CESM2 updates a leading climate model and fixes a major glitch.
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One year ago this month, climate researchers met at a workshop in Boulder, Colo., to fix a big glitch in the second version of the Community Earth System Model (CESM), a computer program that scientists around the world use to simulate Earth’s complex climate system. Last July, Eos reported on that glitch and the befuddlement it had caused the model’s developers.

Now, a year later, at the same annual CESM workshop, held this week again in Boulder, the team behind the model’s development has released the promised second version: CESM2. The new version offers a slew of new features that will help modelers explore the climate in far greater detail than CESM1 ever could.

“You’re driving this car, and you know it doesn’t work as well as it could.”

The glitch, however, meant that the ride to this new version was not exactly smooth. Jean-François Lamarque, an atmospheric chemist at the National Center for Atmospheric Research (NCAR) who was the chief scientist behind CESM a year ago, likened the glitch to having car trouble: “You’re driving this car, and you know it doesn’t work as well as it could,” he said. Fixing it, he added, would take a great deal of work.

Fixing the Glitch

Lamarque and his team had hoped that CESM2 would debut in August of last year, but their CESM2 car kept sputtering. The issue arose when the program ran climate simulations and returned results that did not match those seen in reality—a problem if the main aim of the model is to mimic Earth’s actual climate.

“We spent 4–5 months really digging into the model.”

Specifically, in CESM2 simulations, there was a stretch of about 2 decades in the middle of the 20th century that showed global temperatures minutely falling by 0.3°C or 0.4°C, despite real-world observations pointing toward a steady rise in global temperatures over the same 20-year period. This contrary trend occurred when the model calculated how sulfate aerosols changed the properties of clouds, a phenomenon known as the “aerosol indirect effect.” When sufficiently strong, this effect can cause cooling on a global scale.

To fix the glitch, a team of about 10 climate experts assembled soon after last year’s workshop to reexamine emissions data sets and to tinker with the model. “We spent 4–5 months really digging into the model,” Lamarque said.

A screenshot from a CESM2 simulation of the Arctic climate system
A screenshot from a CESM2 simulation of the Arctic climate system. Warmer colors on the Greenland ice sheet indicate regions of faster ice flow. This simulation, which covered the end of the 20th century and the beginning of the 21st, shows that the model’s output matches observational data from satellites; that is, both show Arctic sea ice cover steadily decreasing over time. Credit: Alice DuVivier, Gunter Leguy, and Ryan Johnson/NCAR, ©UCAR

The researchers thoroughly reviewed how the model captured cloud-aerosol interactions and compared their parameterizations against current knowledge from observations and high-resolution simulations. Through that scrutiny, they identified several problems with their real-world emissions data. They reported these problems to the data suppliers, who then gave them a new, corrected version of the data. This work revealed that “our initial choice of parameters could, and should, be modified to reduce the strength of the aerosol indirect effect,” Lamarque explained.

Despite their efforts, the contrary trend still crops up in CESM2. “But it’s much, much reduced from last year,” Lamarque said, adding that it will take many more years of work “by very smart people” to untangle what is really going on under the model’s hood. The cloud-aerosol mechanism currently outputs a temperature drop of about 0.1°C, effectively curtailing the glitch by more than half.

New Ride

“We went from a standard car to a car with more features.”

Despite that lingering glitch, CESM2 boasts several never-before-seen features. “We went from a standard car to a car with more features,” Lamarque said. These features “include quite substantial improvements in the representation of the physics that they are using,” added Gokhan Danabasoglu, an ocean and climate modeler at NCAR who is the current chief scientist behind CESM.

One of those new features is a capability that will allow users to model the behavior of Greenland’s ice sheet in greater detail. “You can have prognostic evolution of the Greenland ice sheet,” Danabasoglu said. This means that when the model runs, the parts of the ice sheet abutting the ocean melt at a relatively faster clip than ice farther inland, a process that more closely matches reality. This mechanism, Danabasoglu explained, is rather new among today’s climate models.

This week at the workshop in Boulder, researchers from around the world discussed the new features. One attendee, Gretchen Keppel-Aleks, an atmospheric scientist at the University of Michigan, described some of the features that she thinks will help advance her own research into the ways elements like carbon and nitrogen cycle through the environment.

“The new representation of carbon–nitrogen cycling in CESM2 will likely yield more robust projections for how terrestrial carbon cycling will change in the future,” she said. Such projections should help reduce one of the largest uncertainties for our future climate: how much anthropogenic carbon dioxide will remain in the atmosphere over time.  This, she said, means that CESM2 offers a “much more sophisticated framework compared to CESM1.”

Climate researchers, it seems, are liking their new wheels.

A full list of features new to CESM2 can be found on NCAR’s website.

—Lucas Joel (email: lucasvjoel@gmail.com), Freelance Journalist


Joel, L. (2018), New version of popular climate model released, Eos, 99, https://doi.org/10.1029/2018EO101489. Published on 22 June 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
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