[MCN] Have climate scientists underestimated risk? Is a 6C rise in temps out of the question?

[Lance Olsen]

Science Advances  05 Jul 2017
http://advances.sciencemag.org/content/3/7/e1602821 <http://advances.sciencemag.org/content/3/7/e1602821>

Excerpts from plain language reporting in The Guardian 

Computer models have long indicated a high level of sensitivity, up to 4.5C for a doubling of CO2 in the atmosphere.

However in recent years estimates of climate sensitivity based on historical temperature records from the past century or so have suggested the response might be no more than 3C. 

But the new work, using both models and paleoclimate data from warming periods in the Earth’s past, shows that the historical temperature measurements do not reveal the slow heating of the planet’s oceans that takes place for decades or centuries after CO2 has been added to the atmosphere.

“The hope was that climate sensitivity was lower and the Earth is not going to warm as much,” said Cristian Proistosescu, at Harvard University in the US, who led the new research. 

“There was this wave of optimism.”

The new research, published in the journal Science Advances <http://advances.sciencemag.org/content/3/7/e1602821>, has ended that. “The worrisome part is that all the models show there is an amplification of the amount of warming in the future,” he said. The situation might be even worse, as Proistosescu’s work shows climate sensitivity could be as high as 6C.

Prof Bill Collins, at the University of Reading, UK, and not part of the new research, said: “Some have suggested that we might be lucky and avoid dangerous climate change without taking determined action if the climate is not very sensitive to CO2 emissions. This work provides new evidence that that chance is remote.” 

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Science April 16, 2019
https://www.sciencemag.org/news/2019/04/new-climate-models-predict-warming-surge <https://www.sciencemag.org/news/2019/04/new-climate-models-predict-warming-surge>

New climate models predict a warming surge

By Paul Voosen <https://www.sciencemag.org/author/paul-voosen>


For nearly 40 years, the massive computer models used to simulate global climate have delivered a fairly consistent picture of how fast human carbon emissions might warm the world. But a host of global climate models developed for the United Nations’s next major assessment of global warming, due in 2021 <https://wg1.ipcc.ch/AR6/AR6.html>, are now showing a puzzling but undeniable trend. They are running hotter <https://www.carbonbrief.org/guest-post-why-results-from-the-next-generation-of-climate-models-matter> than they have in the past. Soon the world could be, too.

In earlier models, doubling atmospheric carbon dioxide (CO2) over preindustrial levels led models to predict somewhere between 2°C and 4.5°C of warming once the planet came into balance. 

But in at least eight of the next-generation models <https://cmip6workshop19.sciencesconf.org/data/CMIP6_CMIP6AnalysisWorkshop_Barcelona_190325_FINAL.pdf>, produced by leading centers in the United States, the United Kingdom, Canada, and France, that “equilibrium climate sensitivity” has come in at 5°C or warmer. Modelers are struggling to identify which of their refinements explain this heightened sensitivity before the next assessment from the United Nations’s Intergovernmental Panel on Climate Change (IPCC). But the trend “is definitely real. There’s no question,” says Reto Knutti, a climate scientist at ETH Zurich in Switzerland. “Is that realistic or not? At this point, we don’t know.”

That’s an urgent question: If the results are to be believed, the world has even less time than was thought to limit warming to 1.5°C or 2°C above preindustrial levels—a threshold many see as too dangerous to cross. With atmospheric CO2 already at 408 parts per million (ppm) and rising, up from preindustrial levels of 280 ppm, even previous scenarios suggested the world could warm 2°C within the next few decades. The new simulations are only now being discussed at meetings, and not all the numbers are in, so “it’s a bit too early to get wound up,” says John Fyfe, a climate scientist at the Canadian Centre for Climate Modelling and Analysis in Victoria, whose model is among those running much hotter than in the past. “But maybe we have to face a reality in the future that’s more pessimistic than it was in the past.”



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