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SOURCE: Bloomberg Green

DATE: February 3, 2020

SNIP: 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 encyclopedic 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, policy makers 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?