. Since electrons cannot give up energy by moving to lower energy states, no thermal energy can be extracted. The momentum of the fermions in the fermion gas nevertheless generates pressure, termed "degeneracy pressure".

Under high densities the matter becomes a degenerate gas when the electrons are all stripped from their parent atoms. In the core of a star, once hydrogen burning in nuclear fusion reactions stops, it becomes a collection of positively charged ions, largely helium and carbon nuclei, floating in a sea of electrons, which have been stripped from the nuclei. Degenerate gas is an almost perfect conductor of heat and does not obey the ordinary gas laws. White dwarfs are luminous not because they are generating any energy but rather because they have trapped a large amount of heat which is gradually radiated away. Normal gas exerts higher pressure when it is heated and expands, but the pressure in a degenerate gas does not depend on the temperature. When gas becomes super-compressed, particles position right up against each other to produce degenerate gas that behaves more like a solid. In degenerate gases the kinetic energies of electrons are quite high and the rate of collision between electrons and other particles is quite low, therefore degenerate electrons can travel great distances at velocities that approach the speed of light. Instead of temperature, the pressure in a degenerate gas depends only on the speed of the degenerate particles; however, adding heat does not increase the speed of most of the electrons, because they are stuck in fully occupied quantum states. Pressure is increased only by the mass of the particles, which increases the gravitational force pulling the particles closer together. Therefore, the phenomenon is the opposite of that normally found in matter where if the mass of the matter is increased, the object becomes bigger. In degenerate gas, when the mass is increased, the particles become spaced closer together due to gravity (and the pressure is increased), so the object becomes smaller. Degenerate gas can be compressed to very high densities, typical values being in the range of 10,000 kilograms per cubic centimeter.

There is an upper limit to the mass of an electron-degenerate object, the Chandrasekhar limit, beyond which electron degeneracy pressure cannot support the object against collapse. The limit is approximately 1.44 solar masses for objects with typical compositions expected for white dwarf stars (carbon and oxygen with two baryons per electron). This mass cutoff is appropriate only for a star supported by ideal electron degeneracy pressure under Newtonian gravity; in general relativity and with realistic Coulomb corrections, the corresponding mass limit is around 1.38 solar masses. The limit may also change with the chemical composition of the object, as it affects the ratio of mass to number of electrons present. The object's rotation, which counteracts the gravitational force, also changes the limit for any particular object. Celestial objects below this limit are white dwarf stars, formed by the gradual shrinking of the cores of stars that run out of fuel. During this shrinking, an electron-degenerate gas forms in the core, providing sufficient degeneracy pressure as it is compressed to resist further collapse. Above this mass limit, a neutron star (primarily supported by neutron degeneracy pressure) or a black hole may be formed instead.

Neutron degeneracy

Neutron degeneracy is analogous to electron degeneracy and is demonstrated in neutron stars, which are partially supported by the pressure from a degenerate neutron gas. The collapse happens when the core of a white dwarf exceeds approximately 1.4 solar masses, which is the Chandrasekhar limit, above which the collapse is not halted by the pressure of degenerate electrons. As the star collapses, the Fermi energy of the electrons increases to the point where it is energetically favorable for them to combine with protons to produce neutrons (via inverse beta decay, also termed electron capture). The result is an extremely compact star composed of nuclear matter, which is predominantly a degenerate neutron gas, sometimes called neutronium, with a small admixture of degenerate proton and electron gases.

Neutrons in a degenerate neutron gas are spaced much more closely than electrons in an electron-degenerate gas because the more massive neutron has a much shorter wavelength at a given energy. In the case of neutron stars and white dwarfs, this phenomenon is compounded by the fact that the pressures within neutron stars are much higher than those in white dwarfs. The pressure increase is caused by the fact that the compactness of a neutron star causes gravitational forces to be much higher than in a less compact body with similar mass. The result is a star with a diameter on the order of a thousandth that of a white dwarf.

There is an upper limit to the mass of a neutron-degenerate object, the Tolman–Oppenheimer–Volkoff limit, which is analogous to the Chandrasekhar limit for electron-degenerate objects. The theoretical limit for non-relativistic objects supported by ideal neutron degeneracy pressure is only 0.75 solar masses; however, with more realistic models including baryon interaction, the precise limit is unknown, as it depends on the equations of state of nuclear matter, for which a highly accurate model is not yet available. Above this limit, a neutron star may collapse into a black hole or into other dense forms of degenerate matter.

Proton degeneracy

Sufficiently dense matter containing protons experiences proton degeneracy pressure, in a manner similar to the electron degeneracy pressure in electron-degenerate matter: protons confined to a sufficiently small volume have a large uncertainty in their momentum due to the Heisenberg uncertainty principle. However, because protons are much more massive than electrons, the same momentum represents a much smaller velocity for protons than for electrons. As a result, in matter with approximately equal numbers of protons and electrons, proton degeneracy pressure is much smaller than electron degeneracy pressure, and proton degeneracy is usually modeled as a correction to the equations of state of electron-degenerate matter.

Quark degeneracy

At densities greater than those supported by neutron degeneracy, quark matter is expected to occur. Several variations of this hypothesis have been proposed that represent quark-degenerate states. Strange matter is a degenerate gas of quarks that is often assumed to contain strange quarks in addition to the usual up and down quarks. Color superconductor materials are degenerate gases of quarks in which quarks pair up in a manner similar to Cooper pairing in electrical superconductors. The equations of state for the various proposed forms of quark-degenerate matter vary widely, and are usually also poorly defined, due to the difficulty of modeling strong force interactions.

Quark-degenerate matter may occur in the cores of neutron stars, depending on the equations of state of neutron-degenerate matter. It may also occur in hypothetical quark stars, formed by the collapse of objects above the Tolman–Oppenheimer–Volkoff mass limit for neutron-degenerate objects. Whether quark-degenerate matter forms at all in these situations depends on the equations of state of both neutron-degenerate matter and quark-degenerate matter, both of which are poorly known. Quark stars are considered to be an intermediate category between neutron stars and black holes.

Atomic nucleus

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Atomic_nucleus 

 

Nuclear physics
Nucleus · Nucleons (p, n) · Nuclear matter · Nuclear force · Nuclear structure · Nuclear reaction

A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue). In this diagram, protons and neutrons look like little balls stuck together, but an actual nucleus (as understood by modern nuclear physics) cannot be explained like this, but only by using quantum mechanics. In a nucleus which occupies a certain energy level (for example, the ground state), each nucleon can be said to occupy a range of locations.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. An atom is composed of a positively-charged nucleus, with a cloud of negatively-charged electrons surrounding it, bound together by electrostatic force. Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force.

The diameter of the nucleus is in the range of 1.7566 fm (1.7566×10⁻¹⁵ m) for hydrogen (the diameter of a single proton) to about 11.7142 fm for uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 26,634 (uranium atomic radius is about 156 pm (156×10⁻¹² m)) to about 60,250 (hydrogen atomic radius is about 52.92 pm).

The branch of physics concerned with the study and understanding of the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics.

Introduction

History

The nucleus was discovered in 1911, as a result of Ernest Rutherford's efforts to test Thomson's "plum pudding model" of the atom. The electron had already been discovered by J.J. Thomson. Knowing that atoms are electrically neutral, J.J.Thomson postulated that there must be a positive charge as well. In his plum pudding model, Thomson suggested that an atom consisted of negative electrons randomly scattered within a sphere of positive charge. Ernest Rutherford later devised an experiment with his research partner Hans Geiger and with help of Ernest Marsden, that involved the deflection of alpha particles (helium nuclei) directed at a thin sheet of metal foil. He reasoned that if J.J Thomson's model were correct, the positively charged alpha particles would easily pass through the foil with very little deviation in their paths, as the foil should act as electrically neutral if the negative and positive charges are so intimately mixed as to make it appear neutral. To his surprise, many of the particles were deflected at very large angles. Because the mass of an alpha particle is about 8000 times that of an electron, it became apparent that a very strong force must be present if it could deflect the massive and fast moving alpha particles. He realized that the plum pudding model could not be accurate and that the deflections of the alpha particles could only be explained if the positive and negative charges were separated from each other and that the mass of the atom was a concentrated point of positive charge. This justified the idea of a nuclear atom with a dense center of positive charge and mass.

Etymology

The term nucleus is from the Latin word nucleus, a diminutive of nux ("nut"), meaning the kernel (i.e., the "small nut") inside a watery type of fruit (like a peach). In 1844, Michael Faraday used the term to refer to the "central point of an atom". The modern atomic meaning was proposed by Ernest Rutherford in 1912. The adoption of the term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule, that "the atom is composed of the kernel and an outer atom or shell"

Nuclear makeup

A figurative depiction of the helium-4 atom with the electron cloud in shades of gray. In the nucleus, the two protons and two neutrons are depicted in red and blue. This depiction shows the particles as separate, whereas in an actual helium atom, the protons are superimposed in space and most likely found at the very center of the nucleus, and the same is true of the two neutrons. Thus, all four particles are most likely found in exactly the same space, at the central point. Classical images of separate particles fail to model known charge distributions in very small nuclei. A more accurate image is that the spatial distribution of nucleons in a helium nucleus is much closer to the helium electron cloud shown here, although on a far smaller scale, than to the fanciful nucleus image.

The nucleus of an atom consists of neutrons and protons, which in turn are the manifestation of more elementary particles, called quarks, that are held in association by the nuclear strong force in certain stable combinations of hadrons, called baryons. The nuclear strong force extends far enough from each baryon so as to bind the neutrons and protons together against the repulsive electrical force between the positively charged protons. The nuclear strong force has a very short range, and essentially drops to zero just beyond the edge of the nucleus. The collective action of the positively charged nucleus is to hold the electrically negative charged electrons in their orbits about the nucleus. The collection of negatively charged electrons orbiting the nucleus display an affinity for certain configurations and numbers of electrons that make their orbits stable. Which chemical element an atom represents is determined by the number of protons in the nucleus; the neutral atom will have an equal number of electrons orbiting that nucleus. Individual chemical elements can create more stable electron configurations by combining to share their electrons. It is that sharing of electrons to create stable electronic orbits about the nucleus that appears to us as the chemistry of our macro world.

Protons define the entire charge of a nucleus, and hence its chemical identity. Neutrons are electrically neutral, but contribute to the mass of a nucleus to nearly the same extent as the protons. Neutrons can explain the phenomenon of isotopes (same atomic number with different atomic mass). The main role of neutrons is to reduce electrostatic repulsion inside the nucleus.

Composition and shape

Protons and neutrons are fermions, with different values of the strong isospin quantum number, so two protons and two neutrons can share the same space wave function since they are not identical quantum entities. They are sometimes viewed as two different quantum states of the same particle, the nucleon. Two fermions, such as two protons, or two neutrons, or a proton + neutron (the deuteron) can exhibit bosonic behavior when they become loosely bound in pairs, which have integer spin.

In the rare case of a hypernucleus, a third baryon called a hyperon, containing one or more strange quarks and/or other unusual quark(s), can also share the wave function. However, this type of nucleus is extremely unstable and not found on Earth except in high energy physics experiments.

The neutron has a positively charged core of radius ≈ 0.3 fm surrounded by a compensating negative charge of radius between 0.3 fm and 2 fm. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.

Nuclei can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped.

Forces

Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is a minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms together internally (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus).

The nuclear force is highly attractive at the distance of typical nucleon separation, and this overwhelms the repulsion between protons due to the electromagnetic force, thus allowing nuclei to exist. However, the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential); thus only nuclei smaller than a certain size can be completely stable. The largest known completely stable nucleus (i.e. stable to alpha, beta, and gamma decay) is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximum are unstable and tend to be increasingly short-lived with larger numbers of nucleons. However, bismuth-209 is also stable to beta decay and has the longest half-life to alpha decay of any known isotope, estimated at a billion times longer than the age of the universe.

The residual strong force is effective over a very short range (usually only a few femtometres (fm); roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons. For example, between protons and neutrons to form [NP] deuteron, and also between protons and protons, and neutrons and neutrons.

Halo nuclei and nuclear force range limits

The effective absolute limit of the range of the nuclear force (also known as residual strong force) is represented by halo nuclei such as lithium-11 or boron-14, in which dineutrons, or other collections of neutrons, orbit at distances of about 10 fm (roughly similar to the 8 fm radius of the nucleus of uranium-238). These nuclei are not maximally dense. Halo nuclei form at the extreme edges of the chart of the nuclides—the neutron drip line and proton drip line—and are all unstable with short half-lives, measured in milliseconds; for example, lithium-11 has a half-life of 8.8 ms.

Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons [NN, NNN] or protons [PP, PPP]. Nuclei which have a single neutron halo include ¹¹Be and ¹⁹C. A two-neutron halo is exhibited by ⁶He, ¹¹Li, ¹⁷B, ¹⁹B and ²²C. Two-neutron halo nuclei break into three fragments, never two, and are called Borromean nuclei because of this behavior (referring to a system of three interlocked rings in which breaking any ring frees both of the others). ⁸He and ¹⁴Be both exhibit a four-neutron halo. Nuclei which have a proton halo include ⁸B and ²⁶P. A two-proton halo is exhibited by ¹⁷Ne and ²⁷S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive electromagnetic forces of the excess proton(s).

Nuclear models

Although the standard model of physics is widely believed to completely describe the composition and behavior of the nucleus, generating predictions from theory is much more difficult than for most other areas of particle physics. This is due to two reasons:

• In principle, the physics within a nucleus can be derived entirely from quantum chromodynamics (QCD). In practice however, current computational and mathematical approaches for solving QCD in low-energy systems such as the nuclei are extremely limited. This is due to the phase transition that occurs between high-energy quark matter and low-energy hadronic matter, which renders perturbative techniques unusable, making it difficult to construct an accurate QCD-derived model of the forces between nucleons. Current approaches are limited to either phenomenological models such as the Argonne v18 potential or chiral effective field theory.
• Even if the nuclear force is well constrained, a significant amount of computational power is required to accurately compute the properties of nuclei ab initio. Developments in many-body theory have made this possible for many low mass and relatively stable nuclei, but further improvements in both computational power and mathematical approaches are required before heavy nuclei or highly unstable nuclei can be tackled.

Historically, experiments have been compared to relatively crude models that are necessarily imperfect. None of these models can completely explain experimental data on nuclear structure.

The nuclear radius (R) is considered to be one of the basic quantities that any model must predict. For stable nuclei (not halo nuclei or other unstable distorted nuclei) the nuclear radius is roughly proportional to the cube root of the mass number (A) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations:

The stable nucleus has approximately a constant density and therefore the nuclear radius R can be approximated by the following formula,

${\displaystyle R=r_{0}A^{1/3}\,}$

where A = Atomic mass number (the number of protons Z, plus the number of neutrons N) and r₀ = 1.25 fm = 1.25 × 10⁻¹⁵ m. In this equation, the "constant" r₀ varies by 0.2 fm, depending on the nucleus in question, but this is less than 20% change from a constant.

In other words, packing protons and neutrons in the nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight spherical or almost spherical bag (some stable nuclei are not quite spherical, but are known to be prolate).

Models of nuclear structure include :

Liquid drop model

Early models of the nucleus viewed the nucleus as a rotating liquid drop. In this model, the trade-off of long-range electromagnetic forces and relatively short-range nuclear forces, together cause behavior which resembled surface tension forces in liquid drops of different sizes. This formula is successful at explaining many important phenomena of nuclei, such as their changing amounts of binding energy as their size and composition changes, but it does not explain the special stability which occurs when nuclei have special "magic numbers" of protons or neutrons.

The terms in the semi-empirical mass formula, which can be used to approximate the binding energy of many nuclei, are considered as the sum of five types of energies (see below). Then the picture of a nucleus as a drop of incompressible liquid roughly accounts for the observed variation of binding energy of the nucleus:

Volume energy. When an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. So, this nuclear energy is proportional to the volume.

Surface energy. A nucleon at the surface of a nucleus interacts with fewer other nucleons than one in the interior of the nucleus and hence its binding energy is less. This surface energy term takes that into account and is therefore negative and is proportional to the surface area.

Coulomb Energy. The electric repulsion between each pair of protons in a nucleus contributes toward decreasing its binding energy.

Asymmetry energy (also called Pauli Energy). An energy associated with the Pauli exclusion principle. Were it not for the Coulomb energy, the most stable form of nuclear matter would have the same number of neutrons as protons, since unequal numbers of neutrons and protons imply filling higher energy levels for one type of particle, while leaving lower energy levels vacant for the other type.

Pairing energy. An energy which is a correction term that arises from the tendency of proton pairs and neutron pairs to occur. An even number of particles is more stable than an odd number.

Shell models and other quantum models

Main article: Nuclear shell model

A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory. These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells.

In the above models, the nucleons may occupy orbitals in pairs, due to being fermions, which allows explanation of even/odd Z and N effects well-known from experiments. The exact nature and capacity of nuclear shells differs from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nuclei) is quite different from the central electromagnetic potential well which binds electrons in atoms. Some resemblance to atomic orbital models may be seen in a small atomic nucleus like that of helium-4, in which the two protons and two neutrons separately occupy 1s orbitals analogous to the 1s orbital for the two electrons in the helium atom, and achieve unusual stability for the same reason. Nuclei with 5 nucleons are all extremely unstable and short-lived, yet, helium-3, with 3 nucleons, is very stable even with lack of a closed 1s orbital shell. Another nucleus with 3 nucleons, the triton hydrogen-3 is unstable and will decay into helium-3 when isolated. Weak nuclear stability with 2 nucleons {NP} in the 1s orbital is found in the deuteron hydrogen-2, with only one nucleon in each of the proton and neutron potential wells. While each nucleon is a fermion, the {NP} deuteron is a boson and thus does not follow Pauli Exclusion for close packing within shells. Lithium-6 with 6 nucleons is highly stable without a closed second 1p shell orbital. For light nuclei with total nucleon numbers 1 to 6 only those with 5 do not show some evidence of stability. Observations of beta-stability of light nuclei outside closed shells indicate that nuclear stability is much more complex than simple closure of shell orbitals with magic numbers of protons and neutrons.

For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict the magic numbers of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly-inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element. Similarly, the distance from shell-closure explains the unusual instability of isotopes which have far from stable numbers of these particles, such as the radioactive elements 43 (technetium) and 61 (promethium), each of which is preceded and followed by 17 or more stable elements.

There are however problems with the shell model when an attempt is made to account for nuclear properties well away from closed shells. This has led to complex post hoc distortions of the shape of the potential well to fit experimental data, but the question remains whether these mathematical manipulations actually correspond to the spatial deformations in real nuclei. Problems with the shell model have led some to propose realistic two-body and three-body nuclear force effects involving nucleon clusters and then build the nucleus on this basis. Three such cluster models are the 1936 Resonating Group Structure model of John Wheeler, Close-Packed Spheron Model of Linus Pauling and the 2D Ising Model of MacGregor.

Consistency between models

As with the case of superfluid liquid helium, atomic nuclei are an example of a state in which both (1) "ordinary" particle physical rules for volume and (2) non-intuitive quantum mechanical rules for a wave-like nature apply. In superfluid helium, the helium atoms have volume, and essentially "touch" each other, yet at the same time exhibit strange bulk properties, consistent with a Bose–Einstein condensation. The nucleons in atomic nuclei also exhibit a wave-like nature and lack standard fluid properties, such as friction. For nuclei made of hadrons which are fermions, Bose-Einstein condensation does not occur, yet nevertheless, many nuclear properties can only be explained similarly by a combination of properties of particles with volume, in addition to the frictionless motion characteristic of the wave-like behavior of objects trapped in Erwin Schrödinger's quantum orbitals.