As climatologists peer into the future to see whether global warming will turn the Earth into a hothouse, as some predict, they have a problem.
Certainly, they know that people's addiction to gas-powered vehicles and the clearing of forests is pumping unprecedented amounts of carbon dioxide into the atmosphere. Certainly, they agree that CO2 is a potent "greenhouse gas" -- so named because it traps heat like the glass of a greenhouse. And certainly, a warmer atmosphere will hold more water, and water vapor makes a more potent heat trap than even CO2.
But when it comes to predicting whether the twin influences of CO2 and water would actually culminate in a global climate catastrophe, the climatologists are less sure. It turns out that a single pivotal unknown in the climate equation could spell the difference between an Earth that heats up like the inside of a car on a hot day, an ice age, and the temperate Earth we enjoy now.
That missing ingredient is clouds.
Clouds "are the biggest unsolved problem that we have to deal with," says Tom Ackerman, a meteorologist at Penn State University.
Only recently have climate modelers even begun to factor clouds into their predictions, and only primitively. The critical question is whether a warmer Earth would produce clouds that act like parasols, shading the earth's surface from the sun's rays, or that act like greenhouse glass, trapping heat inside. For now, researchers are unsure.
This enormous uncertainty over something so basic leads many climatologists to approach long-term forecasting with "a certain amount of trepidation," says Gerald Stokes, the chief scientist in the Department of Energy's Atmospheric Radiation Measurement program.
"Our climate models could have the intellectual equivalent of the Challenger disaster built into them," he says. "All of a sudden, you realize you've forgotten something fundamental."
What's at stake is nothing less than future global climate. The worst-case scenarios predict temperatures high enough to melt icebergs, flood coastal cities and turn farmlands into deserts.
No one expects pinning down clouds to be easy. "You can't catch a cloud and measure what you want to know," says Ackerman, an adviser to the Atmospheric Radiation Measurement program. "They keep changing." Normally, scientists can learn by doing experiments under controlled conditions. "But you just can't make a good cloud in a laboratory."
The program is part of a $40 million-a-year effort to get a handle on clouds.
It involves 50 teams of scientists in more than 70 institutions, doing everything from computer simulations to satellite observation. They have installed cloud sensors in remote
outposts from the Oklahoma plains to the tropical Pacific, and flown "cloud sandwiches" in which planes fly above and below a single cloud to study how much of the sun's energy seeps through.
In addition, the program is tracking other critical ingredients in the global climate recipe that interact with clouds in unknown ways, and may produce enormous effects on their own.
Steven Schwartz and several of his colleagues at the Brookhaven National Laboratory recently published research showing that the smog, dust and dirt put into the air, along with CO2, could create enough of a haze to eventually divert huge amounts of solar energy back into space -- enough energy, he says, to make the difference between a muggy greenhouse and a "whitehouse" that would stay relatively cool.
What intrigues scientists even more is that these airborne particles also make excellent nuclei for cloud droplets -- thereby increasing the number of clouds in the atmosphere. No one knows exactly what kinds of clouds would form in a warmer, smoggier world, however, and the kind of cloud can make a critical difference.
High cirrus clouds over the tropics tend to have a warming effect, while thick, low coastal clouds reflect the sun's energy back to space, cooling things down. "We'd like to know which of those effects wins the game," says climate modeler Jeff Keihl of the National Center for Atmospheric Research in Boulder, Colo.
Since the sun drives everything from global climate to local weather, the fundamental puzzle facing climatologists is "how much sunlight really falls on our heads," says atmospheric scientist V. Ramanathan of the Scripps Institute of Oceanography in San Diego. But just as different kinds of clouds have opposite effects, the same cloud can have different effects different times of day.
At night, the Earth reradiates some of the heat it has soaked up from the sun during the day. If the night is clear, the sky is an open window, letting the heat escape; but cloud cover can act like a blanket holding the heat in.
"To understand what's going to happen in the future, you need to understand why real clouds act the way they do," says global climate expert Richard Somerville of the Scripps Institute of Oceanography in San Diego, "and that's the kind of thing we don't know yet."
Satellite measurements in the late 1980s established that overall, clouds appear to have a net cooling effect on Earth.
There may also be a third role for clouds. In addition to keeping out or trapping heat, clouds appear to absorb solar heat like a sponge. Sets of planes flying in the cloud sandwich formations found that clouds might retain as much as 8 percent of all the sunlight falling on Earth.
A much tougher question: Why is a cloud there in the first place?
As long as it is impossible to produce clouds in the laboratory, climatologists will turn to computer models to see how various combinations of clouds might affect the Earth in a hypothetically warmer climate. To make the right prediction, however, the model "has to put the right kind of cloud in the right place at the right time for the right reasons," says Stokes. And for now no one knows how to do that.
Even if the physicists had clouds pinned down, the computer modelers couldn't necessarily deal with them well enough to create virtual climates that reflect the real future.
That's because a single cloud can be complicated enough to fill up a whole computer. "We're just starting to understand the data," Ackerman says.
With so many unknowns, researchers are eager to get their hands on the data coming in from projects such as the Atmospheric Radiation Measurement program.
Meanwhile, though, they aren't recommending that policy-makers do nothing to stop the influx of greenhouse gases. Even without certain predictions, precautions should be taken, because greenhouse gases put into the atmosphere today will hang around for a long time, maybe hundreds of years.
And once the gases are there, says Schwartz, "there's no way to get them out."
The enormous increase in atmospheric CO2 since the start of the Industrial Revolution, he says, "is a major red flag in terms of whether we should just proceed as in the past."
Pub Date: 11/30/96