Unlocking secrets of the Ice

John Overpeck's job is like putting together a huge, complicated jigsaw puzzle. He may find one piece on the bottom of an African lake and another one deep in an Arctic glacier. The puzzle he's working on is a picture of what the Earth's climate was like thousands of years ago. Dr. Overpeck is a paleoclimatologist (PAY-lee-oh-kly-muh-tohl-uh-jist). "Paleo" means "ancient" in Greek. He studies ancient climates.

Why do scientists want to know what the weather was like 1,000 years ago? Weather patterns have a big impact on people. They determine where we live and how our food grows. Scientific measurements suggest that the average temperature of Earth's atmosphere has risen by 0.5 degrees Centigrade (0.9 degrees Fahrenheit) since 1860. That's a tiny but significant amount. Most scientists now think that human activity has contributed to "global warming." But they can't say for sure until they know what ancient weather patterns were like and how they changed.

That's where Overpeck comes in. He's the director of the Institute for the Study of Planet Earth at the University of Arizona at Tucson.

"Climate" and "weather" are different. Weather changes quickly. Thunderstorms come and go, heat waves break, cloudy days clear up. Climate is the "normal" weather we expect over a longer period of time - inches of precipitation per month, average summer temperature for the past 10 years, number of floods per century.

Weather records date back only 140 years in this country. So, how can scientists find out what Earth's climate was like in the days before thermometers, rain gauges, barometers, and anemometers (wind-speed indicators)?

Paleoclimatologists look at substitute, or "proxy" records. These records include the width of growth rings in trees, how thick the layers are in an ice-core sample, the composition of coral, the presence of pollen grains, the thickness of the sediment layers in ocean and lake beds, even old historical records.

"None of the proxies is perfect," Overpeck says. "That's why we want multiple records, so that we can compare them and look for common signals."

Overpeck might look at samples from a lake bed in Ghana. He knows that the width of the bands of sediment that accumulate each year depends on how much dust blows into the lake. Dry years mean more dust and thicker layers. Wet years produce thinner layers. He might also look at the pollen preserved in the samples, to find out what plants were growing in the area. Plants are clues to climate.

Diaries, farmers' planting records, and newspaper accounts might also contain hints about the weather. The writers may not know what the temperature was. But by knowing when crops were planted or harvested, or if the year was wet or dry, scientists can make assumptions about the climate.

By studying ocean coral, scientists learn about the temperature of the water in which it grew. (They look at the oxygen content in the coral's outer shell, which is made of calcium carbonate, a compound of calcium, carbon, and oxygen.)

Every flowering plant produces pollen, and the pollen of each species is different. Scientists study pollen grains preserved in layers of sediments that have settled on the bottoms of ponds, lakes, and oceans. By figuring out what kinds of plants were growing nearby, scientists can speculate about the climate needed to allow those plants to grow.

Ice cores are collected from the polar ice caps or from glaciers high in the mountains where snow has built up over many centuries. Scientists drill deep into the ice - thousands of feet, sometimes - to collect ice cores. The ice contains dust, air bubbles, and other things that can be used to find out what weather patterns were like. A core might even contain layers of ash from volcanic eruptions.

Scientists estimate that between 6 billion and 11 billion metric tons of sediment are washed into oceans and lakes each year. Core samples of ocean and lake beds may contain chemicals or tiny fossils that hint at the climates of the past.

Overpeck works mostly with lake-bed and ocean sediments. There's a trick to getting a good sample, he says. You have to freeze them to minus-78 degrees Centigrade (-109 degrees F.). Frozen cores don't fall apart as easily, and the layers of sediments are preserved.

To do this, dry ice is packed around the core pipe. "When you pull it out about 20 minutes later," Overpeck says, "it's like a popsicle, and everything is preserved."

Earth's climate has changed before. Huge glaciers have advanced and retreated across the Northern Hemisphere in the last million years. These ice ages lasted thousands of years.

The Medieval Warm Period, from AD 800 to 1300, was once thought to be another example of global warming. But research now suggests that the warmer temperatures in Europe were caused by changes in the Gulf Stream. Another warm period 6,000 years ago has been linked to changes in the Earth's orbit.

Proxy data from paleoclimatologists may tell us if we are in the midst of a natural climate change. These measurements may also tell us what effect humanity has had on Earth's climate.

"Our job is to try and pull it all together and answer the big questions," Overpeck says. "What is the role of the sun in climate change? What role do big volcanic eruptions play? Can natural causes account for global warming?"

Whatever the reason, a better understanding of climate change will help us prepare for the future by knowing what kind of weather to expect.

(c) Copyright 2000. The Christian Science Publishing Society

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