{"id":148963,"date":"2019-12-09T12:00:07","date_gmt":"2019-12-09T12:00:07","guid":{"rendered":"https:\/\/www.transcend.org\/tms\/?p=148963"},"modified":"2019-12-04T07:51:47","modified_gmt":"2019-12-04T07:51:47","slug":"climate-tipping-points-too-risky-to-bet-against","status":"publish","type":"post","link":"https:\/\/www.transcend.org\/tms\/2019\/12\/climate-tipping-points-too-risky-to-bet-against\/","title":{"rendered":"Climate Tipping Points \u2014 Too Risky to Bet Against"},"content":{"rendered":"

27 Nov 2019 – The growing threat of abrupt and irreversible climate changes must compel political and economic action on emissions. <\/em><\/p><\/blockquote>\n

\"\"<\/a>

An aeroplane flies over a glacier in the Wrangell St Elias National Park in Alaska.
Credit: Frans Lanting\/Nat Geo Image Collection<\/p><\/div>\n

Politicians, economists and even some natural scientists have tended to assume that tipping points1<\/a><\/sup> in the Earth system \u2014 such as the loss of the Amazon rainforest or the West Antarctic ice sheet \u2014 are of low probability and little understood. Yet evidence is mounting that these events could be more likely than was thought, have high impacts and are interconnected across different biophysical systems, potentially committing the world to long-term irreversible changes.<\/p>\n

Here we summarize evidence on the threat of exceeding tipping points, identify knowledge gaps and suggest how these should be plugged. We explore the effects of such large-scale changes, how quickly they might unfold and whether we still have any control over them.<\/p>\n

In our view, the consideration of tipping points helps to define that we are in a climate emergency and strengthens this year\u2019s chorus of calls for urgent climate action \u2014 from schoolchildren to scientists, cities and countries.<\/p>\n

The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these \u2018large-scale discontinuities\u2019 in the climate system were considered likely only if global warming exceeded 5\u2009\u00b0C above pre-industrial levels. Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September this year)2<\/a>,3<\/a><\/sup> suggests that tipping points could be exceeded even between 1 and 2\u2009\u00b0C of warming (see \u2018Too close for comfort\u2019).<\/p>\n

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Source: IPCC<\/p><\/div>\n

If current national pledges to reduce greenhouse-gas emissions are implemented \u2014 and that\u2019s a big \u2018if\u2019 \u2014 they are likely to result in at least 3\u2009\u00b0C of global warming. This is despite the goal of the 2015 Paris agreement to limit warming to well below 2\u2009\u00b0C. Some economists, assuming that climate tipping points are of very low probability (even if they would be catastrophic), have suggested that 3\u2009\u00b0C warming is optimal from a cost\u2013benefit perspective. However, if tipping points are looking more likely, then the \u2018optimal policy\u2019 recommendation of simple cost\u2013benefit climate-economy models4<\/a><\/sup> aligns with those of the recent IPCC report2<\/a><\/sup>. In other words, warming must be limited to 1.5\u2009\u00b0C. This requires an emergency response.<\/p>\n

Ice collapse<\/strong><\/p>\n

We think that several cryosphere tipping points are dangerously close, but mitigating greenhouse-gas emissions could still slow down the inevitable accumulation of impacts and help us to adapt.<\/p>\n

Research in the past decade has shown that the Amundsen Sea embayment of West Antarctica might have passed a tipping point3<\/a><\/sup>: the \u2018grounding line\u2019 where ice, ocean and bedrock meet is retreating irreversibly. A model study shows5<\/a><\/sup> that when this sector collapses, it could destabilize the rest of the West Antarctic ice sheet like toppling dominoes \u2014 leading to about 3 metres of sea-level rise on a timescale of centuries to millennia. Palaeo-evidence shows that such widespread collapse of the West Antarctic ice sheet has occurred repeatedly in the past.<\/p>\n

The latest data show that part of the East Antarctic ice sheet \u2014 the Wilkes Basin \u2014 might be similarly unstable3<\/a><\/sup>. Modelling work suggests that it could add another 3\u20134\u2009m to sea level on timescales beyond a century.<\/p>\n

The Greenland ice sheet is melting at an accelerating rate3<\/a><\/sup>. It could add a further 7\u2009m to sea level over thousands of years if it passes a particular threshold. Beyond that, as the elevation of the ice sheet lowers, it melts further, exposing the surface to ever-warmer air. Models suggest that the Greenland ice sheet could be doomed at 1.5\u2009\u00b0C of warming3<\/a><\/sup>, which could happen as soon as 2030.<\/p>\n

Thus, we might already have committed future generations to living with sea-level rises of around 10\u2009m over thousands of years3<\/a><\/sup>. But that timescale is still under our control. The rate of melting depends on the magnitude of warming above the tipping point. At 1.5\u2009\u00b0C, it could take 10,000 years to unfold3<\/a><\/sup>; above 2\u2009\u00b0C it could take less than 1,000 years6<\/a><\/sup>. Researchers need more observational data to establish whether ice sheets are reaching a tipping point, and require better models constrained by past and present data to resolve how soon and how fast the ice sheets could collapse.<\/p>\n

Whatever those data show, action must be taken to slow sea-level rise. This will aid adaptation, including the eventual resettling of large, low-lying population centres.<\/p>\n

A further key impetus to limit warming to 1.5\u2009\u00b0C is that other tipping points could be triggered at low levels of global warming. The latest IPCC models projected a cluster of abrupt shifts7<\/a><\/sup> between 1.5\u2009\u00b0C and 2\u2009\u00b0C, several of which involve sea ice. This ice is already shrinking rapidly in the Arctic, indicating that, at 2\u2009\u00b0C of warming, the region has a 10\u201335% chance3<\/a><\/sup> of becoming largely ice-free in summer.<\/p>\n

Biosphere boundaries<\/strong><\/p>\n

Climate change and other human activities risk triggering biosphere tipping points across a range of ecosystems and scales (see \u2018Raising the alarm\u2019).<\/p>\n

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Source: T. M. Lenton et al.<\/p><\/div>\n

Ocean heatwaves have led to mass coral bleaching and to the loss of half of the shallow-water corals on Australia\u2019s Great Barrier Reef. A staggering 99% of tropical corals are projected2<\/a><\/sup> to be lost if global average temperature rises by 2\u2009\u00b0C, owing to interactions between warming, ocean acidification and pollution. This would represent a profound loss of marine biodiversity and human livelihoods.<\/p>\n

As well as undermining our life-support system, biosphere tipping points can trigger abrupt carbon release back to the atmosphere. This can amplify climate change and reduce remaining emission budgets.<\/p>\n

Deforestation and climate change are destabilizing the Amazon \u2014 the world\u2019s largest rainforest, which is home to one in ten known species. Estimates of where an Amazon tipping point could lie range from 40% deforestation to just 20% forest-cover loss8<\/a><\/sup>. About 17% has been lost since 1970. The rate of deforestation varies with changes in policy. Finding the tipping point requires models that include deforestation and climate change as interacting drivers, and that incorporate fire and climate feedbacks as interacting tipping mechanisms across scales.<\/p>\n

With the Arctic warming at least twice as quickly as the global average, the boreal forest in the subarctic is increasingly vulnerable. Already, warming has triggered large-scale insect disturbances and an increase in fires that have led to dieback of North American boreal forests, potentially turning some regions from a carbon sink to a carbon source9<\/a><\/sup>. Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane \u2014 a greenhouse gas that is around 30 times more potent than CO2<\/sub> over a 100-year period.<\/p>\n

Researchers need to improve their understanding of these observed changes in major ecosystems, as well as where future tipping points might lie. Existing carbon stores and potential releases of CO2<\/sub> and methane need better quantification.<\/p>\n

The world\u2019s remaining emissions budget for a 50:50 chance of staying within 1.5\u2009\u00b0C of warming is only about 500\u2009gigatonnes (Gt) of CO2<\/sub>. Permafrost emissions could take an estimated 20% (100\u2009Gt\u2009CO2<\/sub>) off this budget10<\/a><\/sup>, and that\u2019s without including methane from deep permafrost or undersea hydrates. If forests are close to tipping points, Amazon dieback could release another 90\u2009Gt\u2009CO2<\/sub> and boreal forests a further 110\u2009Gt\u2009CO2<\/sub>11<\/a><\/sup>. With global total CO2<\/sub> emissions still at more than 40\u2009Gt\u2009per year, the remaining budget could be all but erased already.<\/p>\n

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Bleached corals on a reef near the island of Moorea in French Polynesia in the South Pacific. Credit: Alexis Rosenfeld\/Getty<\/p><\/div>\n

Global cascade<\/strong><\/p>\n

In our view, the clearest emergency would be if we were approaching a global cascade of tipping points that led to a new, less habitable, \u2018hothouse\u2019 climate state11<\/a><\/sup>. Interactions could happen through ocean and atmospheric circulation or through feedbacks that increase greenhouse-gas levels and global temperature. Alternatively, strong cloud feedbacks could cause a global tipping point12<\/a>,13<\/a><\/sup>.<\/p>\n

We argue that cascading effects might be common. Research last year14<\/a><\/sup> analysed 30 types of regime shift spanning physical climate and ecological systems, from collapse of the West Antarctic ice sheet to a switch from rainforest to savanna. This indicated that exceeding tipping points in one system can increase the risk of crossing them in others. Such links were found for 45% of possible interactions14<\/a><\/sup>.<\/p>\n

In our view, examples are starting to be observed. For example, Arctic sea-ice loss is amplifying regional warming, and Arctic warming and Greenland melting are driving an influx of fresh water into the North Atlantic. This could have contributed to a 15% slowdown15<\/a><\/sup> since the mid-twentieth century of the Atlantic Meridional Overturning Circulation (AMOC) , a key part of global heat and salt transport by the ocean3<\/a><\/sup>. Rapid melting of the Greenland ice sheet and further slowdown of the AMOC could destabilize the West African monsoon, triggering drought in Africa\u2019s Sahel region. A slowdown in the AMOC could also dry the Amazon, disrupt the East Asian monsoon and cause heat to build up in the Southern Ocean, which could accelerate Antarctic ice loss.<\/p>\n

The palaeo-record shows global tipping, such as the entry into ice-age cycles 2.6 million years ago and their switch in amplitude and frequency around one million years ago, which models are only just capable of simulating. Regional tipping occurred repeatedly within and at the end of the last ice age, between 80,000 and 10,000 years ago (the Dansgaard\u2013Oeschger and Heinrich events). Although this is not directly applicable to the present interglacial period, it highlights that the Earth system has been unstable across multiple timescales before, under relatively weak forcing caused by changes in Earth\u2019s orbit. Now we are strongly forcing the system, with atmospheric CO2<\/sub> concentration and global temperature increasing at rates that are an order of magnitude higher than those during the most recent deglaciation.<\/p>\n

Atmospheric CO2<\/sub> is already at levels last seen around four million years ago, in the Pliocene epoch. It is rapidly heading towards levels last seen some 50 million years ago \u2014 in the Eocene \u2014 when temperatures were up to 14\u2009\u00b0C higher than they were in pre-industrial times. It is challenging for climate models to simulate such past \u2018hothouse\u2019 Earth states. One possible explanation is that the models have been missing a key tipping point: a cloud-resolving model published this year suggests that the abrupt break-up of stratocumulus cloud above about 1,200 parts per million of CO2<\/sub> could have resulted in roughly 8\u2009\u00b0C of global warming12<\/a><\/sup>.<\/p>\n

Some early results from the latest climate models \u2014 run for the IPCC\u2019s sixth assessment report, due in 2021 \u2014 indicate a much larger climate sensitivity (defined as the temperature response to doubling of atmospheric CO2<\/sub>) than in previous models. Many more results are pending and further investigation is required, but to us, these preliminary results hint that a global tipping point is possible.<\/p>\n

To address these issues, we need models that capture a richer suite of couplings and feedbacks in the Earth system, and we need more data \u2014 present and past \u2014 and better ways to use them. Improving the ability of models to capture known past abrupt climate changes and \u2018hothouse\u2019 climate states should increase confidence in their ability to forecast these.<\/p>\n

Some scientists counter that the possibility of global tipping remains highly speculative. It is our position that, given its huge impact and irreversible nature, any serious risk assessment must consider the evidence, however limited our understanding might still be. To err on the side of danger is not a responsible option.<\/p>\n

If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilization. No amount of economic cost\u2013benefit analysis is going to help us. We need to change our approach to the climate problem.<\/p>\n

Act now <\/strong><\/p>\n

In our view, the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute (see \u2018Emergency: do the maths\u2019).<\/p>\n

Emergency: do the maths<\/strong><\/p>\n

We define emergency (E<\/em>) as the product of risk and urgency. Risk (R<\/em>) is defined by insurers as probability (p<\/em>) multiplied by damage (D<\/em>). Urgency (U<\/em>) is defined in emergency situations as reaction time to an alert (\u03c4<\/em>) divided by the intervention time left to avoid a bad outcome (T<\/em>). Thus:<\/p>\n

E = R \u00d7 U = p \u00d7 D \u00d7 \u03c4 \/ T <\/em><\/p>\n

The situation is an emergency if both risk and urgency are high. If reaction time is longer than the intervention time left (\u03c4<\/em>\u2009\/\u2009T<\/em>\u2009>\u20091), we have lost control.<\/p>\n

We argue that the intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens. A saving grace is that the rate at which damage accumulates from tipping \u2014 and hence the risk posed \u2014 could still be under our control to some extent.<\/p>\n

The stability and resilience of our planet is in peril. International action \u2014 not just words \u2014 must reflect this.<\/p>\n

References:<\/strong><\/p>\n

Lenton, T. M. et al.<\/em> Proc. Natl Acad. Sci. USA<\/em> 105<\/strong>, 1786\u20131793 (2008).<\/p>\n

IPCC. Global Warming of 1.5\u00b0C<\/em> (IPCC, 2018).<\/p>\n

IPCC. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate<\/em> (IPCC, 2019).<\/p>\n

Cai, Y., Lenton, T. M., & Lontzek, T. S. Nature Clim. Change<\/em> 6<\/strong>, 520\u2013525 (2016).<\/p>\n

Feldmann, J. & Levermann, A. Proc. Natl Acad. Sci. USA<\/em> 112<\/strong>, 14191\u201314196 (2015).<\/p>\n

Aschwanden, A. et al.<\/em> Sci. Adv.<\/em> 5<\/strong>, eaav9396 (2019).<\/p>\n

Drijfhout, S. et al.<\/em> Proc. <\/em>Natl Acad. Sci. USA<\/em> 112<\/strong>, E5777\u2013E5786 (2015).<\/p>\n

Lovejoy, T. E. & Nobre, C. Sci. Adv.<\/em> 4<\/strong>, eaat2340 (2018).<\/p>\n

Walker, X. J. et al.<\/em> Nature<\/em> 572<\/strong>, 520\u2013523 (2019).<\/p>\n

Rogelj, J., Forster, P. M., Kriegler, E., Smith, C. J. & S\u00e9f\u00e9rian, R. Nature<\/em> 571<\/strong>, 335\u2013342 (2019).<\/p>\n

Steffen, W. et al.<\/em> Proc. <\/em>Natl Acad. Sci. USA<\/em> 115<\/strong>, 8252\u20138259 (2018).<\/p>\n

Schneider, T., Kaul, C. M. & Pressel, K. G. Nature Geosci<\/em>. 12<\/strong>, 163\u2013167 (2019).<\/p>\n

Tan, I., Storelvmo, T. & Zelinka, M. D. Science<\/em> 352<\/strong>, 224\u2013227 (2016).<\/p>\n

Rocha, J. C., Peterson, G., Bodin, \u00d6. & Levin, S. Science<\/em> 362<\/strong>, 1379\u20131383 (2018).<\/p>\n

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. & Saba, V. Nature<\/em> 556<\/strong>, 191\u2013196 (2018).<\/p>\n

_____________________________________________________<\/p>\n

Timothy M. Lenton is director of the Global Systems Institute, University of Exeter, UK.<\/em><\/p>\n

Johan Rockstr\u00f6m is director of the Potsdam Institute for Climate Impact Research, Germany.<\/em><\/p>\n

Owen Gaffney is a global sustainability analyst at the Potsdam Institute for Climate Impact Research, Germany; and at the Stockholm Resilience Centre, Stockholm University, Sweden.<\/em><\/p>\n

Stefan Rahmstorf is professor of physics of the oceans at the University of Potsdam; and head of Earth system analysis at the Potsdam Institute for Climate Impact Research, Germany.<\/em><\/p>\n

Katherine Richardson is professor of biological oceanography at the Globe Institute, University of Copenhagen, Denmark.<\/em><\/p>\n

Will Steffen is emeritus professor of climate and Earth System science at the Australian National University, Canberra, Australia.<\/em><\/p>\n

Hans Joachim Schellnhuber – Search for this author in: Pub Med<\/a> – Nature.com<\/a> – Google Scholar<\/a><\/em><\/p>\n

Go to Original \u2013 nature.com<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

27 Nov 2019 – Politicians, economists and natural scientists have tended to assume that tipping points in the Earth system \u2014 such as the loss of the Amazon rainforest or the West Antarctic ice sheet \u2014 are of low probability and little understood. Yet evidence is mounting that these events could be more likely than was thought, have high impacts and are interconnected across different biophysical systems, potentially committing the world to long-term irreversible changes.<\/p>\n","protected":false},"author":4,"featured_media":148964,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[61],"tags":[536,1248,686,794,1354,401,1394,993,896,493,1255,304],"class_list":["post-148963","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-environment","tag-amazonia","tag-arctic","tag-climate-change","tag-deforestation","tag-earth","tag-environment","tag-glaciers","tag-global-warming","tag-oceans","tag-paris-climate-agreement","tag-rain-forests","tag-science"],"_links":{"self":[{"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/posts\/148963","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/comments?post=148963"}],"version-history":[{"count":0,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/posts\/148963\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/media\/148964"}],"wp:attachment":[{"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/media?parent=148963"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/categories?post=148963"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.transcend.org\/tms\/wp-json\/wp\/v2\/tags?post=148963"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}