Earth system scientists discover missing piece in climate models

As the Earth warms due to human-induced climate change, accurate computer climate models are essential to clarify exactly how the climate will change in the coming years.

In a study published in the Journal for Geophysical Research: AtmospheresA team led by researchers from UC Irvine’s Department of Earth Sciences and the University of Michigan’s Department of Climate and Space Science and Engineering reveals how a climate model widely used by Earth scientists currently overestimates a key physical property of Earth’s climate system: albedo. This is the degree to which ice reflects planet-warming sunlight back into space.

“We found that in old versions of the model, the ice was about 5 percent too reflective,” said Chloe Clarke, a project scientist in the group of UC Irvine professor Charlie Zender. “The reflectivity of the ice was way too high.”

The amount of sunlight the planet receives and reflects is important for estimating how much the planet will warm in the coming years. Earlier versions of the model, the Energy Exascale Earth System Model (E3SM), overestimated albedo because they didn’t account for what Clarke described as the microphysical properties of ice in a warming world.

These properties include the effects that things like algae and dust have on albedo. Dark algae and dust can make snow and ice less reflective, less able to reflect sunlight.

To conduct the analysis, Clarke and her team studied satellite data that track the albedo of the Greenland Ice Sheet. They found that E3SM’s reflectivity overestimates the reflectivity of the ice sheet, “meaning the model estimates less melt than would be expected based on the microphysical properties of the ice,” Clarke said.

But with the new ice reflectivity incorporated into the model, the Greenland ice sheet is melting at a rate of about six gigatons more than in older model versions, based on albedo measurements that are more consistent with satellite observations.

Clarke hopes her team’s study will highlight the importance of seemingly minuscule features that can have far-reaching consequences for the overall climate. “I think our work is going to help models capture snow and ice-related climate feedbacks much better,” she said.

Next, Clarke wants to study different icy regions of the planet to determine how widespread the albedo anomaly in E3SM is.

“Our next steps are to make it globally functional and not just valid for Greenland,” said Clarke, who also plans to compare the new melt rates of the Greenland ice sheet with observations to gauge how accurate the new ice albedo is. “It would be useful to apply it to glaciers in places like the Andes and Alaska.”

Other authors include Raf Antwerpen (Lamont-Doherty Earth Observatory), Mark G. Flanner (University of Michigan), Adam Schneider (National Oceanic and Atmospheric Administration), Marco Tedesco (Lamont-Doherty Earth Observatory), and Charlie S. Zender (UC Irvine).