Tilting elements in the earth's climate system

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Possible tilting elements in the earth system

As a tilting element ( English Tipping element ) in which is Earth System Research a trans-regional component of the global climate system referred to, which can be set even with low external influences to a new state when a "tipping point" or " tipping point reaches" . These changes can be abrupt and in some cases irreversible. They can also set in motion feedback, induce changes in other subsystems of the system earth and thus trigger cascade effects.

history

The concept of the tilting elements was introduced into the research community by Hans Joachim Schellnhuber around the year 2000. Building on his work on nonlinear dynamics , as one of the coordinating lead authors of Working Group II, in the third assessment report of the Intergovernmental Panel on Climate Change (2001), he pointed to the previously neglected possibility of discontinuous, irreversible and extreme events in connection with global warming . Until then, linear, gradual changes had been assumed.

The specialist article "Tipping elements in the Earth's climate system" published in February 2008 was one of the most frequently cited works in the field of geosciences in 2008 and 2009 and currently (as of April 2019) has over 2500 citations in the specialist literature. Research on the article began in October 2005. At a workshop at the British Embassy in Berlin , 36 British and German climate researchers discussed the concept and identified possible tipping elements in the Earth system. In the following year, 52 other international experts were interviewed and all relevant scientific literature on the topic was evaluated. As a result, nine potential tipping elements were identified for which the tipping point could be reached before the year 2100. In the meantime, further possible tilting elements have been identified.

In 2001, the IPCC assumed that tipping points were only likely to be reached when the temperature rises above 5 degrees, but in the more recent special reports from 2018 and 2019 it came to the conclusion that tipping points were already reached when the temperature rises between 1 and 2 degrees could be exceeded.

Possible tilting elements identified so far

The working group around Schellnhuber named the following nine potential tipping elements in 2008:

Of these nine tipping elements, the experts surveyed believe that the melting of the Arctic sea ice and the Greenland ice sheet currently pose the greatest threat.

Other potential tipping elements were identified later:

  • Melting parts of the East Antarctic Ice Sheet, at the Wilkes Basin
  • Disappearance of the Tibetan glaciers
  • Methane outgassing from the oceans and other methane hydrate storage sites
  • Methane and carbon dioxide emissions from thawing permafrost
  • Desiccation of the North American Southwest
  • Attenuation of the marine carbon pump
  • Coral reef death
  • Destabilization of the jet stream (as well as the monsoons - see above) increases the likelihood of violent floods and droughts
  • Decline in net productivity of the biosphere (NPB), i.e. That is, the ability of the biosphere to bind the greenhouse gas CO 2 .
  • Dissolution of low layers of stratocumulus clouds over the subtropical sea ​​at CO 2 concentrations around 1200 ppm

Melting arctic sea ice

Amount of Arctic sea ice cover over the past 1450 years

Whether the melting of the Arctic sea ice has already passed a tipping point or whether it will occur in the future has been discussed for several years. As a result of global warming , the air temperature in the Arctic has increased by three times the global average in the last few decades, due to the polar amplification . It has been 2 ° C warmer there since the 1970s; summer sea ice cover has since declined by an average of 40%. In addition, the ice layer became thinner in large areas. A temporary change in the Arctic Oscillation and the Pacific Decade Oscillation from 1989 onwards also caused larger parts of the ice cover to loosen. The increasing proportion of the water surface not covered by ice led to greater absorption of solar radiation and thus to further thawing of ice, an increase in sea temperature and less ice formation in the winter months. After 1988, the influence of the ice-albedo feedback became greater than external influences. According to Lindsay and Zhang (2005), the fact that this effect continues despite the normalization of the Arctic Oscillation and the Pacific Decade Oscillation indicates pronounced non-linear effects . They therefore assume that the tipping point for the melting of the Arctic sea ice cover was already exceeded in the late 1980s / early 1990s. Holland et al. (2006), on the other hand, based on their own calculations, assumed that the tipping point would not be reached until 2015 at the earliest. According to calculations by Livina and Lenton (2013), an abrupt and since then sustained change in the amplitude of the seasonal fluctuations in the Arctic sea ice cover took place in 2007, which seems to be due to the internal dynamics of the Arctic climate system (and not to external influences) is viewed by the authors as a tipping point. It is assumed that it is a reversible (reversible) tipping point.

Melting of the Greenland ice sheet

The tipping point for the complete melting of the Greenland Ice Sheet could already be reached from a global warming of 1.5 to 2 ° C. The Greenland Ice Sheet is mostly 3000 meters thick, so its surface, which is high above sea level, is exposed to very low temperatures. The air temperature decreases according to the barometric altitude formula by about 0.5 ° C per 100 m altitude. The thinner the ice sheet, the more frequent there will be periods when the surface begins to thaw. The melting thus accelerates itself and would lead to a rise in sea level of around 7 meters over millennia. It is assumed that below a critical ice thickness, the melting process will continue even if the climate should return to the pre-industrial temperature level. A comparison with the last interglacial , the Eem warm period about 126,000 to 115,000 years ago, however, gives a mixed picture from a scientific point of view. While some studies postulate that the sea level is up to 15 meters higher than at present, with a proportion of meltwater in the Greenland Ice Sheet of 4.2 to 5.9 meters, it is predominantly assumed that during the Eem interglacial, with a partially warmer climate than in the Holocene , the sea level was a maximum of 9 meters above today's level. According to this scenario, the ice sheet would have contributed approximately 1.5 to 2.5 meters to this increase and therefore only lost part of its mass.

Melting of the West Antarctic Ice Sheet

Surface slopes of the Antarctic

In East Antarctica, which comprises most of the Antarctic, no significant meltdown is expected for the foreseeable future. In the case of West Antarctica, however, it is assumed that there will be profound changes there. Some very large glaciers of the West Antarctic Ice Sheet end in the sea. There they are supported several hundred meters below the surface of the sea on a ridge sloping towards the mainland. Since the seawater has warmed up there in the past decades, this led to increased melting and a retreat of the glacier tongue from z. B. the Pine Island Glacier or the Thwaites Glacier . Analyzes showed that the tipping point for a complete melting of the Thwaites Glacier has probably already been reached and that it will melt completely over a period of 200 to 900 years. This would cause the sea level to rise by 3 m. This process is also self-reinforcing, because a higher water level further reduces the stability of the glacier tongues.

The Atlantic thermohaline circulation slows down

Animation of the thermohaline circulation (video)

The increasing melting of the Arctic sea and land ice leads to a greater inflow of fresh water , as well as to increased speed and stability of the Arctic ocean current leading to the south . This could affect the North Atlantic deep water , eventually slowing down the thermohaline circulation . While the collapse of the thermohaline circulation with subsequent abrupt climate change is probably a tipping point distant in time, the slowdown in the thermohaline circulation, which would have a similar but weakened effect, is predicted robustly. The slowdown of the thermohaline circulation is an example of a tipping point that depends not only on the extent but also on the speed of climate change ( rate dependent tipping point ).

Disturbance of the South Pacific Climate Oscillation and amplification of the El Niño phenomenon

Various theories are discussed regarding the effects of global warming on the El Niño phenomenon. In 1999, Mojib Latif's working group assumed that the increased absorption of heat into the ocean would lead to a sustained lowering of the thermocline (water layers) in the eastern equatorial Pacific, and consequently to a greater amplitude of the El Niño-Southern Oscillation (ENSO) and / or more common El Niño phenomena. In 1997, a working group at NASA Goddard Space Flight Center postulated sustained La Niña conditions due to greater warming of the western compared to the eastern equatorial Pacific, which could lead to stronger easterly winds and an increased rise in cold water in the eastern equatorial Pacific. Lenton et al. In their summary, based on recent paleoclimatic studies, assumed that the most likely development is an increase in the intensity of the El Niño phenomena, although an increase in frequency cannot be predicted with certainty. The existence or location of a tipping point is also uncertain. Significant consequences - even with gradual changes - can nevertheless be assumed, for example droughts in Australia and Southeast Asia and increased precipitation on the western coasts of America. A connection between El Niño and unusually cold winters in Europe is also being discussed.

Methane and carbon dioxide emissions from thawing permafrost soils

As soon as permafrost thaws, microorganisms can decompose the fossil remains stored there. The greenhouse gases carbon dioxide and methane are released. These gases, in turn, increase global warming, causing the permafrost to continue melting. A self-reinforcing feedback from warming, progressive thawing and further release of carbon is called permafrost-carbon feedback.

Model studies on permafrost dynamics and greenhouse gas emissions suggest a relatively slow permafrost carbon feedback on time scales of several hundred years. However, some effects are not taken into account in these models, such as further amplification by the abrupt thawing of thermokarst lakes. In 2019, some permafrost soils in the Canadian Arctic were also observed to thaw significantly faster than predicted.

Decline in the net productivity of the biosphere

Today's earth system is a CO 2 sink, it absorbs more CO 2 than it emits. The oceans absorb approx. 25% of the CO 2 produced by humans , the biosphere (trees and other plants and soil) absorb another approx. 25%. However, according to a study by Columbia University in New York, the absorption capacity of our planet will decline from the middle of the century. A destructive feedback is predicted: Heat waves and droughts cause plants to shut down their photosynthesis, which is one of the most important mechanisms for removing CO 2 from the atmosphere. At the same time, many plants die off. This means that more anthropogenic CO 2 remains in the atmosphere and further CO 2 is added (released into the atmosphere) due to the decomposition of the dead biomass . This is driving global warming further, so that heat and drought intensify. Since plants evaporate less water during heat stress, the cooling effect of this perspiration is also missing.

Interactions and cascades

Presumed interactions between some tipping elements (⊕: increases the probability of occurrence, ⊖: reduces it, ⊖ / ⊕: effect in both directions, net effect uncertain)

There can be interactions between tilting elements. Triggering one tilt element can increase or, in some cases, reduce the likelihood that others will tilt. For some interactions, the direction - higher or lower probability of occurrence - is unknown. There is a risk of domino effects and mutually reinforcing feedback through such interactions. In an economic cost-benefit analysis , this risk speaks in favor of stabilizing the climate below 1.5 ° C as an optimal climate policy. The earth system scientist Timothy Lenton points out the possibility that small-scale tilting elements that are not considered in detail and are often not included in models could trigger the tilting of large-scale elements.

An investigation into the risk of self-reinforcing feedback in the climate system roughly divides large-scale tipping elements into three groups after the warming that is likely to trigger them:

1 ° C - 3 ° C
The melting of the Greenland ice sheet, the arctic sea ice cover in summer, the alpine glaciers and the West Antarctic ice sheet and the death of almost all coral reefs
3 ° C - 5 ° C
among other things decline in boreal forests, change in the El Niño-Southern Oscillation (ENSO), slowdown in the Atlantic thermohaline circulation, desertification of the tropical rainforest, collapse of the Indian summer monsoon
> 5 ° C
Extensive melting of the East Antarctic ice sheet and the winter arctic sea ice, sea level rise by several dozen meters, extensive thawing of the permafrost soils

If tilting elements of the first group are triggered, this could activate further tilting elements together with the temperature rise through gradual biogeophysical feedback. This threatens the risk of a cascade that would uncontrollably and irreversibly transform the climate into a warm climate with temperatures comparable to those of the Middle Miocene . A stabilization of the terrestrial climate system in a fluctuation range similar to that of the current Holocene with a temperature corridor of a maximum of ± 1 ° C, in which the human civilizations could develop relatively undisturbed, would then not occur in the foreseeable future on the basis of a thermal-radiative equilibrium . Even if the two-degree target were met , as agreed in the Paris Agreement in 2015 , this risk would exist; if the temperature continued to rise, it would rise sharply. In the course of this very rapid development, including the possible destabilization of the entire biosphere , a climatic condition could arise whose special characteristics would be a novelty in the history of the earth. Occurrence and climatic effects of tipping points during different geochronological periods are considered to be certain and are the subject of research in paleoclimatology .

Computer simulations of climate models often do not adequately depict tilting elements with abrupt, non-linear changes in state. In some cases, the relationships on which the newly discovered tilting elements are based are only included in corresponding climate models over time or are temporarily included as subsequent correction factors.

literature

Web links

Individual evidence

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