Add CO2 to the atmosphere and the climate will get warmer that much is well established. But climate change and carbon aren't in a one-to-one relationship. If they were, climate modeling would be a cinch. How much the globe will warm if we put a certain amount of CO2 into the air depends on the sensitivity of the climate. How vulnerable is the polar sea ice; how rapidly might the Amazon dry up; how fast could the Greenland ice cap disintegrate? That's why models like those from the Intergovernmental Panel on Climate Change spit out a range of predictions for future warming, rather than a single neat number.
One of the biggest questions in climate sensitivity has been the role of low-level cloud cover. Low-altitude clouds reflect some of the sun's radiation back into the atmosphere, cooling the earth. It's not yet known whether global warming will dissipate clouds, which would effectively speed up the process of climate change, or increase cloud cover, which would slow it down.
But a new study published in the July 24 issue of Science is clearing the haze. A group of researchers from the University of Miami and the Scripps Institute of Oceanography studied cloud data of the northeast Pacific Ocean both from satellites and from the human eye over the past 50 years and combined that with climate models. They found that low-level clouds tend to dissipate as the ocean warms which means a warmer world could well have less cloud cover. "That would create positive feedback, a reinforcing cycle that continues to warm the climate," says Amy Clement, a climate scientist at the University of Miami and the lead author of the Science study.
Getting data on cloud cover isn't easy. There is reliable information from satellites, but those only go back a few decades not long enough to provide a reliable forecast for the future. Clement and her colleagues combined recent satellite data with human observations literally, from sailors scanning the sky that go back to 1952, and found the two sets were surprisingly in sync. "It's pretty remarkable," says Clement. "We were almost shocked by the degree of concordance."
The data showed that as the Pacific Ocean has warmed over the past several decades part of the gradual process of global warming low-level cloud cover has lessened. That might be due to the fact that as the earth's surface warms, the atmosphere becomes more unstable and draws up water vapor from low altitudes to form deep clouds high in the sky. (Those types of high-altitude clouds don't have the same cooling effect.) The Science study also found that as the oceans warmed, the trade winds the easterly surface winds that blow near the equator weakened, which further dissipated the low clouds.
The question now is whether this process will continue in the future, as the world keeps warming. Scientists create climate models to try to predict how the earth will respond to higher levels of greenhouse-gas emissions, but only one model created by the Hadley Centre in Britain includes the possible impact of changing cloud behavior. And the bad news is that the Hadley model contains particularly high temperature increases for the 21st century, in part because it sees dissipating cloud cover as a positive-feedback cycle meaning the warmer it gets, the less cloud cover there will be, which will further warm the earth. Though it's just one data set over one part of the earth's surface, the Science study indicates that the pessimistic Hadley model may be right. "These low clouds are like the mirrors of the climate system," says Clement. "If they disappear, you might see that positive-feedback cycle."
Cloud cover is only one element of climate sensitivity. Scientists are also concerned about the earth's ice, which reflects sunlight back into space, making it a cooling factor, while seawater absorbs the sun's heat. That means that as polar sea ice melts because of warming, leaving more open water, the warming process could accelerate which would then melt more ice. There are also concerns that as the permafrost in the Arctic thaws, it could release massive amounts of methane, a powerful greenhouse gas that would further accelerate warming.
And then there's the Amazon. Right now, the rain forest is a huge carbon sink, which compensates for the greenhouse gases we release by burning fossil fuels. But if the climate warms so much that the rain forest begins to die off a distinct possibility we'll lose that carbon sink, and then warming will again accelerate. Scientists, including the authors of the Science study, are still trying to nail down exactly where these tipping points might be but it seems that the more we find out, the more the evidence points to an increasingly sensitive climate. And that's bad news for us.