Climate model's dire predictions don't line up with reality - study

A state-of-the-art climate modelling system which recently predicted an ice-free Arctic by 2050 might be running a bit hot, according to a new study.

CESM2 is one of 27 climate models included in version six of the Coupled Model Intercomparison Project, which will be used by the UN's Intergovernmental Panel on Climate Change in its next major climate report. 

Together they're known as CMIP6. This system was used in a study published in April which said the Arctic would start losing its ice in summer very soon, even if we managed to rapidly reduce carbon emissions in the short-term.

"This really surprised us," said Dirk Notz of the University of Hamburg, who led that research. "The Arctic will become practically sea-ice free in September before the year 2050... in all scenarios."

But a closer look at CESM2 has suggested a potential explanation for the extreme result - it could simply be wrong. 

Climate scientists at the University of Michigan used it to simulate the climate in the Early Eocene, 50 million years ago, a time when the Earth warmed and rainforests thrived around the tropics of the Americas.

They found it predicted temperatures of over 55C, "much higher than the temperature tolerance of plant photosynthesis". 

In other words, it would have been so hot plants couldn't survive. But the fossil record shows they in fact did quite well. 

"Some of the newest models used to make future predictions may be too sensitive to increases in atmospheric carbon dioxide and thus predict too much warming," said Chris Poulsen, study co-author. 

They said CESM2 predicts warming of about 5.3C if carbon dioxide levels in the atmosphere are sustained at double the pre-industrial level of 285 parts per million. This is higher than its predecessor - CSEM1.2 - which predicts 4.2C of warming. Testing of CSEM1.2 showed it accurately predicted the climate 50 million years ago.

"Our study implies that CESM2's climate sensitivity of 5.3 C is likely too high. This means that its prediction of future warming under a high-CO2 scenario would be too high as well," said co-author Jiang Zhu.

Carbon levels are presently at 418 parts per million. In the Early Eocene, it's estimated they may have been five times higher, with volcanoes probably to blame. Temperatures then were up to 14C higher than today.

But don't think this is an excuse to dismiss dire warnings about the future of our climate entirely. A study that came out in March, just weeks before the headline-grabbing April findings, had the same conclusion - that the Arctic will likely have ice-free summers by 2050. It used the CMIP5 modelling.