Tomorrow's weather: in the movie The Day After Tomorrow, twisters topple buildings, waves wash out cities, and snow blankets streets. Could rapid climate change really happen?

Thunderous tornadoes demolish Los Angeles. Bowling-ball size hailstones rain down on Tokyo. Why the sudden onslaught of deadly weather? Global warming has triggered a series of natural disasters--including a monster storm that's racing south from the Arctic. And when the behemoth hits in 96 hours, beware: It will turn New York City into a frozen wasteland.

Panicked? Don't be--this is just the plot of an upcoming flick, The Day After Tomorrow, to be released next month by Twentieth Century Fox. Even though scenes from the fast-paced blockbuster are more Hollywood fiction than science fact, researchers have long been sounding a similar alarm: Earth has been gradually warming for decades.

Could global warming (an average increase in Earth's temperature) really turn Earth's climate (weather conditions of an area) upside down as it did in The Day After Tomorrow? According to Michael Molitor, an earth-systems scientist and senior technical adviser for the movie, "The basic idea of the film--that human activity leads to an abrupt climate change event with nasty consequences--is based on robust science. Where the film is less than accurate is the speed at which these changes would occur." Rather than battling surprise storms or waking up to an ice age, we'd more likely feel changes over a decade.

TOO MUCH OF A GOOD THING

On the silver screen and in real life, global warming is the villain. What's to blame for this rise in mercury? An out-of-whack greenhouse effect (see "Threats From Climate Change"). After sunlight hits Earth's surface, waves of infrared radiation, or heat energy, radiate back up toward the atmosphere. But this energy doesn't make a quick getaway into space. Instead, it collides with molecules of greenhouse gases such as water vapor, carbon dioxide, methane, and ozone. Some of the heat energy radiates back toward Earth, and some radiates higher into the atmosphere, eventually escaping into space. The energy that returns to Earth heats the planet's surface and gets recycled over and over before finally escaping into space. Do we need the greenhouse effect? You bet your thermal pj's we do. Without it, Earth's average temperature would be a nose-numbing 60 degrees Fahrenheit colder.

The trouble starts when unusually large amounts of greenhouse gases build up in the atmosphere. Then the heat cranks way up. Human activities--such as the burning of fossil fuels, like coal and petroleum--release billions of tons of greenhouse gases into the atmosphere every year. And deforestation (clearing of forests) removes the trees that would otherwise absorb carbon dioxide. Most climate scientists agree: The accumulation of gases is causing warming on a global scale.

In The Day After Tomorrow, rising global temperatures lead to a big freeze. How? The key is a natural ocean pump most people take for granted.

GO WITH THE FLOW

The North Atlantic Ocean would be a lot colder if it weren't for the great ocean conveyor bell, an ocean-current system that slowly wends its way around the globe. It carries warm water to the North Atlantic. As this water reaches the Arctic, it cools and increases in density--its molecules pack together so it has more mass, or amount of matter per unit volume. The surface saltwater freezes. And because salt can't fit into the ice crystals' structure, it gets released into the water below the icy surface. This increase in salinity (saltiness) makes the water even denser. The dense saltwater sinks deep below the ocean surface and flows southward. Warmer water flows north to replace it and the cycle begins again. This movement of the ocean conveyor is called thermohaline circulation, because the water's density depends on temperature (therm, Greek for heat) and salinity (hal, Greek for salt).

Scientists fear global warming could weaken or shut down the ocean's conveyor. "Melting glaciers would put more freshwater into the higher latitudes of the North Atlantic. When the water is fresher, it's less dense and doesn't have as much salt in it, and it doesn't sink," explains Warren Washington, senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. That means warmer water doesn't flow north to replace it.

If the great conveyor were to shut down, scientists say the temperature drop could create abrupt climate change. In the movie, that change takes place in a few days. Could it happen that suddenly?

HOW FAST IS FAST?

Research shows abrupt climate changes in the distant past. But Inez Fung, director of the Berkeley Atmospheric Science Center, reminds us, "In geologic time, 'abrupt' could mean decades to centuries." The ocean holds so much heat that a shutdown of the conveyor wouldn't cause an instant temperature drop.

As for whether or not a shutdown could trigger the weather catastrophes that flash across the screen in The Day After Tomorrow, Molitor says, "Changes in thermohaline circulation would lead to changes in the climate regime globally. You'd see effects everywhere, but we don't know how big, or how bad they would be."

REAL-LIFE DRAMA

Climate change might not come in the form of mammoth storms and widespread destruction. Even so, the changes could alter your way of life. Residents of the island village of Shishmaref, Alaska, know firsthand. Over the past century, the average global temperature has risen one degree Fahrenheit. But in Alaska, the mercury has soared four degrees. That might not sound like much, but it's caused a 4- to 10-inch rise in sea levels. That's because as seawater warms, it expands. Water from melting glaciers also adds to the ocean's volume.

The upshot? Shishmaref is eroding into the sea. Rising sea levels, melting sea ice, and warmer air that holds more moisture are a recipe for brutal sea storms that have already washed away chunks of shoreline. Residents hope their island village can hang on until they find funds to relocate. But that's not their only problem. The native population's way of life depends on the sea freezing over at the right time. With worming temperatures, "we don't freeze up until about two months later now," says Vice Mayor Stanley Tocktoo. "By that time, the fish that we subsist on are way up in the river and we're not able to catch many."

IS IT TOO LATE?

Even if humans were to stop releasing additional greenhouse gases, global warming would continue while the oceans slowly respond to the buildup of gases already in the atmosphere. But Washington believes action now could help avert future disasters, such as coastlines lost to rising sea levels and extreme shrinkage of the polar ice caps (see "Threats From Climate Change").

What can you do to curb global warming? Since most of our home energy is generated by power plants that burn fossil fuels, we can cut down on our share of greenhouse-gas emissions with simple energy-saving measures. Turn off the TV, computer, and other electrical devices when you're not using them. And if you plug these devices into a surge protector strip, make sure to switch it off too. Otherwise, the vampire-like gadgets continue to suck electricity from the lines. And flick out the lights when you leave a room. You can even help slow global warming by "taking shorter showers. Remember: it takes energy to heat water. "We all have an individual responsibility to deal with this problem [of global warming]," says Washington.

THREATS FROM CLIMATE CHANGE

As humans burn more and more fossil fuels, the amount of greenhouse gases--such as carbon dioxide--in the atmosphere rises. Most scientists agree: The added C[O.sub.2] exaggerates the greenhouse effects, leading to a warmer world--and potentially big changes for the planet.

GREENHOUSE EFFECT

The sun's rays pass through the atmosphere, hitting Earth's surface. The planet radiates the heat as infrared rays. Some of this heat energy escapes into space. The rest is trapped by gases in the atmosphere only to heat Earth's surface again.

RISING SEA LEVELS

As Earth heats, glaciers melt and seawater warms and expands. Result: Sea levels have already risen by 10 to 25 cm (4 to 10 in.). Scientists fear that's just the beginning.

SHRINKING ARCTIC ICE CAP

Like an ice cube melting in the summer heat, Earth's polar ice caps have begun to shrink. Researchers predict more dramatic melting in the future.

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Did You Know?

* According to the U.S. Environmental Protection Agency (EPA), each American is responsible for emitting 6.6 tons of greenhouse gases each year. There was a 3 percent increase in U.S. greenhouse-gas emissions between 1990 and 1997.

* Carbon dioxide dissolves more easily in cold water than warm water. With rising ocean temperatures, this could mean an increase in the release of this greenhouse gas into the atmosphere.

* As temperatures rise on Earth, wildlife--such as bird species--might have to relocate to cooler locations found closer to the poles.

Resources

Find out how you can reduce the emissions of carbon dioxide, a greenhouse gas: www.greenhousenet.org/action/20steps.html

Visit this EPA kids' Web site for great facts and games on global warming: www.epa.gov/globalwarming/kids/index.html

Learn more about global warming and the ocean conveyor belt with "Tiki the Penguin": www.oneworld.net/penguin/features/global_warming.html