Cloudy Clues
With heads in the clouds and hands on their computers,
scientists are trying to predict Earth's climate.
by Chana Freiman, Scholastic Science World
When atmospheric physicist Anthony DelGenio looks at clouds, he isn't planning today's
weather report, or even the five-day forecast. He studies clouds to predict Earth's climate.
Clouds play a powerful role in warming and cooling our planet, DelGenio explains.
"Even a one percent change in cloud cover can have dramatic effects on global
temperature," he says.
In the next century, a slight increase or decrease in clouds could determine whether or not
your hometown will be a "hot" place to live. That's because clouds have a dual role in
balancing Earth's heat budget. On the one hand, clouds spread out like global umbrellas,
blocking the sun's heat energy and reflecting much of it back to space. That cools the
planet, says DelGenio.
But clouds can also act like giant, insulating blankets. The water droplets they're made of
absorb and trap heat radiating from Earth, which helps to keep the planet warm and
livable.
All life processes on Earth depend on a delicate balance between this warming and
cooling. If the balance shifts, the effects could be drastic. Colder winters or hotter
summers could threaten entire species of plants and animals, and force whole
populations–humans included–to move to different latitudes, says DelGenio.
He and other scientists are now studying clouds in an attempt to forecast such climatic
changes. Many of these scientists predict global warming, a rise in Earth's temperature
during the next century. They presume this warming will occur as levels of carbon
dioxide and other greenhouse gases spewed by the burning of fossil fuels continue to
build up in the atmosphere. When these gases accumulate, they trap heat, and cause
temperatures to rise above normal.
But a looming questions remains: If global warming does occur, how severe will it be?
No one yet knows, says DelGenio, and clouds are a puzzling variable in the equation.
What is known is how clouds form: Water vapor in warm air rises. As it gains altitude, it
cools and condenses on particles of dust. This condensation causes either water droplets
or ice crystals to form. When they crowd together, presto! A cloud.
Applying this recipe for cloud formation, scientists hope to find answers to these
questions: If Earth's oceans warm, as predicted, will more water evaporate, rise, and form
more clouds? If so, will an increase in cloud cover block more sunlight and offset global
warming? Or, will the added clouds blanket the Earth, trapping more heat and causing
temperature to rise even more?
These "unknowns" are the heart of DelGenio's studies. But finding answers raises another
problem: How on earth do scientists even study clouds?
Cloud Variety
Just by looking up in the sky, you can begin to grasp why clouds make such difficult "lab
specimens." For one thing, masses of clouds cover half the globe during any given
month. In addition, individual clouds differ widely in shape and size. Some stretch out
over the horizon and pile up 10 km thick. Others are fleeting wisps.
What's more, each cloud type reflects and absorbs the sun's energy in varying amounts.
For example, smooth sheets of stratus clouds, which bring light rain, tend to reflect more
sunlight than they absorb. Result: a cooling effect on the land below. Yet feathery, highaltitude cirrus clouds, composed of tiny ice crystals, allow more sunlight through,
thereby warming the Earth.
In addition, clouds can disappear as quickly — or slowly — as they form. "Some last 10
minutes, some 10 days," says atmospheric scientist Stephen Cox. "Some clouds last an
entire season."
Finally, consider this: To make effective cloud/climate predictions, scientists must look at
how the whole variety of clouds will perform in the future.
What tool can possibly take all these factors into account? Look no further than a
personal computer.
Clearing the Fog
Computers can store reams of climate data — past, present, and future — and solve
equations faster than any scientist, Cox says. Just as you might build a cotton-and-clay
diorama to illustrate the science of cloud formation, scientists construct their climate
models with computer programs.
Instead of using hobby shop materials, though, computer modelers simulate atmospheric
conditions using numbers. Each number represents a variable in the cloud/climate
equation – things like heat, air pressure, winds, and water vapor.
Next, the scientists write formulas to predict, for example, how increasing carbon dioxide
levels will affect these variables and therefore cloud formation, in the future. They then
plug the variables into the formulas, and wait while the computer performs all the
necessary calculations.
Soon the computer prints out its results, pages and pages, with rows of the predicted
numbers for each variable.
These numbers bear little resemblance to actual clouds. To make sense of this dizzying
data, climate modelers display it visually: in maps, on graphs, or as pictures of clouds,
says Cox. By identifying trends or patterns in those charts or pictures, scientists then try
to make their climate predictions.
For instance, one computer model might show that if carbon dioxide levels rise, cloud
cover will decrease by the middle of the next century. A scientist might then predict that
with fewer heat-reflecting clouds, more heat will reach the Earth, thereby worsening
global warming. Based on such predictions, nations might increase efforts to reduce their
use of fossil fuels.
Of course, computer modeling is not a perfect science. "There are hundreds of different
processes that the models simulate at once," says Cox. Scientists must often base their
formulas on estimates, which combined can oversimplify atmospheric conditions and
throw off climate predictions.
Under these circumstances, says DelGenio, there are no guarantees. "I think it's possible
that, before the decade is over, we will have the cloud problem sorted out." But, he adds,
"You can never know for sure if you've gotten your prediction right until it happens."