Tipping Elements - the Achilles Heels of the Earth System

 

Tipping elements are large-scale components of the Earth system, which are characterized by a threshold behavior. When relevant aspects of the climate approach a threshold, these components can be tipped into a qualitatively different state by small external perturbations. To compare them with the human body, tipping elements could be described as organs which drastically alter or stop functioning normally if certain requirements, such as oxygen supply, are not met.

The threshold behavior is often based on self-reinforcing processes which, once tipped, can continue without further forcing. It is thus possible that a component of the Earth system remains ‘tipped’, even if the background climate falls back below the threshold. The transition resulting from the exceedance of a system-specific tipping point can be either abrupt or gradual.  Crossing single tipping points has severe impacts on the environment and threatens the livelihood of many people1.

There is a risk that additional tipping points in the Earth system might be triggered through self-reinforcing feedbacks and unleash a domino-like chain reaction. A so-called “tipping cascade” could push the Earth system towards a new hothouse earth pathway2.

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Figure: Map of the most important tipping elements in the Earth System overlain on the Köppen climate classification. There are three groups of tipping elements: ice bodies (cryosphere entities), circulations of the ocean and atmosphere (circulation patterns), and large-scale ecosystems (biosphere components. Question marks indicate systems whose status as tipping elements is particularly uncertain. Source: PIK, 2017.

  The tipping elements map is licensed under a Creative Commons BY-ND 3.0 DE license.

 Ice Masses (cryosphere entities)

When ice melts it exposes a generally darker underlying surface, whether the rocky bed of a glacier or the sea. This darker surface absorbs more radiation from the sun, in turn accelerating the melting of the remaining ice. This mechanism, known as the ice-albedo feedback, is a classic example of a self-reinforcing process where the same phenomenon, namely ice loss, is both the driver and the result of temperature rise. However, this is not the only mechanism (described below) that makes the Earth's large ice masses a tipping element.

For several decades, the Arctic sea ice has been melting at an unprecedented rate, reducing in thickness and extent. The extent of Arctic sea ice quickly recovers during the cold season, but it remains relatively thin and highly sensitive to warm summers. It can thus be expected that the Arctic will be ice-free in summer by the end of this century. The ice-albedo feedback, together with other processes, contributes to an amplification of the regional warming in high northern latitudes, which is currently twice as fast as the global mean3.

The Greenland ice loss due to glacier flow into the sea and enhanced melting during summer has considerably increased in recent years. As a consequence, the ice sheet (three kilometers thick in some places) is becoming thinner and thereby loses height. As its surface, which today still reaches into high, cold air layers, sinks it will be increasingly exposed to lower and warmer layers of air. This accelerates the melting process. Scientific evidence indicates that the tipping point to a complete loss of ice could be already reached if global temperature rises by slightly less than 2°C4. The complete collapse of the Greenland ice sheet would cause a sea-level rise of up to seven meters over a timescale of hundreds to thousands of years5.

Large parts of the base of the West Antarctic ice sheet rest on the continental bedrock beneath the sea surface – the further upstream, the deeper. Due to this topography, the West Antarctic ice sheet can be destabilized by certain flow dynamics6. If the ice sheet were to break up, sea level would rise by about three meters in the course of several centuries. Scientific evidence indicates that such a process – with or without human input7 – has already been initiated8–11.

While the East Antarctic ice sheet, which stores the biggest part of Earth’s frozen reservoir of freshwater, seems stable today, large basins are also characterised by unstable topographic configurations. Recent research indicates that the melting of a relatively small “cork” – a rim of ice at the east Antarctic coast which holds the ice further inland in place – could trigger a self-sustained discharge of the entire basin, similar to the process in the West Antarctic region12. In the long term, a global warming of 2-3°C could trigger this instability, with the potential to raise sea level by 3-4 meters13.

The arctic permafrost, which has been frozen for centuries or even millennia, is located in Siberia and North America. If it were to thaw, it would potentially release large amounts of carbon dioxide and methane. Around a thousand billion tons of carbon are estimated to be stored in the upper three meters of the frozen soil14. But permafrost can reach even deeper: in so-called Yedoma-soils, in depths of more than three meters, additional hundreds of billion tons of carbon may be stored 14. These gas compounds originate from organic material which was stored during the last Ice Age.. The heat caused by microbial decomposition of the carbon compound accelerates thawing and degradation of the soil. Degradation of the surface layer exposes deeper soil layers to thawing and decomposition and therefore accelerates the so-called thermokarst formation15. Such self-reinforcing degradation processes, further amplified by a 2.5-fold arctic warming compared to global warming16, would be irreversible at time scales of a few centuries, since the original carbon storesin these soils built up over thousands of years17.

Methane hydrates are solid compounds of methane trapped in frozen water which are stored in sediments of the Arctic sea floor, especially in East Siberia. Estimating the total amount of organic carbon stored in these compounds remains challenging14. Methane hydrates are a slowly acting tipping element. Throughout the last millennia, the heat input from the warmer sea water has led to a slow decomposition of methane hydrates, resulting in methane being released from the sea sediments. Methane is a short-lived but very potent greenhouse gas. Most of it oxidizes to CO2 within a decade, which then contributes to warming in the atmosphere over a scale of thousands of years.

 Circulation patterns

There are some prominent examples of atmospheric and ocean circulation with marked (but variable) annual or seasonal patterns – but these can change. Throughout the history of our planet’s climate, there have been multiple phases of disruption and re-organization. This section gives a brief outline of potentially abrupt changes in circulation systems which could occur in the future.

The overturning circulation of the Atlantic is like a huge conveyor belt, transporting warm surface water northwards and, after cooling and sinking in high latitudes, cold deep water southwards. The Gulf Stream, which is responsible for the mild climate of northwestern Europe, is part of this large-scale system of Atlantic currents. One of its main motors is the cold, dense (and therefore heavy) salt water which sinks near Greenland and the Labrador coast. If the amount of freshwater from melting ice in the northern latitudes increases, this deep water formation could cease, slowing down the circulation motor. Scientific evidence indicates that a weakening of approximately 15% has already occurred.18-19. This can have severe impacts on marine ecosystems, result in distinct cooling of the north Atlantic region, and enhance sea-level rise, especially on the US-Atlantic coast.

Normally, trade winds cause an upwelling of cold water in the Pacific near South America. Warm surface water then flows, driven by wind, from South America to South-East Asia. During the El Niño weather phenomenon, the trade winds are weakened and the surface water flows in the opposite direction, warming the southeastern Pacific in the region of South America. Strong versions of this phenomenon, which recurs every 2-7 years, could become more frequent under continuing climate change20. The impacts of this circulation pattern, such as persistent droughts in Australia and South-East Asia or enhanced rainfall on the west coast of South America, span the entire globe. Such a change of large-scale ocean-atmosphere circulation patterns could also be associated with alterations of the monsoon dynamics21, for example in West India and South Africa.

The Jet Stream, a fast-flowing zonal air current meandering above the mid-latitudes of the Northern Hemisphere at a latitude of about 7 to 12 kilometers, separates cold Arctic air masses from the warmer air of the temperate south. Its air waves migrate eastwards and control synoptic-scale weather systems (i.e. formation of high pressure and low pressure systems) in the mid-latitudes. The air mass movement due to the Jet Stream seems to be slowing down. Moreover, its waves can stagnate leading to persistent weather systems which can last several weeks. This can cause prolonged extreme weather conditions such as cold periods and heat waves, floods or droughts22–26.

 Ecosystems (biosphere components)

Climatic changes might cause an area to become too warm or too dry for particular types of plants or animals living in it, so that their ecological niches shrinks and they can no longer exist there. Some species are able to migrate, for example further towards the poles or to a greater altitude. For species that already live in highly specialized environments such as polar or mountain regions, there is no alternative habitat to migrate to. Suitable habitats are in any case rare in a world so thoroughly exploited by humans. Climate change could alter whole regions as ecosystems, dependent on their typical climate and adapted plant and animal communities, disappear.

A large part of the rainfall in the Amazon basin originates from water evaporating over the rainforest. A warmer global climate with declining regional precipitation in combination with deforestation and forest fire could push the rainforest towards a critical threshold31. Noticeable effects of a destabilized rainforest system may occur with a time lag of several decades after the threshold has been exceeded. A transformation of the Amazon rainforest into a seasonal forest, adapted to drier conditions, or to grassland, would have fundamental impacts on global climate, since around 25% of the global atmosphere-biosphere carbon-exchange takes place here. Moreover, one of the most important terrestrial carbon sinks for carbon dioxide would be lost. The transformation would also mean a massive loss of biodiversity. And biodiversity, at the same time, could be an important component of a possible recovery of the system32.

The coniferous forests of the nordic regions represent almost a third of the global forest area. Climate change increases the stress on forests caused by spreading pests, increasing fires and storm damage, while at the same time a lack of water, enhanced evaporation and human exploitation inhibit their capacity to regenerate33. Once a critical threshold has been crossed, forests can be transformed back to scrub or grassland ecosystems. A loss of these forests would not only mean a destruction of habitats for animals and plants, but also massive release of carbon dioxide, which in turn contributes to accelerated global warming34-35.

Coral reefs are extremely sensitive ecosystems which are damaged by slight changes in water temperature and acidity. Warmer water is the most common cause of “coral bleaching” which has been increasingly observed in recent years. Bleaching means that the corals expel the algal organisms living within them and then often die themselves36. Even with warming limited to 2°C, most of the current coral systems are expected to disappear37. Once a coral system has collapsed, it takes several thousand years for the reef to regrow.

The world's oceans absorb large amounts of carbon. Around 40% of all anthropogenic CO2 emissions are extracted from the atmosphere by the oceans. Much of this is used by algae for growth, and the carbon sinks with them to the ocean floor after they die. The function of this marine biological carbon pump could be impeded by the warming and acidification of sea water as well as by the more frequently occurring oxygen depletion.

 

General Information on Tipping Elements

Schellnhuber, Hans Joachim. Selbstverbrennung: Die fatale Dreiecksbeziehung zwischen Klima, Mensch und Kohlenstoff. C. Bertelsmann Verlag, Kapitel 21, 2015.

Levermann, Anders, et al. "Potential climatic transitions with profound impact on Europe." Climatic Change 110.3-4 (2012): 845-878.

Schellnhuber, Hans Joachim. "Tipping elements in the Earth System." Proceedings of the National Academy of Sciences 106.49 (2009): 20561-20563.

Lenton, Timothy M., et al. "Tipping elements in the Earth's climate system." Proceedings of the national Academy of Sciences 105.6 (2008): 1786-1793.

Lenton, Timothy M., and Hans Joachim Schellnhuber. "Tipping the scales." Nature Climate Change 1.712 (2007): 97-98.

Lenton, Timothy M., et al. "Climate tipping points—too risky to bet against." (2019): 592-595.

Schellnhuber, Hans Joachim "Facing Climate Change: Tipping Points and U-Turns." In Buckland, D. et al. (Eds.), Burning Ice. Cape Farewell, London (2006): 112

Schellnhuber, Hans Joachim, and Held, Hermann. "How Fragile is the Earth System?" In Briden, J. C. and Downing, T. E. (Eds.), Managing the Earth. The Linacre Lextures 2001. Oxford University Press, Oxford, (2002): 5

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Köppen Climate Classification

              

Köppen climate classification modified by Trewartha and Rudloff.

Köppen, W. (1936). Das geographische System der Klimate. In: Köppen W., Geiger R. (eds) Handbuch der Klimatologie, Bd. I, Borntraeger, Berlin.

Trewartha, G. T. (1968). An introduction to climate, McGraw-Hill, New York.

Rudloff, W. (1981). World-Climates, with tables of climatic data and
practical suggestions, Wissenschaftliche Verlagsgesellschaft, Stuttgart.