SOURCE: Yale e360

DATE: February 5, 2020

SNIP: It is the most worrying development in the science of climate change for a long time. An apparently settled conclusion about how sensitive the climate is to adding more greenhouse gases has been thrown into doubt by a series of new studies from the world’s top climate modeling groups.

The studies have changed how the models treat clouds, following new field research. They suggest that the ability of clouds to keep us cool could be drastically reduced as the world warms — pushing global heating into overdrive.

Clouds have long been the biggest uncertainty in climate calculations. They can both shade the Earth and trap heat. Which effect dominates depends on how reflective they are, how high they are, and whether it is day or night. Things are made more complicated because cloud dynamics are complex and happen on small scales that are hard to include in the models used to predict future climate.

Recent concern about how accurately the models handle clouds has focused on the blankets of low clouds that any international flyer will have seen extending for hundreds of miles below them across the oceans. Marine stratus and stratocumulus clouds predominantly cool the Earth. They shade roughly a fifth of the oceans, reflecting 30 to 60 percent of the solar radiation that hits them back into space. In this way, they are reckoned to cut the amount of energy reaching the Earth’s surface by between 4 and 7 percent.

But it seems increasingly likely that they could become thinner or burn off entirely in a warmer world, leaving more clear skies through which the sun may add a degree Celsius or more to global warming. As Mark Zelinka of the Lawrence Livermore National Laboratory, lead author of a review of the new models published last month, has put it: The models “are shedding their protective sunscreen in dramatic fashion.”

The new predictions overturn a consensus about the planet’s climate sensitivity that has persisted for the entire 32-year history of the UN’s Intergovernmental Panel on Climate Change. All five of the IPCC’s scientific assessments have agreed that doubling carbon dioxide, the most critical planet-warming greenhouse gas, from pre-industrial levels will eventually warm us by about 3 degrees C (5.4 degrees F), with an error bar extending from a low of 1.5 degrees C (2.7 degrees F) to a high of 4.5 degrees C (8 degrees F). This is known in the jargon as the equilibrium climate sensitivity.

That consensus was reaffirmed in 2018 when a widely cited review, headed by Peter Cox of the University of Exeter, found that the chance of climate sensitivity exceeding 4.5 degrees was “less than 1 percent.”

But the consensus has now been blown apart. Most leading climate models — including those from the U.S. National Center for Atmospheric Research (NCAR) and Britain’s Hadley Center — are now calculating the climate’s sensitivity to doubling CO2 levels as a degree or more higher, ranging up to 5.6 degrees C (10 degrees F). Their findings are almost certain to feature strongly in the next IPCC assessment of the physical science of climate change, due to be signed off in April next year.

In the meantime, the new findings should make disturbing reading for climate negotiators preparing a new deal on reducing greenhouse-gas emissions to be agreed in Glasgow, Scotland, this November. That’s because the revised estimates make the prospect of keeping global warming below 2 degrees C — let alone the aspiration of 1.5 degrees C agreed in the Paris climate accord five years ago — even more unlikely.

[Read the full article to get more details about clouds and how they are affecting the projections for climate sensitivity.]