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....
...Researchers are starting to put together a*nswers, 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....
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.”
...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.