In the climate lexicon, tipping points are thresholds beyond which certain impacts can no longer be avoided even if temperatures are brought down later. Examples include the loss of the Amazon rainforest or of the West Antarctic ice sheet.
When the Intergovernmental Panel on Climate Change (IPCC) introduced the concept two decades ago, it was thought that these “large-scale discontinuities” in the climate system were likely only if global mean temperatures increased by more than 5 °C over pre-industrial levels.
Recent IPCC reports, including last year’s Global Warming of 1.5°C and this year’s Special Report on the Ocean and Cryosphere in a Changing Climate, suggest that tipping points could be exceeded even between 1 and 2 °C of warming. This is worrying as we are on track to a 3.2 degree warmer world, suggests the UNEP’s Emissions Gap Report released last week.
Nevertheless, many politicians and even scientists assume that tipping points have a low probability of occurrence. A new article in Nature by a group of scientists summarises the evidence to challenge this notion. The article has been co-authoured by climate scientists from the Potsdam Institute for Climate Impact Research in Germany, the University of Exeter in the UK, Stockholm University in Sweden, University of Copenhagen in Denmark and Australian National University in Australia.
On ice, the article notes, Amundsen Sea embayment in West Antarctica may have already reached a tipping point, which could destabilise the rest of the West Antarctic ice sheet like toppling dominoes and lead to a sea-level rise of about 3 metres on a millennial timescale.
Together, with similar developments in the East Antarctic and Greenland, this may already have committed the future generation to 10 metres (m) of sea-level rise. But, as the paper emphasises, despite breaching a tipping point, the pace of change, which can partially mitigate the catastrophe, is still in our hands. If warming is limited to 1.5 °C, the 10 m rise could take 10,000 years to unfold, while warming of over 2 °C could mean getting there in just a 1,000 years.
The report addresses biosphere tipping points such as Amazonian deforestation, which can trigger abrupt carbon releases into the atmosphere, amplify climate change and reduce remaining emission budgets. Permafrost emissions could total 100 Gigatonnes (Gt), Amazon dieback emissions could be as high as 90 Gt and emissions from boreal forests a further 110 Gt.
These three phenomena could thus alone consume 60 per cent of the 500 Gt carbon budget remaining to maintain a 50 per cent chance of limiting warming to 1.5 °C. While estimates of an Amazonian tipping point vary from 40 per cent deforestation to just 20 per cent forest cover loss, about 17 per cent has been lost since 1970.
The paper argues that finding the precise tipping point requires understanding deforestation and climate change as interacting drivers, as well as fire and climate feedbacks as interacting tipping mechanisms.
Exceeding tipping points in one system can increase the risk of crossing them in others. Thus a “global cascade” of such tipping points could trigger a “global tipping point” leading to a less habitable, “hothouse” climate state. Noting that such regional cascades had occurred in the geological past when only natural forces were at play, the paper makes the case for a potentially significant probability of the future global tipping point, in the backdrop of atmospheric CO2 concentration and global temperature increasing at rates that are an order of magnitude higher than those in the past.
The article also claims that the case for a potential global tipping point is strengthened by the early results of studies for the IPCC’s Sixth Assessment Report due to be published in 2021, which indicate a much larger climate sensitivity (the temperature response to doubling of atmospheric CO2) than in previous models.
Some economists assume that climate tipping points are of very low probability and, as a result, suggest that 3 °C warming is optimal from a cost-benefit perspective. The article challenges this notion and argues that as tipping points are more likely, the “optimal policy” recommendations of climate-economy models would require warming to be limited to 1.5 °C.
To scientists who argue that a global tipping point is highly speculative, the paper’s authors respond that given its huge impact and irreversible nature, one must err on the side of caution. “This is an existential threat to civilization. No amount of economic cost-benefit analysis is going to help us. We need to change our approach to the climate problem,” the article notes.
To evaluate if the world is indeed in a climate emergency, the paper defines an emergency as the product of risk and urgency. Urgency is the ratio between the reaction time to an alert and the intervention time left to avoid a bad outcome. A situation is an emergency if both risk and urgency are high.
If the reaction time is longer than the intervention time left, we have lost control. The paper argues 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. Thus we might already have lost control and the world is indeed in a climate emergency.
Hans Joachim Schellnhuber, one of the authors of the report, told The Guardian Australia that the formula was just the “tip of a mathematical iceberg” in defining the climate emergency. He argued: “It can be illustrated by the Titanic disaster… Yet there are options to avoid the disaster.” Noting that little time was left to act, Schellnhuber warned that beyond the critical point “only some sort of adaptation option is left, such as moving the Titanic passengers into rescue boats (if available).”
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