The Hadley Centre team reported last month that, in the model, the aerosols had an exceptionally large effect on North Atlantic sea surface temperatures. And it was an indirect aerosol effect that made the bulk of the difference. The sulfate particles attracted water vapor to create a vast supply of tiny droplets within clouds, brightening them and reducing the amount of sunlight reaching the sea surface.
Overall, North Atlantic sea surface temperatures climbed throughout the simulation, from 1860 to 2005. But an increase in aerosols slowed the ocean warming during the mid-twentieth century, when rapid industrialization caused extreme levels of air pollution. After restrictions on sulfur emissions in the United States and Europe started to kick in the 1970s, the skies grew clearer and sea surface temperatures increased.
In the end, Booth says, the changing output of industrial aerosols explains two-thirds of the long-term swings ob served in sea surface temperatures in the North Atlantic. “It's only in the current generation of models that we can see that relationship physically,” says Booth.
The Hadley Centre's results seem to overturn the prevailing wisdom in climate circles, which holds that the ups and downs in sea surface temperatures result from a natural ocean cycle dubbed the Atlantic multidecadal oscillation (AMO). Earlier research suggested that the cooler Atlantic temperatures associated with the AMO could have contributed to droughts over the Sahel in Africa during the latter half of the twentieth century; the same cooling effect may have led to a reduction in the force of tropical storms and hurricanes steaming towards America. But on the basis of the new picture, human pollution could be causing these climate disruptions instead.
The question now is whether the results will hold up. Researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, say that they see hints of similar effects in their new simulations. But not everybody is convinced that aerosol pollution could have such a profound effect on ocean temperatures — and consequently on climate. NCAR climate scientist Kevin Trenberth says that the results depend on uncertain estimates of aerosol pollution and cloud distributions around the Atlantic. At the same time, satellite observations do not find the indirect aerosol effect to be as strong as the models seem to suggest, he says. “It would be surprising to me if the ocean is not playing a substantial role” through natural cycles.
Researchers are also struggling to tease apart the roles of natural cycles and human-caused changes in the melting Arctic. The sea ice there has taken a beating during the past few decades and coverage reached a near-record low of 4.33 million square kilometers last September. Because the speed of the ice loss has outstripped all but the most dire model predictions, researchers have wondered what might be missing from their simulations.
Early results from the new models suggest that the addition of the more complex clouds and aerosols to simulations could help to provide an explanation. NCAR's new atmospheric model produced more warming and sea-ice loss than the previous iteration, and the culprit seems to be clouds — a result that caught researchers by surprise. “I'm a cloud girl, but I didn't go into this thinking that clouds were going to play the lead role,” says Jennifer Kay, an atmospheric scientist at NCAR.
To figure out what was happening, the team built new diagnostic tools into the model that effectively tell scientists what they would see if they were observing the planet from a pair of US satellites, CloudSat and Calipso. The model's output is translated into a signal that can be compared directly with radar and laser instruments aboard the satellites, Kay explains. “You basically fly a little satellite around inside the model,” she says, “and what it shows is that the clouds are remarkably improved in the new version.” They tend to be thinner and more transparent — more like their physical counterparts in the Arctic skies — although why remains unclear.