A catastrophic 4.0 to 5.0°C of average global warming by 2100 looks extremely unlikely, but so do the most optimistic projections for future temperature levels, in a University of Exeter study published this week in the journal Nature.
The study, “if correct, voids worst-case UN climate change predictions,” notes Agence France-Presse, while reducing the variability in projections of future warming by about 60%.
“Our study all but rules out very low and very high climate sensitivities,” said mathematician and lead author Peter Cox, professor of climate system dynamics at Exeter. By looking at variations in today’s climate that are not related to greenhouse gas emissions, then extrapolating them into the future, Cox and his team reduced the “likely” range of sensitivity by century’s end from the 1.5 to 4.5°C in past United Nations climate assessments to a span of 2.2 to 3.4°—still far beyond the limit that would trigger devastating climate impacts.
That means the research team’s findings are no reason to let up on the global effort to get climate change under control, AFP stresses. “How effectively the world slashes CO2 and methane emissions, improves energy efficiency, and develops technologies to remove CO2 from the air will determine whether climate change remains manageable or unleashes a maelstrom of human misery,” the news agency notes, citing the study results.
“You can think of global warming as the stretching of a spring as we hang weights from it, and climate sensitivity as related to the strength of the spring,” Cox explained. “To relate the observed global warming to climate sensitivity, you need to know the amount of weight being added to the spring, which climate scientists call the ‘forcing’, and also how quickly the spring responds to added weight. Unfortunately, we know neither of these things very well.”
But he said the inability to narrow the range of possible climate impacts has been a challenge until now. “The problem is the low end sort of implies maybe you don’t need to do much about climate change except adapt, and the high end implies it’s kind of too late,” he told the Washington Post.
If Cox and his colleagues are right, their conclusion “precludes the most destructive doomsday scenarios” associated with runaway climate change, AFP states. “These scientists have produced a more accurate estimate of how the planet will respond to increasing CO2 levels,” said Piers Forster, director of the University of Leeds’ Priestley International Centre for Climate. “I should thank Cox and colleagues for helping me to sleep a little easier in my bed at night,” he told the Post, adding that he found the methodology “ingenious”.
The study “asserted that there is less than a 3% possibility that the climate sensitivity is lower than 1.5°C, and less than a 1% possibility that it is higher than 4.5°,” the Post notes. But “the “new finding contrasts with a similar study, also published in the journal Nature, which found that the global climate models that best capture the current climate are also those that predict some of the warmest and most severe outcomes.” That study, by Patrick Brown and Ken Caldeira at the Carnegie Institute of Science, assigned less likelihood to lower levels of average warming but more to the high-end outcomes.
“It’s normal for these types of studies to be apparently contradictory,” climate specialist Ben Sanderson of the U.S. National Center for Atmospheric Research told the Post’s Chris Mooney. “The way forward is to better understand the physical mechanisms for why the correlations exist,” then “look together at all the information to make a holistic assessment of all the evidence as it pertains to the real world.”
Cox acknowledged that his team hadn’t factored in climate “tipping points”, or feedbacks, in which climate change triggers further abrupt system changes that lead to sudden, rapid warming. “The collapse of the Gulf Stream, the thawing of carbon-rich permafrost, or the melting of ice sheets on Greenland and Antarctica—any of these could quickly change the equation, and not in the Earth’s favour,” AFP notes.