risk of human extinction “dangerously underexplored”

The risk of global societal collapse or human extinction is “dangerously under-explored”, climate scientists have warned in an analysis.

They call such a catastrophe the “climate endgame”. Although that had a small chance of happening, given the uncertainty of future emissions and the climate system, catastrophic scenarios could not be ruled out, they said.

“Facing a future of accelerating climate change while blind to worst-case scenarios is naive risk management at best and fatally foolish at worst,” the researchers said, adding that there was “ample reason” to suspect that global warming could result an apocalyptic disaster.

The international team of experts argues that the world must start preparing for the possibility of the climate endgame. “Analyzing the mechanisms of these extreme impacts can help stimulate action, improve resilience and inform policy,” they said.

Explorations in the 1980s of the nuclear winter that would follow a nuclear war spurred public concern and disarmament efforts, the researchers said. The analysis proposes a research agenda, including what they call “the four horsemen” of the climate endgame: famine, extreme weather, war and disease.

They also asked the Intergovernmental Panel on Climate Change to produce a special report on the issue. The IPCC report on the impact of just 1.5C of warming created a “fundamental wave of public concern”, they said.

“There are many reasons to believe that climate change could be catastrophic, even at modest levels of warming,” said Dr Luke Kemp of the University of Cambridge’s Center for the Study of Existential Risk, who led the analysis. “Climate change has played a role in every mass extinction. It has helped fallen empires and shaped history.

“Pathways to disaster are not limited to direct impacts of high temperatures, such as extreme weather events. Consequences such as economic crises, conflicts and new disease outbreaks can trigger other disasters.”

The analysis is published in the journal Proceedings of the National Academy of Sciences and was reviewed by a dozen researchers. It argues that the impacts of global warming beyond 3C have been underestimated, with few quantitative estimates of the total impacts. “We know the least about the scenarios that matter the most,” Kemp said.

A thorough risk assessment would consider how risks spread, interacted and amplified, but had not been attempted, the researchers said. “Yet this is how risk plays out in the real world,” they said. “For example, a cyclone destroys electrical infrastructure, leaving a population vulnerable to a subsequent deadly heat wave.” The Covid pandemic underscored the need to examine rare but large global risks, they added.

Of particular concern are tipping points, where a small increase in global temperature results in a large change in climate, such as huge carbon emissions from an Amazon rainforest suffering major droughts and fires. Tipping points can trigger others in a cascade, and some remained understudied, they said, such as the abrupt loss of stratocumulus cloud cover that could cause another 8C of global warming.

The researchers warn that climate breakdown could exacerbate or trigger other catastrophic risks, such as international wars or infectious diseases, and exacerbate existing vulnerabilities such as poverty, crop failure and water scarcity. The analysis suggests that superpowers may one day battle over geoengineering plans to reflect sunlight or the right to emit carbon.

“There is a striking overlap between current vulnerable states and future areas of extreme warming,” the researchers said. “If current political fragility does not improve significantly over the coming decades, a belt of instability with potentially serious consequences could emerge.”

There were further good reasons to be concerned about the potential for a global climate catastrophe, the researchers said: “There are warnings from history. Climate change has played a role in the collapse or transformation of a number of past societies and in each of the five mass extinction events in Earth’s history.”

Related: The domino effect of climate events can move the Earth into a “greenhouse” state

New modeling in the analysis shows that extreme heat – defined as an annual average temperature of more than 29C – could affect 2 billion people by 2070 if carbon emissions continue.

“Such temperatures are currently affecting about 30 million people in the Sahara and Gulf Coast,” said Chi Xu, of Nanjing University in China, who was part of the team. “By 2070, these temperatures and the social and political consequences will directly affect two nuclear powers and seven maximum containment laboratories housing the most dangerous pathogens. There is a serious potential for catastrophic consequences.”

The current trend of greenhouse gas emissions will lead to an increase of 2.1-3.9C by 2100. However, if existing action pledges are fully implemented, the range will be 1.9-3C. Achieving all long-term targets set to date would mean 1.7-2.6 C of warming.

“Even these optimistic assumptions lead to dangerous Earth system orbits,” the researchers said. Temperatures more than 2C above pre-industrial levels had not been sustained on Earth for more than 2.6 million years, they said, well before the rise of human civilization, which had risen in a “narrow climatic envelope” over the past 10,000 years .

“The more we learn about how our planet works, the greater the cause for concern,” said Professor Johan Rockström, of the Potsdam Institute for Climate Impact Research in Germany. “We increasingly understand that our planet is a more sophisticated and fragile organism. We must calculate disaster to avoid it.”

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