thumb|300x300px|Possible tipping elements in the climate system. thumb|300px|Interactions of climate tipping points (bottom) with associated tipping points in the socioeconomic system (top) on different time scales.
A tipping point in the climate system is a critical threshold that, when exceeded, leads to large and often irreversible changes in the state of the system. The term 'tipping point' is used by climate scientists to identify vulnerable features of the climate system. If they 'tip', they are likely to have severe impacts on human society.
The Intergovernmental Panel on Climate Change began considering the possibility of tipping points 20 years ago. At that time the IPCC concluded they would only be likely in the event of unmitigated global warming of 4 degrees C or more above preindustrial times. Tipping points are now considered to have significant probability at today's warming level of just over 1 degree C, with high probability above 2 degrees C of global warming.
Large-scale components of the Earth system that may pass a tipping point are called tipping elements. At least 15 different elements of the climate system, such as the Greenland and Antarctic ice sheets, have been identified as possible tipping points. A danger is that if the tipping point in one system is crossed, this could lead to a cascade of other tipping points. If a cascade occurs, this could cause a hothouse Earth in which global average temperatures would be higher than at any period in the past 1.2 million years.
Tipping points are not necessarily abrupt. For example, with a temperature rise between 1.5 and 2 degrees Celsius, large parts of the Greenland ice sheet are likely to melt – but the melting process is set to take millennia. A 2021 study of the Antarctic ice sheet has shown that tipping into full ice sheet retreat could take as little as ten years.
The 2021 IPCC Sixth Assessment Report says that a tipping point in the climate system is a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly". It can be brought about by a small disturbance causing a disproportionately large change in the system. Tipping points also require self-reinforcing feedbacks and lead to changes in the climate system which are irreversible on a human timescale. For any particular climate component, the shift from one state to a new state may take many decades or centuries - although palaeoclimate data and global climate models, suggest that the "climate system may abruptly 'tip' from one regime to another in a comparatively short time."
The Special Report on the Ocean and Cryosphere in a Changing Climate released by the Intergovernmental Panel on Climate Change (IPCC) in 2019 defines a tipping point as: "A level of change in system properties beyond which a system reorganises, often in a non-linear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold at which global or regional climate changes from one stable state to another stable state. Tipping points are also used when referring to impact: the term can imply that an impact tipping point is (about to be) reached in a natural or human system".
The term 'tipping point' has become a foundational concept in climate change science discussions and is used by climate scientists and the news media as a metaphor for "drastic, irreversible and dangerous climate change". 
= 地质记录 =
The geologic record of temperature and greenhouse gas concentration allows climate scientists to gather information on climate feedbacks that lead to different climate states. A key finding is that when the concentration of carbon dioxide in the atmosphere goes up, the average global temperature goes up with it. In the last 100 million years, global temperatures have peaked twice, tipping the climate into a hothouse state. During the Cretaceous period, roughly 92 million years ago, 模板:CO2 levels were around 1,000 ppm. The climate was so hot that crocodile-like reptiles lived in what is now the Canadian Arctic, and forests thrived near the South Pole. The second hothouse period was the Paleocene-Eocene Thermal Maximum (PETM) 55-56 million years ago. Records suggest that during the PETM, the average global temperature rose between 5 and 8 °C; there was no ice at the poles, allowing palm trees and crocodiles to live above the Arctic Circle.
温度和温室气体浓度的地质记录使气候科学家能够手机导致不同气候状态的气候反馈信息。一个重要的发现是，当大气中二氧化碳浓度升高时，全球平均温度也随之上升。 在过去的一亿年中，全球温度两次达到峰值，使气候进入热室状态。在大约9200万年前的白垩纪时期，大气二氧化碳水平达到了约1000ppm。 气候如此炎热，以至于像鳄鱼一样的爬行动物生活在如今的加拿大北极地区，南极附近也森林茂密。第二个热室时期是距今5500-5600万年前的古新世-始新世极热事件（PETM）。记录显示在此期间全球平均温度上升了5-8℃；两极没有冰雪覆盖，使得棕榈树和鳄鱼能在北极圈以内生活。
However, as recently as three million years ago, atmospheric concentrations of 模板:CO2 matched today's levels. At that time, average global temperatures were 3C higher than they are now and sea levels were 5-to-25 metres higher.
Combining this historical information with the understanding of current climate change resulted in the finding published in 2018 in Proceedings of the National Academy of Sciences that "a 2 °C warming could activate important tipping elements, raising the temperature further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures".
Speed at which tipping occurs 发生转变的速度
The geologic record fails to provide clarity as to whether past temperature changes have taken only a few decades or many millennia. In March 2020, researchers showed that larger ecosystems can 'collapse' faster than previously thought, the Amazon rainforest for example (to a savanna) within ~50 years and the coral reefs of the Caribbean within ~15 years once a mode of 'collapse' is triggered, which in case of Amazonia they estimate could be as early as in 2021.
In July 2021, Nature Geoscience published a review illustrating how cascading interactions in the Earth system have led to abrupt changes in climate, ecological and social systems during the past 30,000 years. The authors point out that "the geological record shows that abrupt changes can occur on timescales short enough to challenge the capacity of human societies to adapt to environmental pressures".
Some tipping points may have already been reached. The greatest threat is from rising sea levels and a 2018 study found that tipping points for the Greenland and Antarctic ice sheets will likely occur between 1.5 and 2 °C of warming. In 2021, the Earth has already warmed by 1.2 °C, and 1.5 °C of warming may be less than 15 years away. Based on current projections, experts say at least 模板:Convert of sea-level rise is inevitable, although the speed at which this will occur is uncertain. Oceanographer, John Englander, says the last time sea-levels were rising quickly was 11,000 years ago, by about 15 feet (4.5 metres) per century. A 2021 study of ocean floor sediments in the Antarctic's iceberg alley has shown that that tipping has occurred in the past on several occasions and that tipping can be sudden and full ice sheet retreat can take as little as ten years.
一些临界点可能已经达到了。 最大的威胁来自于海平面上升 ，同时2018年的一项研究发现格陵兰和南极冰盖的临界点将有可能在升温1.5-2℃时发生。直到2021年，地球温度已经上升了1.2℃，升温1.5℃可能就在15年之内。基于目前的预测，专家们表明至少海平面上升是不可避免的 ，尽管这发生的速度暂时未能确定。 海洋学家John Englander表示，上一次海平面快速上升是在11000年之前，每个世纪上升约15英尺（4.5米）。 2021年对南极冰山巷海底沉积物的一项研究表明，在过去曾多次发生临界转变的现象，并且这种转变可以是突然的，冰盖完全退缩可能只需要短短十年的时间。
Cascading tipping points 级联的临界点
Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. These are called cascading tipping points. Ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, and boreal forest dieback. Thawing permafrost poses a multiplier threat because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere. If this is released into the atmosphere, the world will have to cope with emissions generated by the planet itself as well as those generated by human use of fossil fuels.
越过气候系统某个部分的阈值可能会触发另一个临界单元转变为新的状态。这些被称作级联临界点。 南极洲西部和格陵兰的冰层流失将显著改变大洋环流。这一过程将导致北半球高纬度地区的持续变暖，从而可能激活该地区的临界单元，例如永久冻土退化和北方森林枯死。 永久冻土的融化会使威胁倍增，因为冻土中的碳含量是目前大气循环中碳含量的两倍。 如果这些碳被排放到大气中，世界不得不应对来自地球自身以及人类使用化石燃料所产生的碳排放。
In 2019, Timothy Lenton and colleagues at Exeter University, published a study in Nature noting that the two most recent IPCC Special Reports, published in 2018 and 2019, suggest that even 1 and 2 °C of warming might push aspects of the climate past their tipping points. The authors added that the risk of cascading tipping points is "much more likely and much more imminent" and that some "may already have been breached."
In June 2021, Live Science reported that when scientists ran three million computer simulations of a climate model, nearly one-third of those simulations resulted in disastrous domino effects even when temperature increases were limited to 2 °C - the upper limit set by the Paris Agreement in 2015. The authors of the Nature study acknowledge that the science of tipping points is complex such that there is great uncertainty as to how they might unfold, but nevertheless, argue that the possibility of cascading tipping points represents “an existential threat to civilisation”.
= = 公众关注 =
In April and May 2021, Ipsos Mori conducted an opinion survey in the G20 nations on behalf of the Global Commons Alliance (GCA). The results, published in August 2021, found 73% of those surveyed believe "Because of human activities, the Earth is close to ‘tipping points’ in nature where climate or nature may change suddenly, or may be more difficult to stabilise in the future".:34 People in poorer countries such as Indonesia, Turkey, and Brazil were significantly more aware of the risk of triggering tipping points than those in wealthier countries such as the United States, Japan, Great Britain and Australia. This survey was conducted before the northern hemisphere summer of 2021 which saw record-breaking heatwaves, floods and fires, and before the latest Intergovernmental Panel on Climate Change report warned of “inevitable and irreversible” climate change directly attributable to human activity.
在2021年4月和5月，Ipsos Mori代表全球公共资源联盟（GCA）在G20国家中进行了一项民意调查。2021年8月公布的调查结果显示，73%的受访者认为“由于人类活动，地球已接近自然的‘临界点’，气候或自然可能会发生突变，或者在未来可能更难以稳定”。 相比于美国、日本、英国和澳大利亚等较富裕的国家，印度尼西亚、土耳其和巴西等较贫穷国家的人们明显更清楚出发临界点的风险。 这项研究是在2021年北半球夏季之前进行的，当时该夏季出现了破纪录的热浪、洪水和火灾，并且也先于最新的IPCC报告，该报告发出了人类活动直接导致了“不可避免和不可逆转”的气候变化的警告。
Tipping elements 临界单元
Scientists have identified a large set of elements in the climate system which may have a tipping point. It is possible that some tipping points are close to being crossed or have already been crossed, like the ice sheets in West Antarctic and Greenland, warm-water coral reefs, and the Amazon rainforest.
Shutdown of the Atlantic Meridional Overturning Circulation 大西洋经向翻转环流关闭
The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents. It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south. As warm water flows northwards, some evaporates which increases salinity. It also cools when it mixes with fresh water from melting ice in Greenland. Cold, salty water is more dense and slowly begins to sink. Several kilometres below the surface, cold, dense water begins to move south. Increased rainfall and the melting of continental ice due to global warming is diluting surface sea water and warming it up. The lighter water is less able to sink, slowing down the circulation.
大西洋经向翻转环流(AMOC) ，又称湾流系统，是一个巨大的洋流系统。 它是由水体密度差异驱动的；较冷且含盐量较高的水比较温暖的淡水重。 AMOC起到传送带的作用，将温暖的表层水从热带向北运输，并将寒冷的淡水运回南部。 随着温暖的海水向北流动，部分海水被蒸发，使得盐度升高。当它与格陵兰融冰的淡水混合时，水体会被冷却。冰冷的盐水密度更大，开始慢慢下沉。 在几千米的海面以下，高密度的冷水开始向南移动。全球变暖导致的降水增加和大陆冰融化正在稀释表层水体并使其升温。较轻的水不能够下沉，从而减缓的循环。
Theory, simplified models, and reconstructions of abrupt changes in the past suggest the AMOC has a tipping point. If freshwater input from melting glaciers reaches a certain threshold, it could collapse into a state of reduced flow. Even after melting stops, the AMOC may not return to its current state. It is unlikely that the AMOC will tip in the 21st century, but it may do so before 2300 if emissions are very high. A weakening of 24% to 39% is expected depending on greenhouse emissions, even without tipping behaviour. If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.
理论、简化模型以及对过去突变的重建表明，AMOC存在一个临界点。如果冰川融化输入的淡水达到了某个阈值，它可能会退为流量减少的状态。即使是在融化停止后，AMOC也有可能无法恢复到当前的状态。AMOC不太可能在21世纪越过临界点， 但是如果温室气体排放量过高的话，这将在2300年之前发生。即使不发生临界转变，预计也会减弱24%-39%，这取决于温室气体的排放量。 如果AMOC真的关闭了，一个新的稳定状态可能会出现并持续数千年，且有可能引发其它的临界点。
West Antarctic ice sheet disintegration
= = 西南极冰盖解体 =
The West Antarctic Ice Sheet (WAIS) is one of three regions making up Antarctica. In places it is more than 4 kilometres thick and sits on bedrock that largely lies below sea level. As such, it is in contact with ocean heat, as well as warmer air which makes it vulnerable to rapid and irreversible ice loss. A tipping point could be reached if thinning or collapse of the WAIS's ice shelves triggers a feedback loop that leads to rapid and irreversible loss of land ice into the ocean - with the potential to raise sea levels by around 3.3 metres.
Ice loss from WAIS is accelerating. The palaeo record suggests that during the past few hundred thousand years, the WAIS largely disappeared in response to similar levels of warming and 模板:CO2 emission scenarios projected for the next few centuries.
Greenland ice sheet disintegration
= 格陵兰冰盖解体 =
The Greenland ice sheet is the second largest mass of ice in the world, and is three times the size of Texas. It holds enough water, which if it melted, could raise global sea levels by 7.2 metres. Due to global warming, the ice sheet is melting at an accelerating rate adding around 0.7 mm to global sea levels every year. Around half of the ice loss occurs via surface melting, and the remainder occurs at the base of the ice sheet where the ice sheet touches the sea, by the breaking off, or 'calving', of icebergs from its edge.
Snowfall in Greenland is no longer able to compensate for the loss of ice due to this melting, such that the disintegration of the ice sheet may now be inevitable. Melting would not occur abruptly, but would be irreversible over millennia.
Amazon rainforest dieback 亚马逊雨林枯死
The Amazon rainforest is the largest tropical rainforest in the world. It is twice the size of India and spans nine countries in South America. It generates around half of its own rainfall by recycling moisture through evaporation and transpiration as air moves across the forest.
Deforestation of the Amazon began when colonists began establishing farms in the forest in the 1960s. They generally slashed and burned the trees in order to cultivate crops. However, soils in the Amazon are only productive for a short period after the land is cleared, so farmers would simply move and clear more land. Other colonists cleared land to raise cattle, leading to further deforestation and environmental damage. Heatwaves and drought have now become a factor driving additional tree deaths. This indicates that the Amazon is experiencing climatic conditions beyond its adaptative limits.
In 2021, the first long-term study of greenhouse gases in the Amazon rainforest found that in the 2010s the rainforest released more carbon dioxide than it absorbed. The forest had previously been a carbon sink, but is now emitting a billion tonnes of carbon dioxide a year. Deforestation has led to fewer trees which means more severe droughts and heatwaves develop leading to more tree deaths and more fires.
Permafrost and methane hydrates 永久冻土和甲烷水合物
Permafrost is ground containing soil and/or organic material bound together by ice and which has remained frozen for at least two years. It covers around a quarter of the non-glaciated land in the northern hemisphere – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be as much as a kilometre thick. Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean. This frozen ground holds vast amounts of carbon, derived from plants and animals that have died and decomposed over thousands of years. Scientists believe there is nearly twice as much carbon in permafrost than is currently in the Earth's atmosphere.
永久冻土是含有由冰结合在一起的土壤和/或有机质，且至少冻结了两年的土层。 它覆盖了北半球约四分之一的非冰川土地——主要位于西伯利亚、阿拉斯加、加拿大北部和青藏高原——厚度可达一公里。 厚达100米的海底永久冻土层也分布在北冰洋的部分海底。 这些冻土层含有大量的碳，这些碳来源于数千年来死亡和被分解的动植物。科学家们认为，永久冻土中的碳含量几乎是目前地球大气中碳含量的两倍。
As the climate warms and the permafrost begins to thaw, carbon dioxide and methane are released into the atmosphere. Research conducted by the US National Oceanic and Atmospheric Administration (NOAA) in 2019 found that thawing permafrost across the Arctic “could be releasing an estimated 300-600m tonnes of net carbon per year to the atmosphere”. In a Special Report on the Ocean and Cryosphere in a Changing Climate, the IPCC says there is “high confidence” in projections of “widespread disappearance of Arctic near-surface permafrost this century" which is "projected to release 10s to 100s of billions of tonnes [or gigatonnes, GtC], up to as much as 240 GtC, of permafrost carbon as 模板:CO2 and methane into the atmosphere".
随着气候变暖，永久冻土层开始融化，二氧化碳和甲烷被释放到大气中。美国国家海洋和大气管理局（NOAA）于2019年进行的研究发现，北极的永久冻土层融化“估计每年向大气释放约3-6亿吨净碳”。 在一份关于气候变化中的海洋和冰冻圈特别报告中，IPCC表示对“本世纪北极近地表冻土大规模消失”，并“预计将向大气中以二氧化碳和甲烷的形式释放100-1000亿吨（或gigatonnes, GtC），最高可达240GtC的永久冻土碳”的预测存在“高度置信”。
Warming in the Arctic allows the frozen permafrost to thaw, releasing locked up carbon dioxide and methane into the atmosphere. In June 2019, satellite images from around the Arctic showed burning fires that are farther north and of greater magnitude than at any time in the 16-year satellite record, and some of the fires appear to have ignited peat soils. Peat is an accumulation of partially decayed vegetation and is an efficient carbon sink. Scientists are concerned because the long-lasting peat fires release their stored carbon back to the atmosphere, contributing to further warming. The fires in June 2019, for example, released as much carbon dioxide as Sweden's annual greenhouse gas emissions.
北极变暖使得冻土融化，释放被固定的二氧化碳和甲烷进入大气。 2019年6月，来自北极周围的卫星图像显示，北极地区的大火比16年来卫星记录中任何时候都更偏北部，且规模更大，其中一些大火似乎引燃了泥炭土。 泥炭是部分腐烂植被的累积物，是一个高效的碳汇。 科学家们之所以担心，是因为长时间的泥炭火灾将储存的碳释放回大气，进一步加剧全球变暖。例如2019年6月的火灾释放的二氧化碳相当于瑞典每年的温室气体排放量。
Coral reef die-off 珊瑚礁死亡
Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection. Since the 1980s, this is being threatened by the increase in sea surface temperatures which is triggering mass bleaching of coral, especially in sub-tropical regions. A sustained ocean temperature spike of 1 °C (1.8 °F) above average is enough to cause bleaching. Under continued heat stress, corals expel the tiny colourful algae which live in their tissues leaving behind a white skeleton. The algae, known as zooxanthellae, have a symbiotic relationship with coral such that without them, the corals slowly die.
全世界约有5亿人依靠珊瑚礁获得食物、收入、旅游业和海岸保护。 自1980年代以来，由于海洋表面温度上升导致珊瑚大规模白化，这种情况正受到威胁，在亚热带地区尤为严重。持续高于平均水平1℃（1.8℉）的海洋温度峰值足以导致白化。 在持续的热应激下，珊瑚会排出生活在其组织中的五颜六色的微小藻类，留下一副白色的骨架。这种被称为虫黄藻的藻类与珊瑚存在共生关系，没有它们珊瑚就会慢慢死亡。
Between 1979 - 2010, 35 coral reef bleaching events were identified at a variety of locations. Some bleaching events are relatively localised, but the frequency and severity of mass-bleaching events affecting coral over hundreds and sometimes thousands of kilometres has been increasing over the last few decades. Mass bleaching events occurred in 1998, 2010, and between 2014–2017. This three year event affected more than 70 percent of the world's coral reefs, leaving two thirds of the Great Barrier Reef dead or severely bleached. Scientific American reports that the world has lost around 50% of coral reefs in the past 30 years. The Intergovernmental Panel on Climate Change (IPCC) states that by the time temperatures have risen to 1.5C above pre-industrial times, between 70% and 90% of coral reefs that exist today will have disappeared; and that if the world warms by 2 °C, "coral reefs will be vanishingly rare".
1979-2010年间，在不同地点发现了35起珊瑚白化事件。 一些白化事件是相对局地性的，但在过去几十年里，影响数百甚至数千公里范围内珊瑚的大规模白化事件的频率和严重程度一直在增加。 大规模白化事件发生在1998、2010和2014-2017年间。这一为期三年的事件影响了世界上70%以上的珊瑚礁，导致三分之二的大堡礁死亡或严重白化。《科学美国人》报告说，在过去的30年中全球已经损失了大约50%的珊瑚礁。 IPCC指出，当温度上升到超过工业化前水平1.5℃时，目前存在的70%-90%的珊瑚礁将消失；如果全球升温2℃，珊瑚将会变得极为罕见。
West African monsoon shift 西非季风转变
The West African Monsoon (WAM) system brings rainfall to West Africa and is the main source of rainfall in the agriculturally based region of the Sahel, an area of semi-arid grassland between the Sahara desert to the north and tropical rainforests to the south. The monsoon is a complex system in which land, ocean and atmosphere are connected is such a way that the wind direction reverses with the seasons.
However, the monsoon is notoriously unreliable. Between the late 1960s and 1980s, the average rainfall declined by more than 30% plunging the region into an extended drought. This led to a famine that killed tens of thousands of people and triggered an international aid effort. Research has shown the drought was largely due to changes in the surface temperatures of the global oceans, in particular, warming of the tropical oceans in response to rising greenhouse gases combined with cooling in the North Atlantic as a result of air pollution from northern hemisphere countries.
The El Niño–Southern Oscillation
= = 厄尔尼诺-南方涛动 =
The possibility that El Niño–Southern Oscillation (ENSO) is a tipping element has been debated, but remains uncertain. Normally strong winds blow west across the South Pacific Ocean from South America to Australia. Every two to seven years, the winds weaken due to pressure changes and the air and water in the middle of the Pacific warms up, causing changes in wind movement patterns around the globe. This known as El Niño and typically leads to droughts in Indonesia, India and Brazil, and increased flooding in Peru. In 2015/2016, this caused food shortages affecting over 60 million people. El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.
厄尔尼诺-南方涛动作为临界单元的可能性一直存在争议，但仍不确定。 正常情况下，强风从南美洲向西吹过南太平洋，吹到澳大利亚。每隔两到七年，由于气压变化，风力减弱，太平洋中部的空气和海水变暖，导致全球风运动模式发生变化。这就是所谓的厄尔尼诺现象，它通常会导致印度尼西亚、印度和巴西的干旱，以及秘鲁洪水泛滥。在2015/2016年，这导致了影响超过6000万人的粮食短缺。 厄尔尼诺引发的干旱可能会增加亚马逊森林火灾的可能性。
The threshold for tipping is estimated between 3.5 and 7 °C of global warming. After tipping, the system would be in a more permanent El Niño state, rather than oscillating between different states. This has happened in Earth's past, in the Pliocene, but the layout of the ocean was significantly different from now. So far, there is no definitive evidence indicating changes in ENSO behaviour.
Arctic sea ice
= 北极海冰 =
The IPCC finds that Arctic sea ice loss does not represent a tipping point because “projected losses are potentially reversible”. This depends on the time scale. Arctic sea ice has been melting rapidly for several decades. Some climate scientists describe the Arctic as a tipping element.
Tipping point effects 临界点的影响
If the climate tips into a state where tipping points begin to cascade, coastal storms will have greater impact, hundreds of millions of people will be displaced by rising sea levels, there will be food and water shortages, and people will die from unhealthy heat levels and generally unlivable conditions. Climatologist Michael E. Mann, believes a global temperature increase of 3 degrees Celsius or more has the potential to trigger collapse of the current societal organization and set the stage for massive unrest and global conflict – bearing in mind the IPCC describes a high probability that tipping points will occur at temperatures above 2 degrees C of global warming.
If cascading tipping points lead to climate temperature increases of 4–5 °C, this will make swaths of the planet around the equator uninhabitable, and lead to sea levels up to 60 metres (197 ft) higher than they are today. Humans cannot survive if the air is too moist and hot, and billions of people may die. Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research says if the world warms by this amount, it could only sustain about one billion people.
A 2021 meta study, conducted by Simon Dietz, James Rising, Thomas Stoerk, and Gernot Wagner, on the potential economic impact of tipping points found that they raise global risk; the medium estimate was that they increase the social cost of carbon (SCC) by about 25%, with a 10% chance of tipping points more than doubling the SCC. Effects like these have been popularized in books like The Uninhabitable Earth and The End of Nature.
Simon Dietz, James Rising, Thomas Stoerk和Gernot Wagner于2021年进行的一项元研究发现，临界点会增加全球风险。中等估计值是它们将碳的社会成本（SCC）增加了约25%，并有10%的可能性使SCC增加一倍以上。 这样的影响已在The Uninhabitable Earth和The End of Nature等书中得到普及。
Mathematical theory 数学理论
Tipping point behaviour in the climate can be described in mathematical terms. Tipping points are then seen as any type of bifurcation with hysteresis, which is the dependence of the state of a system on its history. For instance, depending on how warm or cold it was in the past, there can be differing amounts of ice on the poles at the same concentration of greenhouse gases or temperature. In a 2012 study inspired by "mathematical and statistical approaches to climate modelling and prediction", the authors identify three types of tipping points in open systems such as the climate system—bifurcation, noise-induced and rate-dependent.
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Bifurcation-induced tipping 分岔引发的转变
This occurs when a particular parameter in the climate, which is observed to be consistently moving in a given direction over a period of time, eventually passes through a critical level - at which point a dangerous bifurcation, or fork takes place - and what was a stable state loses its stability or simply disappears. The Atlantic Meridional Overturning Circulation (AMOC) is like a conveyor belt driven by thermohaline circulation. Slow changes to the bifurcation parameters in this system — the salinity, temperature and density of the water - have caused circulation to slow down by about 15% in the last 70 years or so. If it reaches a critical point where it stops completely, this would be an example of bifurcation induced tipping.
= = = = = 分岔引发的转变 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =。
Noise-induced tipping 噪声引发的转变
This refers to transitions from one state to another due to random fluctuations or internal variability of the system. Noise-induced transitions show none of the early warning signals which occur with bifurcations. This means they are fundamentally unpredictable as there is no systematic change in the underlying parameters. Because they are unpredictable, such occurrences are often described as a ‘one-in-x-year’ event. An example is the Dansgaard–Oeschger events during the last glacial period, with 25 occurrences of sudden climate fluctuations over a 500 year period.
= = = = 噪声引发的转变 = = = = = 这是指由于系统的随机波动或内部变化而从一种状态转变为另一种状态。噪声诱导的转变不会显示任何随分岔出现的早期预警信号。这意味着它们从根本上是不可预测的，因为基础参数没有系统性的变化。由于它们是不可预测的，这类事件通常被描述为“X年一次”事件。 末次冰期的Dansgaard–Oeschger事件就是一个例子，当时在500年的时间里发生了25次突然的气候波动。
Rate-induced tipping 速率引发的转变
This aspect of tipping assumes that there is a unique, stable state for any fixed aspect or parameter of the climate and that, if left undisturbed, there will only be small responses to a ‘small’ stimulus. However, when changes in one of the system parameters begin to occur more rapidly, a very large 'excitable' response may appear. In the case of peatlands, for instance, after years of relative stability, the rate-induced tipping point leads to an "explosive release of soil carbon from peatlands into the atmosphere" - sometimes known as "compost bomb instability".
Early warning signals 早期预警信号
For tipping points that occur because of a bifurcation, it may be possible to detect whether they are getting closer to a tipping point, as the system is getting less resilient to perturbations on approach of the tipping threshold. These systems display critical slowing down, with an increased memory (rising autocorrelation) and variance. Depending on the nature of the tipping system, changes may also be detected in the skewness and kurtosis of time series of relevant variables, with asymmetries in the distributions of anomalies indicating that tipping may be close. Abrupt change is not an early warning signal (EWS) for tipping points, as abrupt change can also occur if the changes are reversible to the control parameter.
= = = 早期预警信号 = = = 对于分岔引起的临界点，由于系统在接近临界点过程中对干扰的恢复力逐渐减弱，据此可以检测出它们是否正在接近临界点。这些系统表现出临界减速，伴随着记忆的增加（自相关性上升）和方差的增加。根据临界系统的性质，相关变量时间序列的偏度和峰度也可能发生变化，异常分布的不对称性表明临界点的接近。 突变不作为临界点的早期预警信号（EWS），因为在控制参数发生可逆变化的情况下也有可能引发突变。
These EWSs are often developed and tested using time series from the paleorecord, like sediments, ice caps, and tree rings, where past examples of tipping can be observed. It is not always possible to say whether increased variance and autocorrelation is a precursor to tipping, or caused by internal variability, for instance in the case of the collapse of the AMOC. Quality limitations of paleodata further complicate the development of EWSs. They have been developed for detecting tipping due to drought in forests in California, the Pine Island Glacier in West Antarctica, among other systems. Using early warning signals (increased autocorrelation and variance of the melt rate time series), it has been suggested that the Greenland ice sheet is currently losing resilience, consistent with modelled early warning signals of the ice sheet.
这些早期预警信号通常是利用诸如沉积物、冰盖和树木年轮此类古记录的时间序列来进行开发和测试的，在这些记录中可以看到过去临界转变的例子。 并不总能认为增加的方差和自相关性是临界转变的前兆，或者是由内部变异性引起的，一个例子便是大西洋经向翻转环流的崩溃。 古资料质量的限制导致早期预警信号的发展进一步复杂化。 它们已被开发利用于检测加利福尼亚森林干旱 、西南极的松岛冰川造成的临界转变。使用早期预警信号（融化速率时间序列中增加的自相关和方差），有人认为格陵兰冰盖正失去恢复力，这与模拟的冰盖早期预警信号一致。
However because the temperature is increasing so quickly there may be no warning.(p1-66)
Runaway greenhouse effect 失控温室效应
The runaway greenhouse effect is used in astronomical circles to refer to a greenhouse effect that is so extreme that oceans boil away and render a planet uninhabitable, an irreversible climate state that happened on Venus. The IPCC Fifth Assessment Report states that "a 'runaway greenhouse effect' —analogous to Venus— appears to have virtually no chance of being induced by anthropogenic activities." Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of percents, which will take a few billion years.
- Greenhouse and icehouse Earth
- Climate sensitivity
- Planetary boundaries
- Climate engineering
- World Scientists' Warning to Humanity
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- ClimateClock: 距离达到1.5 ° c 阈值还有一段时间
Category:Climate change feedbacks Category:Climatology
类别: 气候变化反馈类别: 气候学