Global warming: cloud patrol. (Earth Science: Global Warning * Clouds).

Maybe you never give them a thought, but without clouds almost all life couldn't exist. On any given day, clouds--made of tiny water droplets or ice crystals--cover nearly one third of Earth's surface. Their job: to reflect the sun's life-sustaining warmth. So how do clouds impact global warming? Scientists are trying to find out.

Earth is a natural greenhouse. The sun's rays (solar radiation) penetrate the atmosphere and warm the planet surface. Heat from the surface radiates back into the atmosphere.

The climate where you live depends on the balance between energy from the sun and heat radiating away from the planet. If the atmosphere absorbs a greater amount of heat--by the addition of a heat-trapping gas like carbon dioxide--global warming results. Clouds impact climate in two ways: They can reflect solar radiation back into space before it reaches Earth's surface. Or, like a blanket, clouds can trap heat near the ground, which warms the planet. "Clouds both cool and heat the climate system," says atmospheric scientist Thomas Ackerman of the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Program. Ackerman's mission: to sleuth out clouds' impact on rising global temperatures.

Scientists don't have a complete answer yet, but they do know that clouds are an important piece in the intricate global warming puzzle.

DELICATE BALANCE

By both reflecting and insulating heat, clouds play a key role in the delicate energy balance. Enter human activity, like the burning of fossil fuels--such as coal and natural gas. made from decaying plants and animals--to run factories and cars. A downside of fossil fuels is that they spew greenhouse gases (carbon dioxide, methane, nitrous oxide, for example) into the atmosphere. And greenhouse gases absorb heat energy.

But of all greenhouse gases, the most abundant is water vapor. It forms naturally when water in oceans, lakes, and even puddles, heats up and evaporates (changes from liquid to gas). Clouds form when water vapor rises into the sky. High up, frigid air condenses the gas molecules into visible water droplets clustered around dust and debris. "More water vapor means more opportunity to form clouds," says physicist Timothy Schneider at the Environmental Technology Laboratory in Boulder, Colo.

How could more cloud cover affect Earth's climate? If more wispy, high-flying cirrus clouds form (see diagram), scientists suspect that Earth's temperature could sky-rocket--cirrus clouds let sunlight pass through them but trap Earth's heat from escaping back to space. An increase in low, reflective stratus clouds, on the other hand, might cool the planet by preventing sunlight from reaching Earth. "The models we use to predict climate suggest that if the planet warms, more high cirrus clouds would form," Ackerman says. That in turn would further increase Earth's temperature and create even more clouds.

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But Ackerman cautions, "We still have a lot questions." Clouds' fleeting nature continues to challenge researchers. "We need to get inside clouds to measure temperature and humidity," says Schneider with a sigh. "But sometimes as we fly toward a cloud, it will literally disappear before our eyes."

THINK ABOUT IT

Why in winter are cloudy days warmer than clear days? Why are clear nights cooler than cloudy nights?

Feeling the Heat

U. S. President George Bush recently sparked global controversy when he rejected the 1997 Kyoto Protocol, an international treaty that aims to reduce heat-trapping gas emissions to pre-1990 levels. Why did he reject the treaty? Bush claims it would hinder industrial production and growth, and prove economically disastrous for the U.S. What do you think?

CIRROCUMULUS ABOVE 18,000 FEET High, icy clouds wit grainy appearance

CUMULONIMBUS FROM NEAR GROUND TO ABOVE 50,000 FEET These towering rain clouds have a very short lifespan.

ALTOSTRATUS 6,000 TO 20,000 FEET A vast grayish sheet of clouds

CUMULUS BELOW 6,000 FEET These puffs come and go quickly, so they have little effect on the energy balance.

CIRRUS CLOUDS ABOVE 20,000 FEET Solar radiation passes through these wisps of ice.

ALTOCUMULUS 6,000 TO 20,000 FEET These form textured clusters.

STRATOCUMULUS BELOW 6,000 FEET Cumulus clouds spread into low, lumpy layers.

STRATUS BELOW 6,000 FEET These gray clouds can sprawl over entire regions. They reflect the sun's rays.

HANDS-ON SCIENCE

Cloud in a Jar

Air cools as it rises in the troposphere, the layer of atmosphere closest to Earth. When water vapor in the air cools, it condenses (forms tiny droplets of liquid water) and clouds form. See how it happens!

YOU NEED: large empty jar * I cup very hot water * metal lid or pie pan to cover the jar * empty can * ice cubes * flashlight * watch or stopwatch * paper * pen

TO DO:

1. Note the time or start the stopwatch. Carefully pour half the hot water into the jar and cover the jar to trap rising steam.

2. Place the can atop the jar and fill it with ice cubes. Record your observations.

3. Dim the room lights, place your flashlight near the jar, and shine light into the cloud to depict the sun.

4. Without opening the jar, look at the underside of the lid. Record your observations.

CONCLUSIONS:

Why does a cloud form? What happens to the cloud when you shine a light through it? How long does the cloud last? How long does it take for rain to start falling in the jar?

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Cross-Curricular Connection

English: Look at pictures of clouds, then use figurative language (similes and metaphors) to describe each cloud.

Geography: Research which parts of the world tend to be cloudiest and which parts have mostly clear skies.

Did You Know?

* Why is it difficult to collect cloud samples? For one thing, they can blow in the wind at 40 meters per second.

* Cirrus clouds are like glass windows. Sunlight passes through them, but infrared radiation can't get out. Stratus clouds are like window blinds. They don't let anything in or out.

* Clouds are ephemeral. As the sun comes up, it heats the cloud layer in mid-atmosphere. As air warms, it can hold more water vapor. As a cloud traps energy and warms up, the cloud can evaporate into thin air.

National Science Education Standards

Grades 5-8: structure of the earth system * transfer of energy, populations, resources, and environments * risks and benefits * science and technology in society

Grades 9-12: energy in the earth system * interactions of energy and matter * environmental quality * natural and human-induced hazards * science and technology in local, national, and global challenges

Resources

Greenhouse: The 200-Year Story of Global Warming by Gale E. Christianson, Penguin Books, 1999

USA Today Weather Basics: www.usatoday.com/weather/wcloud0.htm

"Both Sides Now: New Way That Clouds May Cool" by Andrew C. Revkin, The New York Times, June 19, 2001

CHECK FOR UNDERSTANDING

Global Warming: Cloud Patrol

Directions: Match the word(s) in the left column with the correct
phrase on the right.

--1. Kyoto Protocol    a. when the atmosphere absorbs more heat than
                          it releases
--2. Solar radiation   b. layer of atmosphere closest to Earth
--3. Global warming    c. wispy clouds that let sunlight pass through
--4. Fossil fuels      d. most abundant greenhouse gas
--5. Water vapor       e. international treaty, aiming to reduce
                          heat-trapping gas emissions
--6. Cirrus clouds     f. sun's rays
--7. Troposphere       g. resources made from decaying plant and animal
                          matter
--8. Stratus clouds    h. low, gray clouds that reflect solar radiation

ANSWERS

1. e 2. f 3. a 4. g 5. d 6. c 7. b 8. h