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Home / News / World /  Climate models run red hot, nobody knows why

There are dozens of climate models, and for decades they’ve agreed on what it would take to heat the planet by about 3° Celsius. It’s an outcome that would be disastrous—flooded cities, agricultural failures, deadly heat—but there’s been a grim steadiness in the consensus among these complicated climate simulations.

Then last year, unnoticed in plain view, some of the models started running very hot. The scientists who hone these systems used the same assumptions about greenhouse-gas emissions as before and came back with far worse outcomes. Some produced projections in excess of 5°C, a nightmare scenario.

The scientists involved couldn’t agree on why—or if the results should be trusted. Climatologists began “talking to each other like, ‘What’d you get?’, ‘What’d you get?’" said Andrew Gettelman, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado, which builds a high-profile climate model.

“The question is whether they’ve overshot," said Mark Zelinka, staff scientist at Lawrence Livermore National Laboratory. Researchers are starting to put together a­nswers, a task that will take months at best, and there’s not yet agreement on how to interpret the hotter results. The reason for worry is that these same models have successfully projected global warming for a half century. Their output continues to frame all major scientific, policy and private-sector climate goals and debates, including the sixth encyclopaedic assessment by the UN’s Intergovernmental Panel on Climate Change due out next year. If the same amount of climate pollution will bring faster warming than previously thought, humanity would have less time to avoid the worst impacts.

For now, however, there are doubts and worries. A higher warming estimate “probably isn’t the right answer", said Klaus Wyser, senior researcher at the Swedish Meteorological and Hydrological Institute. His model produced a result of about 4.3°C warming, a 30% jump over its previous update. “We hope it’s not the right answer."

This uncertainty over how to read the models highlights one of the central challenges of climate change. On the one hand, policymakers and members of the public are turning to scientists as never before to explain historic wildfires, devastating droughts and spring-like temperatures in mid-winter.

And the bedrock of the science has never been more solid. But the questions vexing experts now are probably the most important of all: Just how bad is it going to get—and how soon?

Earth-system models are the workhorses of climate research, helping scientists test ideas about the impact of ice-sheet melting, soil moisture and clouds, all without waiting for the actual planet to fall apart. There are more than a hundred models used to forecast the relationship between carbon dioxide and warming, developed by about two dozen independent research groups.

One question modelling can help answer is called “climate sensitivity", an estimate of how much warmer the planet will be once it has adjusted to atmospheric CO2 at double the pre-industrial level. (At current rates, CO2 could reach a doubling point in the last decades of this century.) This is the old, reliable number that’s come out to 3°C for 40 years. It was as close as anything gets to certainty.

It takes climate modellers, who run hugely complex calculations on supercomputers, more than a biblical six days to create their virtual worlds. Modules for air, land and sea all churn together and interact, and through early runs the researchers will make adjustments for troubleshooting and debugging that amount to rewiring the whole world. The first step is to replicate actual conditions of the 20th century within the model; then you can trust the software to forecast the future.

The model run by NCAR, one of American’s main climate-science institutions, started producing unusual data last year while trying to reproduce the recent past. “We got some really strange results," Gettelman said.

The scientists went on to try 300 configurations of rain, pollution and heat flows—something they can do as gods of their own digital earth—before matching the model to history. But by solving that puzzle, Gettelman’s team sent future projections upward at an unheard-of rate. NCAR found that CO2 doubling would lead to 5.3°C world, a 33% jump from the model’s past reading on global warming.

Soon there were multiple teams at other institutions putting out new climate-sensitivity numbers that looked like worst-case scenarios on steroids. The Met Office Hadley Centre, the UK’s main research group, found a doubling of CO2 would deliver 5.5°C warming. A team at the US Department of Energy ended up with 5.3°C, and the Canadian model topped out at 5.6°C. France’s National Centre for Meteorological Research saw its estimate jump to 4.9°C from 3.3°C.

In all, as many as a fifth of new results published in the last year have come in with anomalously high climate sensitivity. There are dozens still left to report, and their results will determine whether these grim forecasts are outliers or significant findings.

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