Despite heat, scientists predict new ice age

But global warming could delay 10,000-year cycle of cooling

October 06, 2002|By Faye Flam | Faye Flam,KNIGHT RIDDER/TRIBUNE

PHILADELPHIA - It may be hot now, but it's never too early to start thinking about the next ice age.

Based on Earth's historical cycle of warm and cold periods, we're due for a big freeze any millennium now. If the next cold spell is like the last one, which ended 10,000 years ago, glaciers would cover much of North America, creeping as far south as New York City.

Over the whole planet, ice ages reduce temperatures by only about 5 to 9 degrees, but the chill is more pronounced in temperate zones - such as most of the United States.

"If you were living in Philadelphia, you could have taken a day trip to see the ice sheet," said Duke University climatologist Tom Crowley. A 50-foot thick glacier covered Long Island back then.

But there's the possibility that global warming could delay the onset of the next big freeze by thousands of years, according to Belgian researchers, writing in the journal Science.

"We've shown that the input of greenhouse gas could have an impact on the climate 50,000 years in the future," said Marie-France Loutre of the Universite Catholique de Louvain in Belgium.

Ice ages and warmer "interglacials" alternate in cycles. In the past few cycles, the relatively warm "interglacials" lasted about 10,000 years. Since the current interglacial started about 10,000 years ago, it's due to end any time now.

The ice ages last much longer - 80,000 to 100,000 years.

But factoring in the higher concentration of carbon dioxide in the atmosphere, Loutre and colleague Andre Barger found the deep freeze might not come for a few more tens of thousands of years. The increase in carbon dioxide, many scientists believe, has come primarily from the increased burning of fossil fuels, such as coal, oil and gas.

Different time scales

Scientists don't normally connect global warming with ice ages since they happen on very different time scales - decades for global warming, compared with tens of thousands of years for ice ages, said Princeton climatologist Jorge Sarmiento.

Still, he said, "it's not an unreasonable idea. ... It's something I've contemplated."

His own work, he said, has backed Loutre's claim that today's increased carbon dioxide should linger for millennia - long enough to influence the far-future forecast.

Some of the extra carbon will dissolve in the oceans, he said, "but that takes a long time."

This summer not withstanding, we're in a relatively cold period compared with most of the planet's history. When dinosaurs roamed New Jersey, 100 million years ago, it was about 15 degrees warmer than today, with little ice anywhere. But it's considerably warmer now than during the last ice age.

Very long-term changes happen, in part, because the continents are slowly drifting around. When there's a lot of land near the poles, ice accumulates there, and that reflects sunlight and generally cools the planet. Earth started getting cold over the last 4 million years, accumulating permanent ice over much of the Arctic and Antarctica.

But the geologic record shows that the ice has retreated every 100,000 years or so for the last several million years, each cycle giving us warmer interglacial periods that last about 10,000 years.

Why this cycle repeats is not known, but the prevailing theory attributes it to the elliptical nature of Earth's orbit and a slight wobble in its tilt on its axis.

Seasons occur because Earth's tilt gives one hemisphere much more sun exposure than the other at any given time of year. But we also feel a small influence from changes in distance from the sun, which varies from 91.5 million miles away to 94.5 million.

Earth's orbit actually gets more elliptical, making the seasonal variations more extreme. Right now the orbit is relatively round. Currently, Earth is closest to the sun in the northern hemisphere's winter, making the winters milder and summers cooler.

Effects of snow

Most of Earth's land is now in the northern hemisphere, so this situation means more snow can stay on the ground all summer, reflecting more sunlight away and thereby pulling the temperature further down, which encourages still more snow and more cooling. That could start to pull us back into the next ice age.

Without human influence, the cycle is likely to repeat. But now the total concentration of carbon dioxide in the atmosphere is more than 30 percent higher than it was at the beginning of the century, and temperatures are rising.

"The warming will certainly launch us into a new interval in terms of climate, far outside what we've seen before," said Crowley. He said it's a big enough influence to cause the cycle of ice ages to "skip a beat."

Loutre and Berger estimated that human activity will double the concentration of carbon dioxide in the atmosphere over the next century, raising temperatures as much as 10 degrees Fahrenheit. Still, "it could get much worse," said Crowley. There's a huge reservoir of coal and if people keep burning it, they could more than quadruple the present carbon dioxide concentrations, he said.

"I find it hard to believe we will restrain ourselves," he said. "It's really rather startling the changes that people will probably see."

Despite the temptation to blame this summer's heat on the carbon-dioxide buildup, Princeton's Sarmiento said it's impossible to attribute a single summer's heat spell to that. The seasonal temperatures in different places normally vary from year to year, while global warming is a long-term trend.

The ice ages, on the other hand, come over a much longer time scale.

"The silliest thing people could say is: We've got an ice age coming, so why are we worrying about global warming?" Sarmiento said. Whether Loutre and Berger's theory is right or not, "We're going to get a lot of global warming before the ice age kicks in."

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