Climate change in China

From Wikipedia, the free encyclopedia

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

 

Warming stripes of China between 1901-2019

Climate change in China is having major effects on the economy, society and the environment. China is the largest emitter of carbon dioxide, through an energy infrastructure heavily focused on fossil fuels and coal. Also, other industries, such as a burgeoning construction industry and industrial manufacturing contribute heavily to carbon emissions. However, like other developing countries, on a per-capita basis, China's carbon emissions were considerably less than countries like the United States: as of 2016, they were the 51st most per capita emitter. It has also been noted that higher-income countries have outsourced emissions-intensive industries to China. On the basis of cumulative CO₂ emissions measured from 1751 through to 2017, China is responsible for 13% globally and about half of the United State's cumulative emissions.

China is suffering from the negative effects of global warming in agriculture, forestry and water resources, and is expected to continue to see increased impacts. China's government is taking some measures to increase renewable energy, and other decarbonization efforts, vowing to hit peak emissions before 2030 and be carbon neutral by 2060 by adopting “more vigorous policies and measures.”

Greenhouse gas emissions

This section is an excerpt from Greenhouse gas emissions by China.

 

China's Emissions per person are above the world average

Greenhouse gas emissions by China are the largest of any country in the world both in production and consumption terms, and stem mainly from coal burning in China, including coal-fired power stations, coal mining, and blast furnaces producing iron and steel. When measuring production-based emissions, China emitted over 14 gigatonnes (Gt) CO_2eq of greenhouse gases in 2019, 27% of the world total. When measuring in consumption-based terms, which adds emissions associated with imported goods and extracts those associated with exported goods, China accounts for over 27% of global emissions.

Despite having the largest emissions in the world, China's large population means its per person emissions have remained considerably lower than those in the developed world. This corresponds to over 10.1 tonnes CO_2eq emitted per person each year, slightly over the world average and the EU average but significantly lower than the second largest emitter of greenhouse gases, the United States, with its 17.6 tonnes per person. In consumption terms, China emits slightly less, with over 6 tonnes in 2016, slightly above the world average, but less than the EU average (close to 8 tonnes) and less than the United States by more than a half, with close to 18 tonnes per person.^[14][15] Accounting for historic emissions, OECD countries produced four times more CO2 in cumulative emissions than China, due to developed countries' early start in industrialization.

Impacts on the natural environment

China has and will suffer some of the effects of global warming, including sea level rise, glacier retreat and air pollution.

Temperature and weather changes

There has also been an increased occurrence of climate-related disasters such as drought and flood, and the amplitude is growing. These events have grave consequences for productivity when they occur, and also create serious repercussions for the natural environment and infrastructure. This threatens the lives of billions and aggravates poverty.

A study published in 2017, using continuous and coherent severe weather reports from over 500 manned stations from 1961 to 2010, found a significant decreasing trend in severe weather occurrence across China, with the total number of severe weather days that have either thunderstorm, hail and/or damaging wind decreasing about 50% from 1961 to 2010. The reduction in severe weather occurrences correlated strongly with the weakening of the East Asian summer monsoon.

China observed a ground average temperature increase of 0.24℃/decade from 1951 to 2017, exceeding the global rate. The average precipitation of China was 641.3 mm in 2017, 1.8% more than the average precipitation of previous years. There was an annual increase in concentrations of carbon dioxide from 1990 to 2016. The annual mean concentration of atmospheric carbon dioxide, methane, and nitrous oxide at Wanliguan Station were 404.4 ppm, 1907 ppb, and 329.7 ppb separately in 2016, slightly higher than the global mean concentration in 2016.

Current Köppen–Geiger climate classification map for China (1980-2016)

 

Predicted future Köppen–Geiger climate classification map for China (2071-2100)

Sea level rise

Coastal cities such as Guangzhou are vulnerable to sea level rise

The sea level rise was 3.4mm/year from 1980 to 2019 compared to the global average of 3.2mm/year.

China's first National Assessment of Global Climate Change, released in the 2000s by the Ministry of Science and Technology (MOST), states that China already suffers from the environmental impacts of climate change: increase of surface and ocean temperature, rise of sea level. Temperatures in the Tibetan Plateau of China are rising four times faster than anywhere else (data from 2011). Rising sea level is an alarming trend because China has a very long and densely populated coastline, with some of the most economically developed cities such as Shanghai, Tianjin, and Guangzhou situated there. Chinese research has estimated that a one-meter rise in sea level would inundate 92,000 square kilometers of China's coast, thereby displacing 67 million people.

Climate change caused an increase in sea level, threatening to impair the functions of harbors.

Rising sea levels affect China's coastal land.  Cities along the coast such as Shanghai, only 3–5 meters above sea level leaves its 18 million residents vulnerable.  Sea levels in Victoria Harbor in Hong Kong have already risen .12 meters in the last 50 years.

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  Yellow Sea

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  Shanghai

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  Tianjin and Yingkou

Ecosystems

Climate change increases forest belt limits and frequencies of pests and diseases, decreases frozen earth areas, and threatens to decrease glacial areas in northwest China. The vulnerability of ecosystems may increase due to future climate change. In the years 1970-2016 the occurrence of crop pest and diseases increased 4 times. 22% of that rise are due to climate change. By the year 2100 the occurrence will rise 243% under a low emission scenario and by 460% under a high emissions scenario. China is the biggest producer of wheat and rice in the world. It is in the second place in maize production.

Desertification Control Project, Ningxia China

China is home to 17,300 species of plants and animals: 667 vertebrates, ancient flora and fauna. Due to rising global temperatures, within the next century 20-30% of species will go extinct.

More than one fourth of China is covered by desert, which is growing due to desertification.  Desertification in China destroys farmland, biodiversity, and exacerbates poverty.

Water resources

Climate change decreased total water resources in North China while increasing total water resources in South China. There were more floods, drought, and extreme weather events. There may be a big impact on the spatial and temporal distribution in China's water resources, increasing extreme weather events and natural disasters.

Glacier melting in the Northern Region of China causes flooding in the upper parts of the Yangtze River.  This ruins soil and arable land.  The glacial melting causes lower parts of the Yangtze River to have lower volumes of water, also disrupting farming.

Furthermore, climate change will worsen the uneven distribution of water resources in China. Outstanding rises in temperature would exacerbate evapotranspiration, intensifying the risk of water shortage for agricultural production in the North. Although China's southern region has an abundance of rainfall, most of its water is lost due to flooding. As the Chinese government faces challenges managing its expanding population, increased demand for water to support the nation's economic activity and people will burden the government. In essence, a water shortage is indeed a large concern for the country.

Overfishing and rising ocean temperatures are killing the coral reefs in the South China Sea.  This lowers biodiversity, and negatively affects the fish market economy in China.

Impacts on people

At least 72% of Chinese, American and European respondents to a 2020−2021 European Investment Bank climate survey stated that climate change had an impact on everyday life.

Health impacts

Climate change has a significant impact on the health of Chinese people. The high temperature has caused health risks for some groups of people, such as older people (≥65 years old), outdoor workers, or people living in poverty. In 2019, each person who is older than 65 years had to endure extra 13 days of the heatwave, and 26,800 people died because of the heatwave in 2019.

In the future, the probability rate of malaria transmission will increase 39-140 percent if the temperature increase of 1-2 degrees Celsius in south China.

Economic impacts

According to the IPCC Sixth Assessment Report the country that will pay the highest financial cost if the temperature continue to rise is China. The impacts will include food insecurity, water scarcity, flooding, especially in coastal areas where most of the population lives due to higher than average sea level rise, and more powerful cyclones. At some point part of the country can face wet-bulb temperatures higher than humans and other mammals can tolerate more than six hours.

Agriculture

The negative effects on China's agriculture caused by climate change have appeared. There was an increase in agricultural production instability, severe damages caused by high temperature and drought, and lower production and quality in the prairie. In the near future, climate change may cause negative influences, causing a reduction of output in wheat, rice, and corn, and change the agricultural distribution of production. China is also dealing with agricultural issues due global demands of products such as soybeans. This global demand is causing coupled effects that stretch across oceans which in turn is affecting other countries. Environmental factor#Socioeconomic Drivers

Fishing Industry

Due to overfishing, pollution, global temperature increase, and change in pH to the world's oceans, the South China Sea is suffering from a lack in biodiversity among marine life. Historically, China was the world's largest capture fisheries and aquaculture producer, making the fish market a significant part of the Chinese economy. Due to the environmental impacts, coral reefs in the South China Sea are dying, decreasing the amount of marine life in the South China Sea. Fisheries are not able to catch the amount of fish that was once brought to the fish market, making that part of the economy suffer. The amount of fishing in China is unsustainable, and therefore declining. The fishing industry supplies a significant amount of jobs, exports, and domestic consumption, which will disappear if the fishing industry collapses.

Mitigation and adaptation

In general, the climate policy of China can be described as "underpromise so that it can overdeliver". China sets itself low climate targets that cannot surely prevent a 2 degrees temperature rise, but it mostly achieves and even overachieve its targets. China wants to peak its carbon emissions before 2030 and became carbon neutral by 2070. The president of China declared that his country will stop financing coal power plants abroad. China achieved 9 of its 15 climate targets in the Paris agreement before it was planned to happen. The climate policy of China can become more effective as a result of higher climate ambition of other countries and better cooperation with the USA. In March 2022 China increased its fossil fuel production "amid “growing” fears of global energy shortages and “rising” concerns of an economic slump"

Mitigation approaches

Main article: Greenhouse gas emissions by China § Mitigation

In 2022, China issued its climate targets in the 14th Five-Year Plan. Those include:

Reduce the economy's energy intensity by 13.5%.

Reduce the CO2 intensity of the economy by 18%.

Increase in the share of nonfossil energy to about 20%.

All three targets should be achieved by 2025. The change is in comparison to the numbers for the year 2021.

In the beginning of the year 2022 a government-supported research said China will peak CO2 emissions in the year 2027 at 12.2Gt and reach net zero carbon emissions before 2060 if it will change its development model.

Renewable energy

Ensuring adequate energy supply to sustain economic growth has been a core concern of the Chinese government since 1949. The country is the world's largest emitter of greenhouse gases, and coal in China is a major cause of global warming. However, from 2010 to 2015 China reduced energy consumption per unit of GDP by 18%, and CO₂ emissions per unit of GDP by 20%. On a per-capita basis, it was the world's 51st largest emitter of greenhouse gases in 2016.

China is the world's leading country in electricity production from renewable energy sources, with over triple the generation of the second-ranking country, the United States. 

China's renewable energy sector is growing faster than its fossil fuels and nuclear power capacity, and is expected to contribute 43 percent of global renewable capacity growth. China's total renewable energy capacity exceeded 1,000GW in 2021, accounting for 43.5 per cent of the country's total power generation capacity, 10.2 percentage points higher than in 2015. The country aims to have 80 per cent of its total energy mix come from non-fossil fuel sources by 2060, and achieve a combined 1,200GW of solar and wind capacity by 2030.

Energy efficiency

A 2011 report by a project facilitated by World Resources Institute stated that the 11th five-year plan (2005 to 2010), in response to worsening energy intensity in the 2002-2005 period, set a goal of a 20% improvement of energy intensity. The report stated that this goal likely was achieved or nearly achieved. The next five-year plan set a goal of improving energy intensity by 16%. In 2022 China published a plan of energy conservation for the 14th five-year plan (2021 to 2025) with a target of cutting energy consumption per unit of GDP by 13.5% by the year 2025 in comparison to the level of 2020. The plan regards 17 different sectors in the economy. In some sectors 20% - 40% of the capacities are not meeting the standards they need to meet by 2025. This policy expects to benefit the biggest companies who have the possibility to reach the targets.

==== Mitigation examples during the provinces of China, there are various projects held aiming to solve emissions reduction and energy-saving, which is a big step in tackling climate change. Beijing is developing in replacing traditional bulbs with energy-saving light bulbs. Provinces such as Rizhao and Dezhou are promoting solar energy in the building heating system. Besides, Tsinghua University launched a lead on low-carbon city development. The city is currently working with Tsinghua University to improve the urban environment by introducing renewable energy into industries and households. More than 360 Chinese cities have dockless bike-sharing systems that deploy nearly 20 million bicycles that travel an average of 47 million kilometres per day. According to the World Resources Institute report, dockless bike-sharing systems reduced China's GHG emissions by 4.8 million tonnes of CO2 annually.

Adaptation approaches

Pictured here is the conversion of three large rivers in Ningbo, China. The country is taking substantial measures to combat the flash floods predicted to intensify in the future.

China has experienced a seven-fold increase in the frequency of floods since the 1950s, rising every decade. The frequency of extreme rainfall has increased and is predicted to continue to increase in the western and southern parts of China. The country is currently undertaking efforts to reduce the threat of these floods (which have the potential effect of completely destroying vulnerable communities), largely focusing on improving the infrastructure responsible for tracking and maintaining adequate water levels. That being said, the country is promoting the extension of technologies for water allocation and water-saving mechanisms. In the country's National Climate Change Policy Program, one of the goals specifically set out is to enhance the ability to bear the impacts of climate change, as well as to raise the public awareness on climate change. China's National Climate Change Policy states that it will integrate climate change policies into the national development strategy. In China, this national policy comes in the form of its "Five Year Plans for Economic and Social Development". China's Five Year Plans serve as the strategic road maps for the country's development. The goals spelled out in the Five Year Plans are mandatory as government officials are held responsible for meeting the targets.

Policies and legislation

Climate change has not been a priority to China until recently (around 2008), when this issue was brought to a higher platform. Chinese state affairs operate as a central system, not a federal system. For example, the central government makes decisions and the local governments fulfill them. As a result, the local governments receive constraints and are measured by their performance from the central governments. Solving environmental issues such as climate change requires long-term investments in money, resources, and time. It is believed that these efforts will be detrimental to economic growth, which is of particular importance to the promotion of local government executives. This is why local governments have no engagement in addressing this issue.

In China's first NDC submission, key areas were identified for climate change adaptation, including agriculture, water resources, and vulnerable areas. It also mentioned that an adaptation strategy should be implemented through regional strategies. Flooding in cities is being tackled by collecting and recycling rainwater. In 2013, China issued its National Strategy for Climate Change Adaptation and set goals of reducing vulnerability, strengthening monitoring, and raising public awareness. Efforts on implementation have been put in adapting forestry, meteorological management, infrastructure, and risk planning.

The development of technology and economy in China share more responsibility in tackling climate change. After facing the 2011 smog issue, China's government launched an extensive strategy, which is to improve air quality by reducing the growth of coal consumption. Nevertheless, the trade war that involved China as one of the leading participants has resulted in the loss of control of polluting industries, especially in the steel and cement during 2018. Fortunately, nearly 70 multinational and local brands implemented the monitoring data by The Institute of Public & Environmental Affairs (IPE) in China, stimulating nearly 8,000 suppliers approaching regulatory violations.

Paris agreement

The Paris agreement is a legally binding international agreement. Its main goal is to limit global warming to below 1.5 degrees Celsius, compared to pre-industrial levels. The Nationally Determined Contributions (NDCs) are the plans to fight climate change adapted for each country, which outlines specific goals and targets for the upcoming five years to help mitigate the effects of climate change. Every party in the agreement has different targets based on its own historical climate records and country's circumstances and all the targets for each country are stated in their NDC.

China is currently a member of the United Nations Framework Convention on Climate Change, the Paris Agreement. As a part of this agreement it has agreed to the 2016 Nationally Determined Contributions (NDC).

The NDC target regarding the China against climate change and greenhouse gas emissions under the Paris agreement are the following:

• Peak of carbon dioxide emissions around 2030.
• 60% to 65% reduction of Carbon dioxide emission per unit of its gross domestic product (GDP), compared to 2005.
• Increase the forest stock volume by around 4.5 billion cubic meters on the 2005 level.

In the NDC of China there is a list of things that have been achieved by 2014:

• Proactive approach to climate change (for example enhancing mechanisms to effectively defend key areas).

Progress

Climate action tracker (CAT) is an independent scientific analysis that tracks government climate action and measures it against the globally agreed Paris Agreement. Climate action tracker found China actions to be "Highly insufficient".

National carbon trading scheme

The Chinese national carbon trading scheme is an intensity-based trading system for carbon dioxide emissions by China, which started operating in 2021. This emission trading scheme (ETS) creates a carbon market where emitters can buy and sell emission credits. From this scheme, China can limit emissions, but allow economic freedom for emitters to reduce emissions or purchase emission allowances from other emitters. China is the largest emitter of greenhouse gases (GHG) and many major Chinese cities have severe air pollution. The scheme is run by the Ministry of Ecology and Environment, which eventually plans to limit emissions from six of China's top carbon dioxide emitting industries. In 2021 it started with its power plants, and covers 40% of China's emissions, which is 15% of world emissions. China was able to gain experience in drafting and implementation of an ETS plan from the United Nations Framework Convention on Climate Change (UNFCCC), where China was part of the Clean Development Mechanism (CDM). From this experience with carbon markets, and lengthy discussions with the next largest carbon market, the European Union (EU), as well as analysis of small scale pilot markets in major Chinese cities and provinces, China's national ETS is the largest of its kind and will help China achieve its Nationally Determined Contribution (NDC) to the Paris Agreement. In July 2021 permits were being handed out for free rather than auctioned, and the market price per tonne of CO2e was around RMB 50, far less than the EU ETS and the UK ETS.

International cooperation

Main article: Debate over China's economic responsibilities for climate change mitigation

Attitudes of the Chinese government on climate change, specifically regarding the role of China in climate change action, have shifted notably in recent years. Historically, climate change was largely seen as a problem that has been created by and should be solved by industrialized countries; in 2015, China said it supports the "common but differentiated responsibilities" principle, which holds that since China is still developing, its abilities and capacities to reduce emissions are comparatively lower than developed countries', but China became the world's largest emitter of carbon dioxide in 2006 and is now responsible for more than a quarter of the world's overall greenhouse gas emissions. 

In 2018, the government has urged countries to continue to support the Paris agreement, even in the wake of the United States' withdrawal in 2017. In 2020, Chinese leader Xi Jinping announced at the UN General Assembly in New York that his country will end its contribution to global heating and achieve carbon neutrality by 2060 by adopting “more vigorous policies and measures.”

Both internationally and within the People's Republic of China, there has been an ongoing debate over China's economic responsibilities for climate change mitigation. The argument has been made that China has a crucial role to play in keeping global warming under 2 °C, and that this cannot be accomplished unless coal use, which accounts for the majority of China's emissions, falls sharply. President Xi says China will "phase down" coal use from 2026 - and will not build new coal-fired projects abroad - but some governments and campaigners say the plans are not going far enough.

The People's Republic of China is an active participant in the climate change talks and other multilateral environmental negotiations, and claims to take environmental challenges seriously but is pushing for the developed world to help developing countries to a greater extent.

However the Belt and Road Initiative is constructing coal-fired power stations (for example Emba Hunutlu power station in Turkey) thus increasing greenhouse gas emissions from other countries.

China is a part of the United Nations Framework Convention on Climate Change, BASIC Alliance.  This alliance is an international commitment to work in partnership with Brazil, South Africa, and India. BASIC’s international commitments and goals are to be carbon net-zero before 2060, and to help achieve the global goal from the UNFCCC of reducing emissions to 1.5% degrees celsius before pre-industrial levels. In 2021, at the UN General Assembly, Chinese leader Xi Jinping stated that China will no longer fund coal-fired power plants abroad. Xi also repeated the country’s commitment to achieving carbon neutrality by 2060.

Society and culture

Public opinion

According to a study from 2017 conducted by the China Climate Change Communication program, 94% of interviewees supported fulfilling the Paris agreement, 96.8% of interviewees supported international cooperation on global climate change, and more than 70% of interviewees were willing to purchase products environmentally friendly . 98.7% of interviewees supported implementing climate change education at schools. Respondents were most concerned about the air pollution caused by climate change. The investigation included 4025 samples.

The investigation showed that Chinese citizens agreed that they were experiencing climate change and that it was caused by human activities.

Furthermore, most Chinese citizens believe individual action on climate change can help, although the government is still seen as the entity most responsible for dealing with climate change. If the government does take action, fiscal and taxation policies are seen as potentially effective.

Activism

Calculations in 2021 showed that for giving the world a 50% chance of avoiding a temperature rise of 2 degrees or more China should increase its climate commitments by 7%. For a 95% chance it should increase the commitments by 24%. For giving a 50% chance of staying below 1.5 degrees China should increase its commitments by 41%.

Activists such as Howey Ou have done school strikes for climate.

Lunar eclipse

From Wikipedia, the free encyclopedia

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

Totality during the lunar eclipse of 21 January 2019. Direct sunlight is being blocked by the Earth, and the only light reaching it is sunlight refracted by Earth's atmosphere, producing a reddish color.

 

Latter phases of the partial lunar eclipse on 17 July 2019 taken from Gloucestershire, United Kingdom

A lunar eclipse occurs when the Moon moves into the Earth's shadow. This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned (in syzygy) with Earth between the other two, and only on the night of a full moon. The type and length of a lunar eclipse depend on the Moon's proximity to either node of its orbit.

The reddish color of totally eclipsed Moon is caused by Earth completely blocking direct sunlight from reaching the Moon, with the only light reflected from the lunar surface has been refracted by Earth's atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of blue light.

Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth. A total lunar eclipse can last up to nearly 2 hours, while a total solar eclipse lasts only up to a few minutes at any given place, because the Moon's shadow is smaller. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full Moon.

For the date of the next eclipse, see § Recent and forthcoming lunar eclipses.

Types of lunar eclipse

A schematic diagram of the shadow cast by Earth. Within the umbra, the central region, the planet totally shields direct sunlight. In contrast, within the penumbra, the outer portion, the sunlight is only partially blocked. (Neither the Sun, Moon, and Earth sizes nor the distances between the bodies are to scale.)

 

A total penumbral lunar eclipse dims the Moon in direct proportion to the area of the Sun's disk covered by Earth. This comparison of the Moon (within the southern part of Earth's shadow) during the penumbral lunar eclipse of January 1999 (left) and the Moon outside the shadow (right) shows this slight darkening.

Earth's shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun's diameter appears about one-quarter of Earth's in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.

Penumbral lunar eclipse

This occurs when the Moon passes through Earth's penumbra. The penumbra causes a subtle dimming of the lunar surface, which is only visible to the naked eye when about 70% of the Moon's diameter has immersed into Earth's penumbra. A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth's penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.

Partial lunar eclipse

This occurs when only a portion of the Moon enters Earth's umbra, while a total lunar eclipse occurs when the entire Moon enters the planet's umbra. The Moon's average orbital speed is about 1.03 km/s (2,300 mph), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon's limb with Earth's shadow is much longer and could last up to 236 minutes.

Total lunar eclipse

This occurs when the moon falls entirely within the earth's umbra. Just prior to complete entry, the brightness of the lunar limb-- the curved edge of the moon still being hit by direct sunlight-- will cause the rest of the moon to appear comparatively dim. The moment the moon enters a complete eclipse, the entire surface will become more or less uniformly bright. Later, as the moon's opposite limb is struck by sunlight, the overall disk will again become obscured. This is because as viewed from the Earth, the brightness of a lunar limb is generally greater than that of the rest of the surface due to reflections from the many surface irregularities within the limb: sunlight striking these irregularities is always reflected back in greater quantities than that striking more central parts, and is why the edges of full moons generally appear brighter than the rest of the lunar surface. This is similar to the effect of velvet fabric over a convex curved surface which to an observer will appear darkest at the center of the curve. It will be true of any planetary body with little or no atmosphere and an irregular cratered surface (e.g., Mercury) when viewed opposite the Sun.

Central lunar eclipse

This is a total lunar eclipse during which the Moon passes through the centre of Earth's shadow, contacting the antisolar point. This type of lunar eclipse is relatively rare.

The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth's umbra does not decrease appreciably within the changes in the Moon's orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.

Selenelion

A view of the October 2014 lunar eclipse from Minneapolis, with the setting and partially eclipsed Moon appearing squashed just above the horizon just after sunrise (seen as sunlight shining on the tree in the right image)

A selenelion or selenehelion, also called a horizontal eclipse, occurs where and when both the Sun and an eclipsed Moon can be observed at the same time. The event can only be observed just before sunset or just after sunrise, when both bodies will appear just above opposite horizons at nearly opposite points in the sky. A selenelion occurs during every total lunar eclipse-- it is an experience of the observer, not a planetary event separate from the lunar eclipse itself. Typically, observers on Earth located on high mountain ridges undergoing false sunrise or false sunset at the same moment of a total lunar eclipse will be able to experience it. Although during selenelion the Moon is completely within the Earth's umbra, both it and the Sun can be observed in the sky because atmospheric refraction causes each body to appear higher (i.e., more central) in the sky than its true geometric planetary position.

Timing

As viewed from Earth, the Earth's shadow can be imagined as two concentric circles. As the diagram illustrates, the type of lunar eclipse is defined by the path taken by the Moon as it passes through Earth's shadow. If the Moon passes through the outer circle but does not reach the inner circle, it is a penumbral eclipse; if only a portion of the moon passes through the inner circle, it is a partial eclipse; and if entire Moon passes through the inner circle at some point, it is a total eclipse.

 

Contact points relative to the Earth's umbral and penumbral shadows, here with the Moon near is descending node

The timing of total lunar eclipses is determined by what are known as its "contacts" (moments of contact with Earth's shadow):

P1 (First contact): Beginning of the penumbral eclipse. Earth's penumbra touches the Moon's outer limb.

U1 (Second contact): Beginning of the partial eclipse. Earth's umbra touches the Moon's outer limb.

U2 (Third contact): Beginning of the total eclipse. The Moon's surface is entirely within Earth's umbra.

Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of Earth's umbra.

U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits Earth's umbra.

U4 (Fifth contact): End of the partial eclipse. Earth's umbra leaves the Moon's surface.

P4 (Sixth contact): End of the penumbral eclipse. Earth's penumbra no longer makes contact with the Moon.

Danjon scale

The following scale (the Danjon scale) was devised by André Danjon for rating the overall darkness of lunar eclipses:

L = 0: Very dark eclipse. Moon almost invisible, especially at mid-totality.

L = 1: Dark eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.

L = 2: Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.

L = 3: Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.

L = 4: Very bright copper-red or orange eclipse. Umbral shadow is bluish and has a very bright rim.

Lunar versus solar eclipse

A solar eclipse occurs in the daytime at new moon, when the Moon is between Earth and the Sun, while a lunar eclipse occurs at night at full moon, when Earth passes between the Sun and the Moon.

 

The Moon does not completely darken as it passes through the umbra because Earth's atmosphere refracts sunlight into the shadow cone.

There is often confusion between a solar eclipse and a lunar eclipse. While both involve interactions between the Sun, Earth, and the Moon, they are very different in their interactions.

Lunar eclipse appearance

In a lunar eclipse, the Moon often passes through two regions of Earth's shadow: an outer penumbra, where direct sunlight is dimmed, and an inner umbra, where indirect and much dimmer sunlight refracted by Earth's atmosphere shines on the Moon, leaving a reddish color. This can be seen in different exposures of a partial lunar eclipse, for example here with exposures of 1/80, 2/5, and 2 seconds.

The Moon does not completely darken as it passes through the umbra because of the refraction of sunlight by Earth's atmosphere into the shadow cone; if Earth had no atmosphere, the Moon would be completely dark during the eclipse. The reddish coloration arises because sunlight reaching the Moon must pass through a long and dense layer of Earth's atmosphere, where it is scattered. Shorter wavelengths are more likely to be scattered by the air molecules and small particles; thus, the longer wavelengths predominate by the time the light rays have penetrated the atmosphere. Human vision perceives this resulting light as red. This is the same effect that causes sunsets and sunrises to turn the sky a reddish color. An alternative way of conceiving this scenario is to realize that, as viewed from the Moon, the Sun would appear to be setting (or rising) behind Earth.

From the Moon, a lunar eclipse would show a ring of reddish-orange light surrounding a silhouetted Earth in the lunar sky.

The amount of refracted light depends on the amount of dust or clouds in the atmosphere; this also controls how much light is scattered. In general, the dustier the atmosphere, the more that other wavelengths of light will be removed (compared to red light), leaving the resulting light a deeper red color. This causes the resulting coppery-red hue of the Moon to vary from one eclipse to the next. Volcanoes are notable for expelling large quantities of dust into the atmosphere, and a large eruption shortly before an eclipse can have a large effect on the resulting color.

Christopher Columbus predicting a lunar eclipse.

Lunar eclipse in culture

Several cultures have myths related to lunar eclipses or allude to the lunar eclipse as being a good or bad omen. The Egyptians saw the eclipse as a sow swallowing the Moon for a short time; other cultures view the eclipse as the Moon being swallowed by other animals, such as a jaguar in Mayan tradition, or a mythical three-legged toad known as Chan Chu in China. Some societies thought it was a demon swallowing the Moon, and that they could chase it away by throwing stones and curses at it. The Ancient Greeks correctly believed the Earth was round and used the shadow from the lunar eclipse as evidence. Some Hindus believe in the importance of bathing in the Ganges River following an eclipse because it will help to achieve salvation.

Inca

Similarly to the Mayans, the Incans believed that lunar eclipses occurred when a jaguar ate the Moon, which is why a blood moon looks red. The Incans also believed that once the jaguar finished eating the Moon, it could come down and devour all the animals on Earth, so they would take spears and shout at the Moon to keep it away.

Mesopotamians

The ancient Mesopotamians believed that a lunar eclipse was when the Moon was being attacked by seven demons. This attack was more than just one on the Moon, however, for the Mesopotamians linked what happened in the sky with what happened on the land, and because the king of Mesopotamia represented the land, the seven demons were thought to be also attacking the king. In order to prevent this attack on the king, the Mesopotamians made someone pretend to be the king so they would be attacked instead of the true king. After the lunar eclipse was over, the substitute king was made to disappear (possibly by poisoning).

Chinese

In some Chinese cultures, people would ring bells to prevent a dragon or other wild animals from biting the Moon. In the 19th century, during a lunar eclipse, the Chinese navy fired its artillery because of this belief. During the Zhou Dynasty (c. 1046–256 BC) in the Book of Songs, the sight of a Red Moon engulfed in darkness was believed to foreshadow famine or disease.

Blood moon

See also: Blood moon prophecy

 

Change to reddish cast

Certain lunar eclipses have been referred to as "blood moons" in popular articles but this is not a scientifically-recognized term. This term has been given two separate, but overlapping, meanings.

The first, and simpler, meaning relates to the reddish color a totally eclipsed Moon takes on to observers on Earth. As sunlight penetrates the atmosphere of Earth, the gaseous layer filters and refracts the rays in such a way that the green to violet wavelengths on the visible spectrum scatter more strongly than the red, thus giving the Moon a reddish cast.

The second meaning of "blood moon" has been derived from this apparent coloration by two fundamentalist Christian pastors, Mark Blitz and John Hagee. They claimed that the 2014–15 "lunar tetrad" of four lunar eclipses coinciding with the feasts of Passover and Tabernacles matched the "moon turning to blood" described in the Book of Joel of the Hebrew Bible. This tetrad was claimed to herald the Second Coming of Christ and the Rapture as described in the Book of Revelation on the date of the first of the eclipses in this sequence on April 15, 2014.

Occurrence

This multi-exposure sequence shows the August 2017 lunar eclipse visible from the ESO headquarters.

 

This collage shows the transitional stages of a lunar eclipse.

 

See also: Saros (astronomy) and Eclipse cycle

At least two lunar eclipses and as many as five occur every year, although total lunar eclipses are significantly less common. If the date and time of an eclipse is known, the occurrences of upcoming eclipses are predictable using an eclipse cycle, like the saros.

Recent and forthcoming lunar eclipses

Main article: List of 21st-century lunar eclipses

Further information: Lists of lunar eclipses

Eclipses occur only during an eclipse season, when the Sun appears to pass near either node of the Moon's orbit.

Quantum circuit

From Wikipedia, the free encyclopedia

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

 

Circuit that performs teleportation of a qubit. This circuit consists of both quantum gates and measurements. Measurement is a quantum phenomena that does not occur in classical circuits.

In quantum information theory, a quantum circuit is a model for quantum computation, similar to classical circuits, in which a computation is a sequence of quantum gates, measurements, initializations of qubits to known values, and possibly other actions. The minimum set of actions that a circuit needs to be able to perform on the qubits to enable quantum computation is known as DiVincenzo's criteria.

Circuits are written such that the horizontal axis is time, starting at the left hand side and ending at the right. Horizontal lines are qubits, doubled lines represent classical bits. The items that are connected by these lines are operations performed on the qubits, such as measurements or gates. These lines define the sequence of events, and are usually not physical cables.

The graphical depiction of quantum circuit elements is described using a variant of the Penrose graphical notation. Richard Feynman used an early version of the quantum circuit notation in 1986.

Reversible classical logic gates

Most elementary logic gates of a classical computer are not reversible. Thus, for instance, for an AND gate one cannot always recover the two input bits from the output bit; for example, if the output bit is 0, we cannot tell from this whether the input bits are 01 or 10 or 00.

However, reversible gates in classical computers are easily constructed for bit strings of any length; moreover, these are actually of practical interest, since irreversible gates must always increase physical entropy. A reversible gate is a reversible function on n-bit data that returns n-bit data, where an n-bit data is a string of bits x₁,x₂, ...,x_n of length n. The set of n-bit data is the space {0,1}ⁿ, which consists of 2ⁿ strings of 0's and 1's.

More precisely: an n-bit reversible gate is a bijective mapping f from the set {0,1}ⁿ of n-bit data onto itself. An example of such a reversible gate f is a mapping that applies a fixed permutation to its inputs. For reasons of practical engineering, one typically studies gates only for small values of n, e.g. n=1, n=2 or n=3. These gates can be easily described by tables.

Quantum logic gates

The quantum logic gates are reversible unitary transformations on at least one qubit. Multiple qubits taken together are referred to as quantum registers. To define quantum gates, we first need to specify the quantum replacement of an n-bit datum. The quantized version of classical n-bit space {0,1}ⁿ is the Hilbert space

${\displaystyle H_{\operatorname {QB} (n)}=\ell ^{2}(\{0,1\}^{n}).}$

This is by definition the space of complex-valued functions on {0,1}ⁿ and is naturally an inner product space. ${\displaystyle \ell ^{2}}$ refers to the 2-norm. This space can also be regarded as consisting of linear combinations, or superpositions, of classical bit strings. Note that H_QB(n) is a vector space over the complex numbers of dimension 2ⁿ. The elements of this vector space are the possible state-vectors of n-qubit quantum registers.

Using Dirac ket notation, if x₁,x₂, ...,x_n is a classical bit string, then

${\displaystyle |x_{1},x_{2},\cdots ,x_{n}\rangle \quad }$

is a special n-qubit register corresponding to the function which maps this classical bit string to 1 and maps all other bit strings to 0; these 2ⁿ special n-qubit registers are called computational basis states. All n-qubit registers are complex linear combinations of these computational basis states.

Quantum logic gates, in contrast to classical logic gates, are always reversible. One requires a special kind of reversible function, namely a unitary mapping, that is, a linear transformation of a complex inner product space that preserves the Hermitian inner product. An n-qubit (reversible) quantum gate is a unitary mapping U from the space H_QB(n) of n-qubit registers onto itself.

Typically, we are only interested in gates for small values of n.

A reversible n-bit classical logic gate gives rise to a reversible n-bit quantum gate as follows: to each reversible n-bit logic gate f corresponds a quantum gate W_f defined as follows:

${\displaystyle W_{f}(|x_{1},x_{2},\cdots ,x_{n}\rangle )=|f(x_{1},x_{2},\cdots ,x_{n})\rangle .}$

Note that W_f permutes the computational basis states.

Of particular importance is the controlled NOT gate (also called CNOT gate) W_CNOT defined on a quantized 2 qubit. Other examples of quantum logic gates derived from classical ones are the Toffoli gate and the Fredkin gate.

However, the Hilbert-space structure of the qubits permits many quantum gates that are not induced by classical ones. For example, a relative phase shift is a 1 qubit gate given by multiplication by the phase shift operator:

${\displaystyle P(\varphi )={\begin{bmatrix}1&0\\0&e^{i\varphi }\end{bmatrix}},}$

so

${\displaystyle P(\varphi )|0\rangle =|0\rangle \quad P(\varphi )|1\rangle =e^{i\varphi }|1\rangle .}$

Reversible logic circuits

Main article: reversible computing

Again, we consider first reversible classical computation. Conceptually, there is no difference between a reversible n-bit circuit and a reversible n-bit logic gate: either one is just an invertible function on the space of n bit data. However, as mentioned in the previous section, for engineering reasons we would like to have a small number of simple reversible gates, that can be put together to assemble any reversible circuit.

To explain this assembly process, suppose we have a reversible n-bit gate f and a reversible m-bit gate g. Putting them together means producing a new circuit by connecting some set of k outputs of f to some set of k inputs of g as in the figure below. In that figure, n=5, k=3 and m=7. The resulting circuit is also reversible and operates on n+m−k bits.

We will refer to this scheme as a classical assemblage (This concept corresponds to a technical definition in Kitaev's pioneering paper cited below). In composing these reversible machines, it is important to ensure that the intermediate machines are also reversible. This condition assures that intermediate "garbage" is not created (the net physical effect would be to increase entropy, which is one of the motivations for going through this exercise).

Note that each horizontal line on the above picture represents either 0 or 1, not these probabilities. Since quantum computations are reversible, at each 'step' the number of lines must be the same number of input lines. Also, each input combination must be mapped to a single combination at each 'step'. This means that each intermediate combination in a quantum circuit is a bijective function of the input.

Now it is possible to show that the Toffoli gate is a universal gate. This means that given any reversible classical n-bit circuit h, we can construct a classical assemblage of Toffoli gates in the above manner to produce an (n+m)-bit circuit f such that

${\displaystyle f(x_{1},\ldots ,x_{n},\underbrace {0,\dots ,0} )=(y_{1},\ldots ,y_{n},\underbrace {0,\ldots ,0} )}$

where there are m underbraced zeroed inputs and

${\displaystyle (y_{1},\ldots ,y_{n})=h(x_{1},\ldots ,x_{n})}$.

Notice that the end result always has a string of m zeros as the ancilla bits. No "rubbish" is ever produced, and so this computation is indeed one that, in a physical sense, generates no entropy. This issue is carefully discussed in Kitaev's article.

More generally, any function f (bijective or not) can be simulated by a circuit of Toffoli gates. Obviously, if the mapping fails to be injective, at some point in the simulation (for example as the last step) some "garbage" has to be produced.

For quantum circuits a similar composition of qubit gates can be defined. That is, associated to any classical assemblage as above, we can produce a reversible quantum circuit when in place of f we have an n-qubit gate U and in place of g we have an m-qubit gate W. See illustration below:

The fact that connecting gates this way gives rise to a unitary mapping on n+m−k qubit space is easy to check. In a real quantum computer the physical connection between the gates is a major engineering challenge, since it is one of the places where decoherence may occur.

There are also universality theorems for certain sets of well-known gates; such a universality theorem exists, for instance, for the pair consisting of the single qubit phase gate U_θ mentioned above (for a suitable value of θ), together with the 2-qubit CNOT gate W_CNOT. However, the universality theorem for the quantum case is somewhat weaker than the one for the classical case; it asserts only that any reversible n-qubit circuit can be approximated arbitrarily well by circuits assembled from these two elementary gates. Note that there are uncountably many possible single qubit phase gates, one for every possible angle θ, so they cannot all be represented by a finite circuit constructed from {U_θ, W_CNOT}.

Quantum computations

So far we have not shown how quantum circuits are used to perform computations. Since many important numerical problems reduce to computing a unitary transformation U on a finite-dimensional space (the celebrated discrete Fourier transform being a prime example), one might expect that some quantum circuit could be designed to carry out the transformation U. In principle, one needs only to prepare an n qubit state ψ as an appropriate superposition of computational basis states for the input and measure the output Uψ. Unfortunately, there are two problems with this:

• One cannot measure the phase of ψ at any computational basis state so there is no way of reading out the complete answer. This is in the nature of measurement in quantum mechanics.
• There is no way to efficiently prepare the input state ψ.

This does not prevent quantum circuits for the discrete Fourier transform from being used as intermediate steps in other quantum circuits, but the use is more subtle. In fact quantum computations are probabilistic.

We now provide a mathematical model for how quantum circuits can simulate probabilistic but classical computations. Consider an r-qubit circuit U with register space H_QB(r). U is thus a unitary map

${\displaystyle H_{\operatorname {QB} (r)}\rightarrow H_{\operatorname {QB} (r)}.}$

In order to associate this circuit to a classical mapping on bitstrings, we specify

• An input register X = {0,1}^m of m (classical) bits.
• An output register Y = {0,1}ⁿ of n (classical) bits.

The contents x = x₁, ..., x_m of the classical input register are used to initialize the qubit register in some way. Ideally, this would be done with the computational basis state

${\displaystyle |{\vec {x}},0\rangle =|x_{1},x_{2},\cdots ,x_{m},\underbrace {0,\dots ,0} \rangle ,}$

where there are r-m underbraced zeroed inputs. Nevertheless, this perfect initialization is completely unrealistic. Let us assume therefore that the initialization is a mixed state given by some density operator S which is near the idealized input in some appropriate metric, e.g.

${\displaystyle \operatorname {Tr} \left({\big |}|{\vec {x}},0\rangle \langle {\vec {x}},0|-S{\big |}\right)\leq \delta .}$

Similarly, the output register space is related to the qubit register, by a Y valued observable A. Note that observables in quantum mechanics are usually defined in terms of projection valued measures on R; if the variable happens to be discrete, the projection valued measure reduces to a family {E_λ} indexed on some parameter λ ranging over a countable set. Similarly, a Y valued observable, can be associated with a family of pairwise orthogonal projections {E_y} indexed by elements of Y. such that

${\displaystyle I=\sum _{y\in Y}\operatorname {E} _{y}.}$

Given a mixed state S, there corresponds a probability measure on Y given by

${\displaystyle \operatorname {Pr} \{y\}=\operatorname {Tr} (S\operatorname {E} _{y}).}$

The function F:X → Y is computed by a circuit U:H_QB(r) → H_QB(r) to within ε if and only if for all bitstrings x of length m

${\displaystyle \left\langle {\vec {x}},0{\big |}U^{*}\operatorname {E} _{F(x)}U{\big |}{\vec {x}},0\right\rangle =\left\langle \operatorname {E} _{F(x)}U(|{\vec {x}},0\rangle ){\big |}U(|{\vec {x}},0\rangle )\right\rangle \geq 1-\epsilon .}$

Now

${\displaystyle \left|\operatorname {Tr} (SU^{*}\operatorname {E} _{F(x)}U)-\left\langle {\vec {x}},0{\big |}U^{*}\operatorname {E} _{F(x)}U{\big |}{\vec {x}},0\right\rangle \right|\leq \operatorname {Tr} ({\big |}|{\vec {x}},0\rangle \langle {\vec {x}},0|-S{\big |})\|U^{*}\operatorname {E} _{F(x)}U\|\leq \delta }$

so that

${\displaystyle \operatorname {Tr} (SU^{*}\operatorname {E} _{F(x)}U)\geq 1-\epsilon -\delta .}$

Theorem. If ε + δ < 1/2, then the probability distribution

${\displaystyle \operatorname {Pr} \{y\}=\operatorname {Tr} (SU^{*}\operatorname {E} _{y}U)}$

on Y can be used to determine F(x) with an arbitrarily small probability of error by majority sampling, for a sufficiently large sample size. Specifically, take k independent samples from the probability distribution Pr on Y and choose a value on which more than half of the samples agree. The probability that the value F(x) is sampled more than k/2 times is at least

${\displaystyle 1-e^{-2\gamma ^{2}k},}$

where γ = 1/2 - ε - δ.

This follows by applying the Chernoff bound.