Star formation

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

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

Stellar nurseries

Hubble telescope image known as Pillars of Creation, where stars are forming in the Eagle Nebula

Interstellar clouds

W51 Nebula - One of the largest Star Factories in the Milky Way (August 25, 2020)

A spiral galaxy like the Milky Way contains stars, stellar remnants, and a diffuse interstellar medium (ISM) of gas and dust. The interstellar medium consists of 10⁻⁴ to 10⁶ particles per cm³ and is typically composed of roughly 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies.

In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H₂) form, so these nebulae are called molecular clouds.^[4] Herschel space observatory have revealed that filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments, which are central to the star formation process, will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments observations have revealed quasi-periodic chains of dense cores with spacing comparable to the filament inner width, and includes embedded protostars with outflows. Observations indicate that the coldest clouds tend to form low-mass stars, observed first in the infrared inside the clouds, then in visible light at their surface when the clouds dissipate, while giant molecular clouds, which are generally warmer, produce stars of all masses. These giant molecular clouds have typical densities of 100 particles per cm³, diameters of 100 light-years (9.5×10¹⁴ km), masses of up to 6 million solar masses (M_☉), and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds, each with more than 100,000 M_☉. The nearest nebula to the Sun where massive stars are being formed is the Orion Nebula, 1,300 ly (1.2×10¹⁶ km) away. However, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex.

A more compact site of star formation is the opaque clouds of dense gas and dust known as Bok globules, so named after the astronomer Bart Bok. These can form in association with collapsing molecular clouds or possibly independently. The Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars. Over half the known Bok globules have been found to contain newly forming stars.

Assembly of galaxy in early Universe.

Cloud collapse

An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy. If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. The mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses. During cloud collapse dozens to ten thousands of stars form more or less simultaneously which is observable in so-called embedded clusters. The end product of a core collapse is an open cluster of stars.

ALMA observations of the Orion Nebula complex provide insights into explosions at star birth.

In triggered star formation, one of several events might occur to compress a molecular cloud and initiate its gravitational collapse. Molecular clouds may collide with each other, or a nearby supernova explosion can be a trigger, sending shocked matter into the cloud at very high speeds. (The resulting new stars may themselves soon produce supernovae, producing self-propagating star formation.) Alternatively, galactic collisions can trigger massive starbursts of star formation as the gas clouds in each galaxy are compressed and agitated by tidal forces. The latter mechanism may be responsible for the formation of globular clusters.

A supermassive black hole at the core of a galaxy may serve to regulate the rate of star formation in a galactic nucleus. A black hole that is accreting infalling matter can become active, emitting a strong wind through a collimated relativistic jet. This can limit further star formation. Massive black holes ejecting radio-frequency-emitting particles at near-light speed can also block the formation of new stars in aging galaxies. However, the radio emissions around the jets may also trigger star formation. Likewise, a weaker jet may trigger star formation when it collides with a cloud.

Dwarf galaxy ESO 553-46 has one of the highest rates of star formation of the 1000 or so galaxies nearest to the Milky Way.

As it collapses, a molecular cloud breaks into smaller and smaller pieces in a hierarchical manner, until the fragments reach stellar mass. In each of these fragments, the collapsing gas radiates away the energy gained by the release of gravitational potential energy. As the density increases, the fragments become opaque and are thus less efficient at radiating away their energy. This raises the temperature of the cloud and inhibits further fragmentation. The fragments now condense into rotating spheres of gas that serve as stellar embryos.

Complicating this picture of a collapsing cloud are the effects of turbulence, macroscopic flows, rotation, magnetic fields and the cloud geometry. Both rotation and magnetic fields can hinder the collapse of a cloud. Turbulence is instrumental in causing fragmentation of the cloud, and on the smallest scales it promotes collapse.

Protostar

LH 95 stellar nursery in Large Magellanic Cloud.

A protostellar cloud will continue to collapse as long as the gravitational binding energy can be eliminated. This excess energy is primarily lost through radiation. However, the collapsing cloud will eventually become opaque to its own radiation, and the energy must be removed through some other means. The dust within the cloud becomes heated to temperatures of 60–100 K, and these particles radiate at wavelengths in the far infrared where the cloud is transparent. Thus the dust mediates the further collapse of the cloud.

During the collapse, the density of the cloud increases towards the center and thus the middle region becomes optically opaque first. This occurs when the density is about 10⁻¹³ g / cm³. A core region, called the first hydrostatic core, forms where the collapse is essentially halted. It continues to increase in temperature as determined by the virial theorem. The gas falling toward this opaque region collides with it and creates shock waves that further heat the core.

Composite image showing young stars in and around molecular cloud Cepheus B.

When the core temperature reaches about 2000 K, the thermal energy dissociates the H₂ molecules. This is followed by the ionization of the hydrogen and helium atoms. These processes absorb the energy of the contraction, allowing it to continue on timescales comparable to the period of collapse at free fall velocities. After the density of infalling material has reached about 10⁻⁸ g / cm³, that material is sufficiently transparent to allow energy radiated by the protostar to escape. The combination of convection within the protostar and radiation from its exterior allow the star to contract further. This continues until the gas is hot enough for the internal pressure to support the protostar against further gravitational collapse—a state called hydrostatic equilibrium. When this accretion phase is nearly complete, the resulting object is known as a protostar.

N11, part of a complex network of gas clouds and star clusters within our neighbouring galaxy, the Large Magellanic Cloud.

Accretion of material onto the protostar continues partially from the newly formed circumstellar disc. When the density and temperature are high enough, deuterium fusion begins, and the outward pressure of the resultant radiation slows (but does not stop) the collapse. Material comprising the cloud continues to "rain" onto the protostar. In this stage bipolar jets are produced called Herbig–Haro objects. This is probably the means by which excess angular momentum of the infalling material is expelled, allowing the star to continue to form.

Star formation region Lupus 3.

When the surrounding gas and dust envelope disperses and accretion process stops, the star is considered a pre-main-sequence star (PMS star). The energy source of these objects is gravitational contraction, as opposed to hydrogen burning in main sequence stars. The PMS star follows a Hayashi track on the Hertzsprung–Russell (H–R) diagram. The contraction will proceed until the Hayashi limit is reached, and thereafter contraction will continue on a Kelvin–Helmholtz timescale with the temperature remaining stable. Stars with less than 0.5 M_☉ thereafter join the main sequence. For more massive PMS stars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track.

Finally, hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away. This ends the protostellar phase and begins the star's main sequence phase on the H–R diagram.

The stages of the process are well defined in stars with masses around 1 M_☉ or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars is studied in stellar evolution.

Protostar
Protostar outburst - HOPS 383 (2015).

Observations

The Orion Nebula is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars.

Key elements of star formation are only available by observing in wavelengths other than the optical. The protostellar stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust left over from the GMC. Often, these star-forming cocoons known as Bok globules, can be seen in silhouette against bright emission from surrounding gas. Early stages of a star's life can be seen in infrared light, which penetrates the dust more easily than visible light. Observations from the Wide-field Infrared Survey Explorer (WISE) have thus been especially important for unveiling numerous galactic protostars and their parent star clusters. Examples of such embedded star clusters are FSR 1184, FSR 1190, Camargo 14, Camargo 74, Majaess 64, and Majaess 98.

Star-forming region S106.

The structure of the molecular cloud and the effects of the protostar can be observed in near-IR extinction maps (where the number of stars are counted per unit area and compared to a nearby zero extinction area of sky), continuum dust emission and rotational transitions of CO and other molecules; these last two are observed in the millimeter and submillimeter range. The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, as the extinction caused by the rest of the cloud in which the star is forming is usually too big to allow us to observe it in the visual part of the spectrum. This presents considerable difficulties as the Earth's atmosphere is almost entirely opaque from 20μm to 850μm, with narrow windows at 200μm and 450μm. Even outside this range, atmospheric subtraction techniques must be used.

Young stars (purple) revealed by X-ray inside the NGC 2024 star-forming region.

X-ray observations have proven useful for studying young stars, since X-ray emission from these objects is about 100–100,000 times stronger than X-ray emission from main-sequence stars. The earliest detections of X-rays from T Tauri stars were made by the Einstein X-ray Observatory. For low-mass stars X-rays are generated by the heating of the stellar corona through magnetic reconnection, while for high-mass O and early B-type stars X-rays are generated through supersonic shocks in the stellar winds. Photons in the soft X-ray energy range covered by the Chandra X-ray Observatory and XMM-Newton may penetrate the interstellar medium with only moderate absorption due to gas, making the X-ray a useful wavelength for seeing the stellar populations within molecular clouds. X-ray emission as evidence of stellar youth makes this band particularly useful for performing censuses of stars in star-forming regions, given that not all young stars have infrared excesses. X-ray observations have provided near-complete censuses of all stellar-mass objects in the Orion Nebula Cluster and Taurus Molecular Cloud.

The formation of individual stars can only be directly observed in the Milky Way Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.

Initial research indicates star-forming clumps start as giant, dense areas in turbulent gas-rich matter in young galaxies, live about 500 million years, and may migrate to the center of a galaxy, creating the central bulge of a galaxy.

On February 21, 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.

In February 2018, astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed - about 180 million years after the Big Bang.

An article published on October 22, 2019, reported on the detection of 3MM-1, a massive star-forming galaxy about 12.5 billion light-years away that is obscured by clouds of dust. At a mass of about 10^10.8 solar masses, it showed a star formation rate about 100 times as high as in the Milky Way.

Notable pathfinder objects

• MWC 349 was first discovered in 1978, and is estimated to be only 1,000 years old.
• VLA 1623 – The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years.
• L1014 – An extremely faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like brown dwarfs or even rogue planets).
• GCIRS 8* – The youngest known main sequence star in the Galactic Center region, discovered in August 2006. It is estimated to be 3.5 million years old.

Low mass and high mass star formation

Star-forming region Westerhout 40 and the Serpens-Aquila Rift- cloud filaments containing new stars fill the region.

Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by observation, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 M_☉, however, the mechanism of star formation is not well understood.

Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses. Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the radiation from a massive protostar can escape without hindering accretion through the disk and onto the protostar. Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.

There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks. Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competitive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region.

Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.

Shutdown of thermohaline circulation

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

 

A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents

A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation.

A 2015 study suggested that the Atlantic meridional overturning circulation (AMOC) has weakened by 15-20% in 200 years.

General

Don Chambers from the University of South Florida College of Marine Science mentioned: "The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast." James Hansen and Makiko Sato stated:

AMOC slowdown that causes cooling ~1 °C and perhaps affects weather patterns is very different from an AMOC shutdown that cools the North Atlantic several degrees Celsius; the latter would have dramatic effects on storms and be irreversible on the century time scale.

Downturn of the Atlantic meridional overturning circulation has been tied to extreme regional sea level rise.

A 2017 review concluded that there is strong evidence for past changes in the strength and structure of the AMOC during abrupt climate events such as the Younger Dryas and many of the Heinrich events.

Slowdown

Lohmann and Dima 2010 found a weakening of the AMOC since the late 1930s. Climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggested that the observed cold pattern during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings in 2015, and concluded that the AMOC circulation showed exceptional slowdown in the last century, and that Greenland melt is a possible contributor, with the slowdown of AMOC since the 1970s being unprecedented over the last millennium.

A study published in 2016 found further evidence for a considerable impact from sea level rise for the U.S. East Coast. The study confirms earlier research findings which identified the region as a hotspot for rising seas, with a potential to divert 3–4 times in the rate of rise, compared to the global average. The researchers attribute the possible increase to an ocean circulation mechanism called deep water formation, which is reduced due to AMOC slow down, leading to more warmer water pockets below the surface. Additionally, the study noted, "Our results suggest that higher carbon emission rates also contribute to increased [sea level rise] in this region compared to the global average."

Shutdown

Global warming could, via a shutdown of the thermohaline circulation, trigger cooling in the North Atlantic, Europe, and North America. This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift. Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen (O
₂) below surface levels of the stagnant oceans becomes completely depleted – a probable cause of past mass extinction events.

Effects on weather

Hansen et al. 2015 found, that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.

Many of the most memorable and devastating storms in eastern North America and western Europe, popularly known as superstorms, have been winter cyclonic storms, though sometimes occurring in late fall or early spring, that generate near-hurricane-force winds and often large amounts of snowfall. Continued warming of low latitude oceans in coming decades will provide more water vapor to strengthen such storms. If this tropical warming is combined with a cooler North Atlantic Ocean from AMOC slowdown and an increase in midlatitude eddy energy, we can anticipate more severe baroclinic storms.

Hansen et al. results at least imply that strong cooling in the North Atlantic from AMOC shutdown does create higher wind speed. The increment in seasonal mean wind speed of the northeasterlies relative to preindustrial conditions is as much as 10–20%. Such a percentage increase of wind speed in a storm translates into an increase of storm power dissipation by a factor ∼1.4–2, because wind power dissipation is proportional to the cube of wind speed. However, the simulated changes refer to seasonal mean winds averaged over large grid-boxes, not individual storms.

Observations

2010 and earlier

In April 2004, the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.

In May 2005, Peter Wadhams reported in The Times (London) about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.

In 2005 a 30% reduction in the warm currents that carry water north from the Gulf Stream was observed from the last such measurement in 1992. The authors noted uncertainties in the measurements. Following media discussions, Detlef Quadfasel pointed out that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results, and implications based on palaeoclimate records show drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concluded that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation. In response Quirin Schiermeier concluded that natural variation was the culprit for the observations but highlighted possible implications.

In 2008, Vage et al. reported "the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008," employing "profiling float data from the Argo program to document deep mixing," and "a variety of in situ, satellite and reanalysis data" to set the context for the phenomenon. This might have a lot to do with the observations of variations in cold water chimney behaviour.

In January 2010, the Gulf Stream briefly connected with the West Greenland Current after fluctuating for a few weeks due to an extreme negative phase of the Arctic oscillation, temporarily diverting it west of Greenland.

Thermohaline circulation and fresh water

The red end of the spectrum indicates slowing in this presentation of the trend of velocities derived from NASA Pathfinder altimeter data from May 1992 to June 2002. Source: NASA.

Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream, a wind-driven gyre, which transports warm water from the Caribbean northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe.

The evaporation of ocean water in the North Atlantic increases the salinity of the water as well as cooling it, both actions increasing the density of water at the surface. Formation of sea ice further increases the salinity and density, because salt is ejected into the ocean when sea ice forms. This dense water then sinks and the circulation stream continues in a southerly direction. However, the Atlantic Meridional Overturning Circulation (AMOC) is driven by ocean temperature and salinity differences. But freshwater decreases ocean water salinity, and through this process prevents colder waters sinking. This mechanism possibly caused the cold ocean surface temperature anomaly currently observed near Greenland (Cold blob (North Atlantic)).

Global warming could lead to an increase in freshwater in the northern oceans, by melting glaciers in Greenland, and by increasing precipitation, especially through Siberian rivers.

An AMOC shutdown may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. 2002 model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F).

Studies of the Florida Current suggest that the Gulf Stream weakens with cooling, being weakest (by ~10%) during the Little Ice Age.

Subpolar gyre

Recent studies (2017) suggest potential convection collapse (heat transport) of the subpolar gyre in the North Atlantic, resulting in rapid cooling, with implications for economic sectors, agriculture industry, water resources and energy management in Western Europe and the East Coast of the United States. Frajka-Williams et al. 2017 pointed out that recent changes in cooling of the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics, increased the spatial distribution of meridional gradient in sea surface temperatures, which is not captured by the AMO Index.

IPCC models

Based on coupled Atmosphere-Ocean General Circulation Models from 2001, the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even over Europe. In the IPCC Fifth Assessment Report, it was reported that it is very unlikely that the AMOC will undergo a rapid transition (high confidence).

In popular culture

The film The Day After Tomorrow exaggerates a scenario related to the AMOC shutdown.

Kim Stanley Robinson's science-fiction novel Fifty Degrees Below, a volume in his Science in the Capital series, depicts a shutdown of thermohaline circulation & mankind's efforts to counteract it by adding great quantities of salt to the ocean.

In Ian Douglas' Star Corpsman novels, an AMOC shutdown triggered an early glacial maximum, covering most of Canada and northern Europe in ice sheet by the mid-22nd century.

Effects of climate change on oceans

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

 

Global mean land-ocean temperature change from 1880–2011, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. The green bars show uncertainty estimates. Source: NASA GISS

Effects of climate change on oceans provides information on the various effects that climate change has on oceans. Climate change can affect sea levels, coastlines, ocean acidification, ocean currents, seawater, sea surface temperatures, tides, the sea floor, weather, and trigger several changes in ocean bio-geochemistry; all of these affect the functioning of a society.

Sea level

Waves on an ocean coast

 

Animated map exhibiting the world's oceanic waters. A continuous body of water encircling Earth, the World Ocean is divided into a number of principal areas with relatively free interchange among them. Five oceanic divisions are usually reckoned: Pacific, Atlantic, Indian, Arctic, and Southern; the last two listed are sometimes consolidated into the first three.

 

Energy (heat) added to various parts of the climate system due to global warming.

Coasts

There are a number of factors affecting rising sea levels, including the thermal expansion of seawater, the melting of glaciers and ice sheets on land, and possibly human changes to groundwater storage.

The consensus of many studies of coastal tide gauge records is that during the past century sea level has risen worldwide at an average rate of 1–2 mm/yr reflecting a net flux of heat into the surface of the land and oceans. Corresponding studies based on satellite altimetry shows that this rate has increased to closer to 3 mm/yr during the more completely monitored past 20 years. A recent review of the literature suggests that 30% of the sea level rise since 1993 is due to thermal expansion and 55% due to continental ice melt, both resulting from warming global temperatures. In another study, results estimate the heat content of the ocean in the upper 700 meters has increased significantly from 1955–2010. It has to be reminded that in this context the usage of the word heat is extremely improper, as heat cannot be stored in a body but only exchanged between bodies. Observations of the changes in "heat content" of the ocean are important for providing realistic estimates of how the ocean is changing with global warming. An even more recent study of the contributions to global sea level due to melting of the two large ice sheets based on satellite measurements of gravity fluctuations suggests that the melting of these alone are causing global sea level to about 1 mm/yr. In a recent modeling study, scientists used an earth system model to study several variables of the ocean, one of which was the "heat content" of the oceans over the past several hundred years. The earth system model incorporated the atmosphere, land surface processes, and other earth components to make it more realistic and similar to observations. Results of their model simulation showed that since 1500, the ocean "heat content" of the upper 500 m has increased.

The connection between sea level rise and ocean thermal expansion follows from Charles's law (also known as the law of volumes) put simply states that the volume of a given mass is proportional to its temperature. This contribution to sea level is monitored by oceanographers using a succession of temperature measuring profiling instruments, which is then compiled at national data centers such as the United States National Oceanographic Data Center. The International Panel on Climate Change (IPCC) Fifth Assessment Report estimates that the upper ocean (surface to 750 m deep) has warmed by 0.09 to 0.13 degrees C per decade over the past 40 years. Other processes important in influencing global sea level include changes to groundwater storage including dams and reservoirs.

Global warming also has an enormous impact with respect to melting glaciers and ice sheets. Higher global temperatures melt glaciers such as the one in Greenland, which flow into the oceans, adding to the amount of seawater. A large rise (on the order of several feet) in global sea levels poses many threats. According to the U.S. Environmental Protection Agency (EPA), “such a rise would inundate coastal wetlands and lowlands, erode beaches, increase the risk of flooding, and increase the salinity of estuaries, aquifers, and wetlands.” The seasonal cycles are closely linked with the seasonal changes in sea ice and sea surface temperatures. The timing and amplitude of the seasonal cycle has been altered by global warming.

Superimposed on the global rise in sea level, is strong regional and decadal variability which may cause sea level along a particular coastline to decline with time (for example along the Canadian eastern seaboard), or to rise faster than the global average. Regions that have shown a rapid rise in sea level during the past two decades include the western tropical Pacific and the United States northeastern seaboard. These regional variations in sea level are the result of many factors, such as local sedimentation rates, geomorphology, post-glacial rebound, and coastal erosion. Large storm events, such as Hurricane Sandy in the eastern Atlantic, can dramatically alter coastlines and affect sea level rise as well.

Coastal regions would be most affected by rising sea levels. The increase in sea level along the coasts of continents, especially North America are much more significant than the global average. According to 2007 estimates by the International Panel on Climate Change (IPCC), “global average sea level will rise between 0.6 and 2 feet (0.18 to 0.59 meters) in the next century. Along the U.S. Mid-Atlantic and Gulf Coasts, however, sea level rose in the last century 5 to 6 inches more than the global average. This is due to the subsiding of coastal lands. The sea level along the U.S. Pacific coast has also increased more than the global average but less than along the Atlantic coast. This can be explained by the varying continental margins along both coasts; the Atlantic type continental margin is characterized by a wide, gently sloping continental shelf, while the Pacific type continental margin incorporates a narrow shelf and slope descending into a deep trench. Since low-sloping coastal regions should retreat faster than higher-sloping regions, the Atlantic coast is more vulnerable to sea level rise than the Pacific coast.

Society

The rise in sea level along coastal regions carries implications for a wide range of habitats and inhabitants. Firstly, rising sea levels will have a serious impact on beaches— a place which humans love to visit recreationally and a prime location for real estate. It is ideal to live on the coast, due to a more moderate climate and pleasant scenery, but beachfront property is at risk from eroding land and rising sea levels. Since the threat posed by rising sea levels has become more prominent, property owners and local government have taken measures to prepare for the worst. For example, “Maine has enacted a policy declaring that shorefront buildings will have to be moved to enable beaches and wetlands to migrate inland to higher ground.” Additionally, many coastal states add sand to their beaches to offset shore erosion, and many property owners have elevated their structures in low-lying areas. As a result of the erosion and ruin of properties by large storms on coastal lands, governments have looked into buying land and having residents relocate further inland. The seas now absorb much of human-generated carbon dioxide, which then affects temperature change. The oceans store 93 percent of that energy which helps keep the planet livable by moderating temperatures.

Another important coastal habitat that is threatened by sea level rise is wetlands, which “occur along the margins of estuaries and other shore areas that are protected from the open ocean and include swamps, tidal flats, coastal marshes and bayous.” Wetlands are extremely vulnerable to rising sea levels, since they are within several feet of sea level. The threat posed to wetlands is serious, due to the fact that they are highly productive ecosystems, and they have an enormous impact on the economy of surrounding areas. Wetlands in the U.S. are rapidly disappearing due to an increase in housing, industry, and agriculture, and rising sea levels contribute to this dangerous trend. As a result of rising sea levels, the outer boundaries of wetlands tend to erode, forming new wetlands more inland. According to the EPA, “the amount of newly created wetlands, however, could be much smaller than the lost area of wetlands— especially in developed areas protected with bulkheads, dikes, and other structures that keep new wetlands from forming inland.” When estimating a sea level rise within the next century of 50 cm (20 inches), the U.S. would lose 38% to 61% of its existing coastal wetlands.

A rise in sea level will have a negative impact not only on coastal property and economy but on our supply of fresh water. According to the EPA, “Rising sea level increases the salinity of both surface water and ground water through salt water intrusion.” Coastal estuaries and aquifers, therefore, are at a high risk of becoming too saline from rising sea levels. With respect to estuaries, an increase in salinity would threaten aquatic animals and plants that cannot tolerate high levels of salinity. Aquifers often serve as a primary water supply to surrounding areas, such as Florida's Biscayne aquifer, which receives freshwater from the Everglades and then supplies water to the Florida Keys. Rising sea levels would submerge low-lying areas of the Everglades, and salinity would greatly increase in portions of the aquifer. The considerable rise in sea level and the decreasing amounts of freshwater along the Atlantic and Gulf coasts would make those areas rather uninhabitable. Many economists predict that global warming will be one of the main economic threats to the West Coast, specifically in California. "Low-lying coastal areas, such as along the Gulf Coast, are particularly vulnerable to sea-level rise and stronger storms—and those risks are reflected in rising insurance rates and premiums. In Florida, for example, the average price of a homeowners’ policy increased by 77 percent between 2001 and 2006."

Global issue

Since rising sea levels present a pressing problem not only to coastal communities but to the whole global population as well, much scientific research has been performed to analyze the causes and consequences of a rise in sea level. The U.S. Geological Survey has conducted such research, addressing coastal vulnerability to sea level rise and incorporating six physical variables to analyze the changes in sea level: geomorphology; coastal slope (percent); rate of relative sea level rise (mm/yr); shoreline erosion and acceleration rates (m/yr); mean tidal range (m); and mean wave height (m). The research was conducted on the various coasts of the U.S., and the results are very useful for future reference. Along the Pacific coast, the most vulnerable areas are low-lying beaches, and “their susceptibility is primarily a function of geomorphology and coastal slope.” With regard to research performed along the Atlantic coast, the most vulnerable areas to sea level rise were found to be along the Mid-Atlantic coast (Maryland to North Carolina) and Northern Florida, since these are “typically high-energy coastlines where the regional coastal slope is low and where the major landform type is a barrier island.” For the Gulf coast, the most vulnerable areas are along the Louisiana-Texas coast. According to the results, “the highest-vulnerability areas are typically lower-lying beach and marsh areas; their susceptibility is primarily a function of geomorphology, coastal slope and rate of relative sea-level rise.”

Many humanitarians and environmentalists believe that political policy needs to have a bigger role in carbon dioxide reduction. Humans have a substantial influence on the rise of sea level because we emit increasing levels of carbon dioxide into the atmosphere through automobile use and industry. A higher amount of carbon dioxide in the atmosphere leads to higher global temperatures, which then results in thermal expansion of seawater and melting of glaciers and ice sheets.

Ocean currents

The currents in the world's oceans are a result of varying temperatures associated with the changing latitudes of our planet. As the atmosphere is warmed nearest the equator, the hot air at the surface of our planet is heated, causing it to rise and draw in cooler air to take its place, creating what is known as circulation cells. This ultimately causes the air to be significantly colder near the poles than at the equator.

Wind patterns associated with these circulation cells drive surface currents which push the surface water to the higher latitudes where the air is colder. This cools the water down enough to where it is capable of dissolving more gasses and minerals, causing it to become very dense in relation to lower latitude waters, which in turn causes it to sink to the bottom of the ocean, forming what is known as North Atlantic Deep Water (NADW) in the north and Antarctic Bottom Water (AABW) in the south. Driven by this sinking and the upwelling that occurs in lower latitudes, as well as the driving force of the winds on surface water, the ocean currents act to circulate water throughout the entire sea. When global warming is added into the equation, changes occur, especially in the regions where deep water is formed. With the warming of the oceans and subsequent melting of glaciers and the polar ice caps, more and more fresh water is released into the high latitude regions where deep water is formed. This extra water that gets thrown into the chemical mix dilutes the contents of the water arriving from lower latitudes, reducing the density of the surface water. Consequently, the water sinks more slowly than it normally would.

It is important to note that ocean currents provide the necessary nutrients for life to sustain itself in the lower latitudes. Should the currents slow down, fewer nutrients would be brought to sustain ocean life resulting in a crumbling of the food chain and irreparable damage to the marine ecosystem. Slower currents would also mean less carbon fixation. Naturally, the ocean is the largest sink within which carbon is stored. When waters become saturated with carbon, excess carbon has nowhere to go, because the currents are not bringing up enough fresh water to fix the excess. This causes a rise in atmospheric carbon which in turn causes positive feedback that can lead to a runaway greenhouse effect.

Ocean acidification

The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because the chemical equilibria that govern seawater pH are temperature-dependent. Greater seawater warming could lead to a smaller change in pH for a given increase in CO₂.

Another effect of global warming on the carbon cycle is ocean acidification. The ocean and the atmosphere constantly act to maintain a state of equilibrium, so a rise in atmospheric carbon naturally leads to a rise in oceanic carbon. When carbon dioxide is dissolved in water it forms hydrogen and bicarbonate ions, which in turn breaks down to hydrogen and carbonate ions. All these extra hydrogen ions increase the acidity of the ocean and make survival harder for planktonic organisms that depend on calcium carbonate to form their shells. A decrease in the base of the food chain will, once again, be destructive to the ecosystems to which they belong. With fewer of these photosynthetic organisms present at the surface of the ocean, less carbon dioxide will be converted to oxygen, thereby allowing the greenhouse gasses to go unchecked.

Steps are being taken to combat the potentially devastating effects of ocean acidification, and scientists worldwide are coming together to solve the problem that is known as “global warming’s evil twin”. 

Between 1750 and 2000, surface-ocean pH has decreased by about 0.1, from about 8.2 to about 8.1. Surface-ocean pH has probably not been below 8.1 during the past 2 million years. Projections suggest that surface-ocean pH could decrease by an additional 0.3–0.4 units by 2100. Ocean acidification could threaten coral reefs, fisheries, protected species, and other natural resources of value to society.

Effects of acidification

The effects of ocean acidification can already be seen and have been happening since the start of the industrial revolution, with pH levels of the ocean dropping by 0.1 since the pre-industrial revolution times. An effect called coral bleaching can be seen on the Great Barrier Reef in Australia, where ocean acidification's effects are already taking place. Coral bleaching is when unicellular organisms that help make up the coral begin to die off and leave the coral giving it a white appearance. These unicellular organisms are important for the coral to feed and get the proper nutrition that is necessary to survive, leaving the coral weak and malnourished. This results in weaker coral that can die more easily and offer less protection to the organisms that depend on coral for shelter and protection. Increased acidity can also dissolve an organism's shell, threatening entire groups of shellfish and zooplankton and in turn, presenting a threat to the food chain and ecosystem.

Without strong shells, surviving and growing becomes more of a challenge for marine life that depend on calcified shells. The populations of these animals becomes smaller and individual members of the species turn weaker. The fish that rely on these smaller shell constructing animals for food now have a decreased supply, and animals that need coral reefs for shelter now have less protection. The effects of ocean acidification decrease population sizes of marine life and may cause an economic disruption if enough fish die off, which can seriously harm the global economy as the fishing industry makes a lot of money worldwide.

Ocean acidification can also have affects on marine fish larvae. It internally affects their olfactory systems, which is a crucial part of their development, especially in the beginning stage of their life. Orange clownfish larvae mostly live on oceanic reefs that are surrounded by vegetative islands. With the use of their sense of smell, larvae are known to be able to detect the differences between reefs surrounded by vegetative islands and reefs not surrounded by vegetative islands. Clownfish larvae need to be able to distinguish between these two destinations to have the ability to locate an area that is satisfactory for their growth. Another use for marine fish olfactory systems is to help in determining the difference between their parents and other adult fish in order to avoid inbreeding.

At James Cook University's experimental aquarium facility, clownfish were sustained in non-manipulated seawater that obtained a pH of 8.15 ± 0.07 which is similar to our current ocean's pH. To test for effects of different pH levels, seawater was manipulated to three different pH levels, including the non-manipulated pH. The two opposing pH levels correspond with climate change models that predict future atmospheric CO2 levels. In the year 2100 the model predicts that we could potentially acquire CO2 levels at 1,000 ppm, which correlates with the pH of 7.8 ± 0.05. Continuing even further into the next century, we could have CO2 levels at 1,700 ppm, which correlates with a pH of 7.6 ± 0.05.

Results of this experiment show that when larvae is exposed to a pH of 7.8 ± 0.05 their reaction to environmental cues differs drastically to larvae's reaction to cues in a non-manipulated pH. Not only did if effect their reaction to environmental cues but their reaction to parental cues was also skewed compared to the larvae reared in a non-manipulated pH of 8.15 ± 0.07. At the pH of 7.6 ± 0.05 larvae had no reaction to any type of cue. These results display the negative outcomes that could possibly be the future for marine fish larvae.

Oxygen depletion

Ocean deoxygenation is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each 1 °C of upper ocean warming.

Marine life

Examples of projected impacts and vulnerabilities for fisheries associated with climate change

Research indicates that increasing ocean temperatures are taking a toll on the marine ecosystem. A study on phytoplankton changes in the Indian Ocean indicates a decline of up to 20% in marine phytoplankton during the past six decades. During the summer, the western Indian Ocean is home to one of the largest concentrations of marine phytoplankton blooms in the world when compared to other oceans in the tropics. Increased warming in the Indian Ocean enhances ocean stratification, which prevents nutrient mixing in the euphotic zone where there is ample light available for photosynthesis. Thus, primary production is constrained and the region's entire food web is disrupted. If rapid warming continues, experts predict that the Indian Ocean will transform into an ecological desert and will no longer be productive. The same study also addresses the abrupt decline of tuna catch rates in the Indian Ocean during the past half century. This decrease is mostly due to increased industrial fisheries, with ocean warming adding further stress to the fish species. These rates show a 50-90% decrease over 5 decades.

A study that describes climate-driven trends in contemporary ocean productivity looked at global-ocean net primary production (NPP) changes detected from satellite measurements of ocean color from 1997 to 2006. These measurements can be used to quantify ocean productivity on a global scale and relate changes to environmental factors. They found an initial increase in NPP from 1997 to 1999 followed by a continuous decrease in productivity after 1999. These trends are propelled by the expansive stratified low-latitude oceans and are closely linked to climate variability. This relationship between the physical environment and ocean biology effects the availability of nutrients for phytoplankton growth since these factors influence variations in upper-ocean temperature and stratification. The downward trends of ocean productivity after 1999 observed in this study can give insight into how climate change can affect marine life in the future.

Effects of Marine Life Danger in Society.

As stated before, marine life has been decreasing in percentage as the time goes on due to the increase in ocean pollution being the main component plastic that is eaten by marine animals. Along with marine life, humans are also being affected by ocean pollution. One of the biggest animal protein industries, as it is the seafood industry, is affected since marine life has been decreasing and it is predicted that if they continue using the harmful techniques that are being used, by 2048 there is the possibility of an ocean without fish. The seafood industry has a big impact in the world’s food industry, proving food for approximately 3 billion people. One of the many famous and trending diets that are out there are the pescatarian diet, in which vegetarian diets followers add fish or other types of seafood in order to obtain the nutrients from the fish. If it comes to the point in which the seafood industry keep growing, as more people are joining this type of food trends and eating more fish (more demand means more production), and using techniques that deteriorate the marine life beyond catching the animals we will end up at the point of no return: where the marine life is extinct and we as humans will not be able to consume such as good source of protein in order to meet the required necessities. The ocean pollution does not mean that only marine life is being damaged, but also that we as humans will deprive ourselves from a great privilege as it is seafood and marine life.

Weather

Global warming also affects weather patterns as they pertain to cyclones. Scientists have found that although there have been fewer cyclones than in the past, the intensity of each cyclone has increased. A simplified definition of what global warming means for the planet is that colder regions would get warmer and warmer regions would get much warmer. However, there is also speculation that the complete opposite could be true. A warmer earth could serve to moderate temperatures worldwide. There is still much that is not understood about the earth's climate, because it is very difficult to make climate models. As such, predicting the effects that global warming might have on our planet is still an inexact science. Global warming is also causing the amount of hazards on the ocean to increase. It has increased the amount of fog at sea level, making it harder for ships to navigate without crashing into other boats or other objects in the ocean. The warmness and dampness of the ground is causing the fog to come closer to the surface level of the ocean. As the rain falls it makes the ground wet, then the warm air rises leaving a layer of cold air that turns into fog causing an unsafe ocean for travel and for working conditions on the ocean. It is also causing the ocean to create more floods due to the fact that it is warming up and the glaciers from the ice age are now melting causing the sea levels to rise, which causes the ocean to take over part of the land and beaches. Glaciers are melting at an alarming rate which is causing the ocean to rise faster than predicted. Inside of this ice there are traces of bubbles that are filled with CO₂ that are then released into the atmosphere when they melt causing the greenhouse effect to grow at an even faster rate.

Regional weather patterns across the globe are also changing due to tropical ocean warming. The Indo-Pacific warm pool has been warming rapidly and expanding during the recent decades, largely in response to increased carbon emissions from fossil fuel burning. The warm pool expanded to almost double its size, from an area of 22 million km² during 1900–1980, to an area of 40 million km² during 1981–2018. This expansion of the warm pool has altered global rainfall patterns, by changing the life cycle of the Madden Julian Oscillation (MJO), which is the most dominant mode of weather fluctuation originating in the tropics.

Seafloor

It is known that climate affects the ocean and the ocean affects the climate. Due to climate change, as the ocean gets warmer this too has an effect on the seafloor. Because of greenhouse gases such as carbon dioxide, this warming will have an effect on the bicarbonate buffer of the ocean. The bicarbonate buffer is the concentration of bicarbonate ions that keeps the ocean's acidity balanced within a pH range of 7.5–8.4. Addition of carbon dioxide to the ocean water makes the oceans more acidic. Increased ocean acidity is not good for the planktonic organisms that depend on calcium to form their shells. Calcium dissolves with very weak acids and any increase in the ocean's acidity will be destructive for the calcareous organisms. Increased ocean acidity will lead to decreased Calcite Compensation Depth (CCD), causing calcite to dissolve in shallower waters. This will then have a great effect on the calcareous ooze in the ocean, because the sediment itself would begin to dissolve.

Predictions

Calculations prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions and regionally divided economic development.

 

The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

A temperature rise of 1.5°C above preindustrial levels is projected to make existence impossible for 10% of fishes in their typical geographical range. A temperature rise of 5°C above this level is projected to make existence impossible for 60% of fishes in their geographical range. The main reason is Oxygen depletion as one of the consequences of the rise in temperature. Further, the change in temperature and decrease in oxygen is expected to occur too quickly for effective adaptation of affected species. Fishes can migrate to cooler places, but there are not always appropriate spawning sites.

If ocean temperatures rise it will have an effect right beneath the ocean floor and it will allow the addition of another greenhouse gas, methane gas. Methane gas has been found under methane hydrate, frozen methane and water, beneath the ocean floor. With the ocean warming, this methane hydrate will begin to melt and release methane gas, contributing to global warming. However, recent research has found that CO2 uptake outpaces methane release in these areas of the ocean causing overall decreases in global warming. Increase of water temperature will also have a devastating effect on different oceanic ecosystems like coral reefs. The direct effect is the coral bleaching of these reefs, which live within a narrow temperature margin, so a small increase in temperature would have a drastic effects in these environments. When corals bleach it is because the coral loses 60–90% of their zooxanthellae due to various stressors, ocean temperature being one of them. If the bleaching is prolonged, the coral host would die.

Although uncertain, another effect of climate change may be the growth, toxicity, and distribution of harmful algal blooms. These algal blooms have serious effects on not only marine ecosystems, killing sea animals and fish with their toxins, but also for humans as well. Some of these blooms deplete the oxygen around them to levels low enough to kill fish.