Potential catastrophic impacts of climate change are "dangerously unexplored" according to an international group of scientists who say the impacts include the possible extinction of humanity.
They say "bad to worse case scenarios", which also include the collapse of food production systems or even whole societies, should be researched as part of prudent risk management.
"There are plenty of reasons to believe climate change could become catastrophic, even at modest levels of warming," said lead author Dr Luke Kemp from Cambridge's Centre for the Study of Existential Risk.
The paper 'Climate Endgame, Exploring Catastrophic Climate Change Scenarios' published in the journal Proceedings of the National Academy of Sciences notes that scientists predict global warming above 2C if all pledges made for emission cuts to be made by 2030 and 2050 are honoured.
It argues that "even these optimistic assumptions lead to dangerous Earth system trajectories".
They say the last time temperatures 2C above pre-industrial norms were sustained was more than 2.6 million years ago, long before human civilisation and "human societies are locally adapted to a specific climatic niche".
"Even if anthropogenic GHG emissions start to decline soon, this does not rule out high future GHG concentrations or extreme climate change", it says, adding that potential tipping points – such as a massive release of methane due to the thawing of Arctic permafrost, carbon loss due to fires or drought in the Amazon – add to the uncertainty in many of the models for predicting our climate's future.
Another possible tipping point listed is the abrupt loss of stratocumulus cloud decks, when CO2 levels reach a certain point, resulting in an additional 8C temperature rise.
The papers says the possibility of a "tipping cascade", where one the occurrence of one tipping point makes others more likely, is "particularly worrying".
It also says there is "deep uncertainty" about the damage that will be caused by climate change and that lack of certainty "should motivate precaution and vigilance, not complacency".
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The authors argue that "knowing the worst cases can compel action", such as the idea of a "nuclear winter" in 1983 galvanising public concern and nuclear disarmament efforts.
Exploring severe risks and higher-temperature scenarios could cement a recommitment to the 1.5 C to 2 C guardrail as the "least unattractive" option.
They say that they "text mined" IPCC reports and concluded that coverage of scenarios with a temperature rise above 3C were "underrepresented relative to their likelihood" with research focused on impacts of 1.5C or 2C.
They call for "thorough risk assessments" which include "compound hazards".
"For example, a cyclone destroys electrical infrastructure, leaving a population vulnerable to an ensuing deadly heat wave."
The authors list four key reasons for concern on catastrophic climate outcomes; the fact that climate change has played a role in the collapse or transformation of societies and in five mass extinction events; climate change could trigger other catastrophic risks such as international conflict or an infectious disease hazard; climate change could lead to system collapse through exacerbating vulnerabilities such as loss of land or water and food security; and that climate change could undermine humanity's ability to recover from another cataclysm "such as nuclear war".
Co-author Prof Johan Rockstrom, director of the Potsdam Institute for Climate Impact Research, said: "The more we learn about how our planet functions, the greater the reason for concern… We increasingly understand that our planet is a more sophisticated and fragile organism.
"We must do the math of disaster in order to avoid it."