The gauzy clouds allow more sunlight through in the summer, which melts more ice and exposes more sea and land surfaces; these effects are enhanced by deposition of dark aerosol particles on the snow. It all adds up to a shift towards darker surfaces that absorb more sunlight and amplify warming. Although the model still tends to underestimate sea-ice loss on average, Kay says, some simulations lined up with satellite observations reasonably well.
Researchers at the GFDL are also seeing greater sea-ice declines with their new climate model. Michael Winton, a modeler at the GFDL, says this is likely to be a theme in the IPCC's fifth assessment, but he warns against premature celebration. The addition of enhanced clouds and aerosols to the simulations is driving the extra warming, but the exact details remain unclear.
In the end, the climate community must confront a basic question about models. “If you made a model and it matched the observations perfectly, would you claim success?” Winton asks. Although the new GFDL model has an enhanced representation of the atmosphere and does a better job of matching satellite observations, Winton warns that modelers could get the right answer for the wrong reasons. There is some evidence, for example, that natural variability in ocean circulation has caused some of the sea-ice loss during the past two decades. “The Arctic has to be understood in the context of the overall climate,” he says.
Taming the monsoon
In satellite images, southeast Asia is often covered by a giant blemish — a brown cloud fed by black carbon emissions from millions of primitive cooking stoves and open fires throughout rural India and neighboring countries. In the atmosphere, those dark particles absorb sunlight and heat the surrounding air while cooling the land below, effectively stabilizing the atmosphere and slowing the regional circulation that draws moisture inland from the northern Indian Ocean. Researchers proposed seven years ago that this mechanism could explain why the south Asian summer monsoon has grown weaker over the past half-century.
However, simulations with one of the new models at the GFDL suggest that the situation might be more complicated, with aerosols and clouds disturbing a much larger hemispheric energy exchange.
The overall system is driven by the summer Sun, which delivers more heat north of the equator than south. In what amounts to a massive heat engine that redistributes energy between the hemispheres, hot air rises in the north and carries heat at altitude to the south, where the air descends and picks up moisture from the Indian Ocean on its return north. It is this last step that brings the summer monsoons, which provide up to 80% of the precipitation to most of India. But the GFDL results, reported in Science last October, showed that aerosols are creating a major disruption.
“Aerosol emissions are like putting up a sunscreen over the Northern Hemisphere, and that reduces the solar imbalance that drives the system,” says Yi Ming, a GFDL climate modeler and an author of the study. “We're trying to argue this from a larger spatial scale.”
Their model also shifts the blame away from the black-carbon emissions of cooking stoves and agricultural fires, and towards sulfur pollution from coal-fired power plants throughout the region. The sulfate particles that develop from such pollution serve as the seeds for water droplets and brighten clouds, cooling the land below. In addition to capturing the 4–5% overall decline in summer rainfall over India since 1950, the model reproduces regional variations in precipitation — more drying over north-central India versus a slight increase in rainfall over southern India and northwestern India and Pakistan. Ming says the indirect aerosol effect included in the new study shows “a different part of the puzzle”.