Leading climate scientists conceded that models used to estimate how much the world will warm with rising levels of carbon dioxide in the atmosphere are running too hot.
“It’s become clear over the last year or so that we can’t avoid this,” Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, told Science magazine.
The admission is seen as a significant development by scientists who argue that not enough attention has been paid to natural cycles in the earth’s climate.
It puts another question mark over the use of the most extreme scenarios generated by models, RCP8.5, to estimate what could be expected in a warming world.
The concession has been made on the eve of this month’s release of the Intergovernmental Panel on Climate Change’s report on the science of climate change.
That report, delayed a year because of Covid-19, is due to be released on August 9 and will outline what can be expected with different levels of warming.
It will play a major role in preparations for the upcoming climate change summit in Glasgow, Scotland, in November.
A Science article published this week said climate scientists faced the alarming reality that “climate models that help them project the future have grown a little too alarmist”.
“Many of the world’s leading models are now projecting warming rates that most scientists, including the model makers themselves, believe are implausibly fast”, the article said.
“In advance of the UN report, scientists have scrambled to understand what went wrong and how to turn the models, which in other respects are more powerful and trustworthy than their predecessors, into useful guidance for policymakers.”
In the past, most models projected a “climate sensitivity” – the warming expected when atmospheric carbon dioxide is doubled over pre-industrial times – of between 2C and 4.5C.
Last year, a landmark paper that used documented factors including ongoing warming trends calculated a likely climate sensitivity of between 2.6C and 3.9C but many of the new models from leading centres showed warming of more than 5C – uncomfortably outside these bounds.
The models were also out of step with records of past climate.
According to Science, the IPCC team will probably use reality – the actual warming of the world over the past few decades – to constrain model projections.
The IPCC report is also likely to present the impacts of different amounts of warming – 2C, 3C, 4C – rather than saying how quickly those impacts will be felt.
Steve Sherwood from the UNSW Climate Change Research Centre said “while it is true some new climate models have surprising climate sensitivities and predict very high future warming, what doesn’t always come through is that most new models have sensitivity values within the range estimated from observations”.
“Those models still predict substantial future weather and climate changes due to carbon dioxide, similar to predictions made by the science community for many years,” Professor Sherwood said.
US climate scientist Judith Curry said the IPCC report would certainly discuss the problem with climate models: “The elephant in the room for the IPCC is they are heavily relying on RCP8.5 emissions scenarios, which are now widely regarded as implausible.”
Michael Asten, an expert reviewer of the IPCC’s AR6 report, said the admission that climate models were running hot was a significant concession.