Most Europeans have probably never read Schar’s report (not least because it was published in the scientific journal Nature in the dead of winter) but they seem to be bracing themselves for the worst. As part of its new national “heat-wave plan,” France issued a level-three alert when temperatures in Provence reached 34 degrees Celsius three days in a row; hospital and rescue workers were asked to prepare for an influx of patients. Italian government officials have proposed creating a national registry of people over 65 so they can be herded into air-conditioned supermarkets in the event of another heat wave. London’s mayor has offered a 100,000 pound reward for anybody who can come up with a practical way of cooling the city’s underground trains, where temperatures have lately reached nearly 40 degrees Celsius. (The money hasn’t been claimed.) Global warming seems to have permanently entered the European psyche.
If the public is more aware, though, experts are more confused. When the Intergovernmental Panel on Climate Change hammered out its last assessment in 2001, scientists pulled together the latest research and made their best estimate of how much the Earth’s atmosphere would warm during the next century. There was a lot they didn’t know, but they were confident they’d be able to plug the gaps in time for the next report, due out in 2007. When they explored the fundamental physics and chemistry of the atmosphere, though, they found something unexpected: the way the atmosphere–and, in particular, clouds–respond to increasing levels of carbon is far more complex and difficult to predict than they had expected. “We thought we’d reduce the uncertainty, but that hasn’t happened,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research and a lead author of the next IPCC report. “As we delve further and further into the science and [gain] a better understanding of the true complexity [of the atmosphere], the uncertainties have gotten deeper.”
This doesn’t mean, of course, that the world isn’t warming. Only the biased or the deluded deny that temperatures have risen, and that human activity has something to do with it. The big question that scientists have struggled with is how much warming will occur over the next century? With so much still unknown in the climate equation, there’s no way of telling whether warnings of catastrophe are overblown or if things are even more dire than we thought.
Why do scientists like Schar make predictions? Because, like economists, it’s their job to hazard a best guess with the resources at hand–namely, vast computer programs that simulate what the Earth’s atmosphere will do in certain circumstances (such as if we continue to burn fossil fuels at the present rate). These models incorporate all the latest research into how the Earth’s atmosphere behaves. But there are problems with the computer models. The atmosphere is very big, but also consists of a multitude of tiny interactions among particles of dust, soot, cloud droplets and trace gases that cannot be safely ignored. Current models don’t have nearly the resolution they need to capture what goes on at such small scales.
Scientists got an inkling that something was missing from the models in the early 1990s when they ran a peculiar experiment. They had the leading models simulate warming over the next century and got a similar answer from each. Then they ran the models again–this time accounting for what was then known about cloud physics. The answers varied so widely as to be nonsensical. By the time the IPCC issued its 2001 report, the range had been narrowed to seven degrees, and now stands at about three (from 1.5 degree Celsius to 4.5 degrees Celsius ). The trouble is, this consensus may be false. Says Bill Collins, an NCAR climate scientist: “Whether or not we’re converging on the truth, we don’t know.”
The more scientists look into the problem, the more they realize how central clouds are to climate change–and how little they understand them. The dynamics of clouds–how they form and how they dissipate, and what happens in between–may be one of the toughest problems in physics. Clouds embody the complexities of air turbulence, which to some degree still mystifies engineers, and adds to it: water is changing from vapor to cloud droplets and back again, gathering in and releasing heat all the while. And then aerosols–particles like soot and dust–interact in complicated ways to change the way clouds behave.
Andrew Heymsfield spends about half his time in his lab at the NCAR, and the other half in the field, observing clouds. He once spent a summer flying in a NASA airplane less than 50 meters over the Indian Ocean, measuring the amount of water vapor rising above the surface and the air pollution wafting over from the Subcontinent. Then he’d fly at an altitude of 15 to 20 kilometers, where thunderheads end in thin wisps, and shoot out probes to sample the ice particles. Heymsfield and his colleagues are trying to bridge the gap between clouds and the computer models by a kind of brute-force taking of measurements. “Clouds could end up having a big effect on the climate models,” he says.
The idea is to gather a lot of data in the field, then come back to the lab and tweak the computer models. What he’s found is that aerosols from the smokestacks and tailpipes of southern India are changing the clouds that form over the Indian Ocean–with potentially dire effects on local weather patterns. When soot particles drift over the ocean, they attract water vapor, which stimulates the formation of clouds. The increased cloud cover reflects sunlight back up into outer space, cooling the region. That might seem like a fine thing to happen, except for one drawback: because these soot-induced clouds have smaller droplets than they normally would, they’re less apt to produce monsoon rains. Greater cloud cover and less rain has already had a detrimental impact on Indian agriculture. And the findings underscore the need to take regional dynamics into account.
Scientists began reporting a few years ago that aerosols cool the atmosphere, but recently there’s been equally convincing evidence that they can have the opposite effect. James Hanson, a climate scientist at NASA’s Goddard Space Laboratory in New York, found that soot particles embedded in snow absorb the sun’s energy and radiate heat, which may explain why temperatures have risen more at the poles than at the equator. Other scientists have found that soot in the atmosphere can either absorb heat or reflect sunlight, depending on the shape of the particles. “If I take a pound of coal,” says Collins, “I can’t tell you how much soot it will release into the atmosphere without knowing how you’re going to burn it.”
Then there’s the whole question of how clouds are going to react to atmospheric warming. Warmer air can absorb more water vapor, a more powerful greenhouse gas than carbon dioxide. Scientists had assumed that higher humidity levels would further warm the climate, but two years ago Bruce Wielicki from NASA’s Langley Research Center in Hampton, Virginia, using satellite data, found that fewer clouds were actually forming over the tropics, and humidity was declining. Wielicki is still assessing the implications for warming.
Scientists are optimistic that they’ll eventually crack the cloud problem, but “it may take seven years,” says Collins. That would be another seven years of political hot air and few facts.
