Since the concept of climate tipping points was introduced two decades ago, there have been calls for a better understanding of their probability and impacts. Tipping points can occur when changes in a system become self-sustaining, lock in irreversible impacts, and are driven by “positive” feedback loops which can quickly amplify small changes.
The first major assessment of potential climate tipping points was published in 2008, finding that nine parts of the climate system (termed “tipping elements”) could feature climate change-driven tipping points. Most climate tipping points were assessed as being possible at around 3-5oC above 20th-century temperatures. Since then, new elements have been proposed (e.g., parts of the East Antarctic ice sheet) and the status of others (e.g., Arctic summer sea ice) has been questioned. In addition, estimates of what warming levels could trigger them have fallen. Some have been reassessed as being at risk within the Paris Agreement range of 1.5-2oC above pre-industrial which is now approaching.
In a paper published recently in Science, I and nine co-authors reassessed proposed climate tipping points based on over 200 scientific papers published since the last major assessment in 2008. These papers cover evidence from recent observations, new records of ancient climate change, and improved climate models. From these, we synthesized estimates for the global warming threshold, timescale, and impact for each proposed tipping element, and categorize them according to their dynamics.
We identified sixteen tipping elements in total, versus nine candidates in 2008. Five are assessed to already be within the lower end of their potential threshold ranges, meaning we cannot rule out them tipping at the current warming level. Four of these pass their central estimates at 1.5oC, making them likely to be triggered in our assessment if warming is not kept to the most ambitious Paris Agreement aim. Triggering any of these climate tipping points would lock in substantial climate impacts in the coming decades and far into the future.
Impacts of climate tipping points
Warming is expected to increase linearly with further emissions, but the impacts of this warming will not be so linear. Climate extremes are quickly becoming more commonplace, with for example extreme temperature events that used to happen once every 50 years happening around 5 times more often at 1oC, and expected to happen 9 times more often at 1.5oC and 14 times more often at 2oC. This escalation in climate impacts motivated the Paris Agreement aims of limiting warming to well below 2oC, and preferably to only 1.5oC.
Climate change and its impacts may not always be steady on a regional scale or in the longer term either—passing “tipping points” can cause abrupt or irreversible changes which would lock in substantial climate impacts for generations to come. While some impacts from tipping points could take hundreds or thousands of years to appear, it is our actions now that will determine the likelihood of them occurring. And some tipping points, such as the die off of tropical and sub-tropical coral reefs and the collapse of ocean circulation in the Labrador and Irminger Seas, are projected to take only a decade or so.
Irreversible collapse of the Greenland and West Antarctic ice sheets could be committed over the coming hundreds to thousands of years if warming remains above 1.5oC by the end of the century. Together they contain enough frozen water to raise sea levels by 10 meters, drastically reshaping global coastlines, committing many low-lying countries to eventual disappearance, and amplifying shorter-term sea level projections. Higher warming could threaten other ice sheets too, such as the subglacial basins in East Antarctica.
Widespread loss of glaciers outside of polar regions could be committed over the next couple of centuries (at c. 2oC). This would threaten water supplies in many regions, with around 2 billion people worldwide relying on glacier water, in particular downstream of high-mountain Asia. While they would take a while to fully disappear, the point at which glacier water supplies start to decline (“peak water”) has already passed in some regions and will come as soon as 2050 in the Himalayas.
Collapse of ocean circulation in the Labrador and Irminger Seas (at c. 1.8oC) or the wider Atlantic meridional overturning circulation (at c. 4oC) would have substantial knock-on effects across the world. Intense regional cooling would occur in Europe as less heat is transported northwards, and weather patterns linked with the jet stream would be disrupted across Europe and North America. Tropical monsoons would also shift southwards, threatening agricultural regions dependent on monsoons. Barents Sea ice loss (at c. 1.6oC) could also drive damaging weather extremes in Europe.
Widespread abrupt thawing of permafrost could occur beyond 1.5oC, with more intense collapse in particularly carbon-rich areas beyond 4oC. Both would amplify carbon emissions from permafrost thaw, creating a potent feedback loop on global warming. Abrupt permafrost thaw is already starting to disrupt northern landscapes, destroying infrastructure and disrupting the livelihoods of Indigenous peoples and local communities, which would become much more substantial beyond 1.5oC. The great northern boreal forest could also start dying back in the south and jumping northwards into the tundra beyond 4oC, causing drastic ecosystem shifts and complex regional climate changes.
Large parts of the Amazon rainforest could die back to a degraded savannah-like state beyond 3.5oC of warming, or likely sooner if combined with deforestation. This is because some of the Amazon is only wet enough to remain as rainforest due to intense moisture recycling in upwind rainforests, and so losing some forest to drought or deforestation can cascade downwind. This would be devastating for both biodiversity and the Indigenous peoples and local communities in these areas, substantially amplify regional warming, and cut rainfall in the countries south of the Amazon.
Tropical and subtropical coral reefs are facing “mass bleaching” events increasingly often, with marine heatwaves pushing them to expel the symbiotic algae that provide their food as a result of heat stress. Beyond 1.5oC it is likely that these heatwaves will become too frequent for most reefs to be able to recover in between bleaching events, leading to mass reef die offs. This would be a massive blow to biodiversity, and damage the livelihoods of the hundreds of millions of people that depend on coral reefs for their rich fisheries and coastal protection.
Complex shifts in warming and rainfall in West Africa could result in an abrupt increase in vegetation in the currently arid Sahel region south of the Sahara desert beyond around 2.8oC. Similar shifts happened in the past, when large parts of the Sahara greened after the end of the last Ice Age glacial period. This could be a positive change, but it would also be associated with rainfall declines in other parts of West Africa and significant changes to the traditional livelihoods of Sahelians.
Implications for climate action
These tipping points would lock in substantial long-term climate impacts, creating a much different world in the near and far future. This would disrupt the livelihoods and well-being of hundreds of millions of people, especially given the global population is projected to reach 10 billion people between 2050 and 2100. Our assessment therefore provides strong scientific support for rapid emission cuts in line with limiting warming to 1.5oC—the more ambitious aim of the Paris Agreement—as this would reduce the likelihood of passing climate tipping points. Even if 1.5oC is missed or some tipping points are triggered though, every fraction of a degree avoided beyond 1.5oC still reduces the likelihood of passing more.
Average warming will reach 1.5oC in the 2030s, giving the world a rapidly shrinking window for keeping warming close to this level, and current policies are projected to result in a clearly unsafe 2.6oC of warming by 2100. Limiting warming to 1.5oC would require emissions to fall by around 45 percent from 2020 to 2030, reaching net zero (where any remaining emissions from hard-to-decarbonize sectors are balanced by CO2 removal) by 2050. The parallel ecological crisis should not be ignored either, with it being important to find synergies between climate action and ecosystem regeneration.
Even a 1.5oC warmer world will be challenging to adapt to, especially as we cannot rule out some climate tipping points being passed even in best-case scenarios. That means adaptation and climate-resilient development plans need to incorporate the possibility of tipping point damages being locked in, for example from the consequences of coral reef die-off, substantial long-term sea level rise, North Atlantic weather disruption, or northern landscapes riven by permafrost thaw.
It is clear that far more ambitious action is required to keep the Paris Agreement alive and minimize the chances of triggering climate tipping points. Climate change due to human-driven greenhouse gas emissions is already creating havoc around the world. Even at today’s 1.2oC of global warming, we are seeing increased weather extremes like this year’s floods in Pakistan and Nigeria and heat waves in India, Europe, and China. The projected sea level rise by 2050 will threaten the homes of 300 million people with annual flooding, particularly in South and East Asia and small island states.
Both rapid emission cuts to reduce the chances of climate tipping points and investment in climate-resilient development are needed. Progress is currently too incremental on both mitigation and adaptation though, making the need for transformative change—potentially accelerated by triggering “positive” societal tipping points—ever more urgent.
David Armstrong McKay is a climate and biosphere scientist, and a Research Impact Fellow at the University of Exeter working on a new ‘State of Tipping Points’ report and with the Earth Commission. He also runs the science communication site climatetippingpoints.info, which explains and analyses the science and claims around climate tipping points.