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?

Science World, April 5, 2004 by Jacqueline Adams

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."