UCSC Review Winter 1995
Earth's Delicate Thermostat
The planet's ancient climate reveals some surprises--and perhaps a grim warning of things to come.
Imagine a world where crocodiles stomp through swamps in Wyoming, scattering flamingos as they prowl. Not far away, giant ferns and palms thrive in Montana and North Dakota. Lush forests grow north of the Arctic Circle in this world, and the ocean near Antarctica is fit for a dip at 60°F.
If earth's climate goes haywire from global warming, this hothouse could become more than just a dream. Scientists already know our planet has looked like this several times in the distant past. But these sultry eras have waxed and waned, often yielding to icy cold that has driven tropical animals and plants back toward the equator.
Indeed, like a feverish child who sweats and then shivers, earth swings erratically from warm temperatures to cold and back again. Some episodes last centuries or millennia; major warm or cold spells might linger millions of years. To their surprise, scientists have found that earth's average temperature also can change much more quicklyperhaps several degrees within a human lifetime. That doesn't sound like much, but it might be enough to wreak havoc on sea level, weather, and growing seasons.
Many researchers think we are on the brink of such a heat wave. As carbon dioxide and other industrial gases build up in the air, they may trap heat like the glass panes of a greenhouse. However, the causes of climate change are so mysterious that no one feels confident about predicting how toasty it could get. So, some scientists look to the past, reasoning that earth's ancient climate might contain lessons. Their burgeoning field is called "paleoclimatology."
James Zachos and Lisa Sloan of UCSC's Institute of Marine Sciences are leading practitioners of this science. Although they often work together, their approaches couldn't seem more different. Zachos, an assistant professor of earth sciences, studies sediment from the seafloor. Sloan, an assistant research scientist, relies upon not sediment but siliconin the chips of powerful supercomputers.
Zachos's work hinges on a curious fact: The shells of some marine animals act as thermometers, recording the temperature of the water in which they live. When the animals die, their shells sink, eventually making thick layers of sediment. "These sediments essentially represent pages of a history book for the oceans," Zachos says. "As you look deeper in the sediments, you move further back in time."
Scientists drill into the ocean floor to retrieve the sediments and the fossilized shells they contain. Careful analysis exposes both the age of the shells and slight chemical differences, which reveal whether the animals lived in warm or cold water. In this way, Zachos and others are creating temperature maps for the world's oceans at various times in the past.
Researchers have learned that during the last 100 million years, earth's climate often was much warmer than it is today. Several torrid intervals stand out, including the Cretaceous period of dinosaur fame and the early stages of an odd geologic epoch, the Eocene, about 55 million years ago.
"We were surprised to find that the planet's high latitudes, near the poles, were extremely warm during the early Eoceneabout 15°C [27°F] warmer than today," Zachos says. Those figures jibe with the crocodiles, palms, and other land fossils from the same era. In another surprise, scientists found that the warming may have happened rapidly on at least one occasion. Other scientists have shown that in the last 100,000 years, noticeable climate changes took decades or even less.
Further, Zachos demonstrated that the tropical oceans were not warmer during the early Eocene than they are today. That puzzled climate experts, for it seemed that increases in greenhouse gases from volcanic eruptions, for instanceshould heat the tropics as well.
Enter Sloan, who uses supercomputer models of the planet to study its climate. It may verge on hubris to believe that one can mimic earth's climate with a computer model, even more so to call the model "GENESIS." Nevertheless, GENESIS and Sloan may have solved the puzzle of the early Eocene.Developed at the National Center for Atmospheric Research in Boulder, Colorado, GENESIS contains simulated oceans, currents, winds, soils, plants, and other ingredients. Sloan asks the program to change a certain factor, such as the amount of carbon dioxide in the atmosphere. After scores of hours of computationfifteen "years" in the modelGENESIS forecasts how the climate might respond.
Sloan could not reproduce the Eocene's uneven temperature pattern simply by adding greenhouse gases. However, she found that a second change did the trick: a more vigorous flow of heat through the oceans, from the tropics toward the North and South Poles. When GENESIS considered both elements, it matched the Eocene temperatures all over the globe.
"I wouldn't ever call these models 'reality,' but they are becoming more complex, faster, and more realistic," says Sloan. "Plus, the new geologic records allow us to check our results." More work remains, she notes, to create models that properly account for "feedbacks" from clouds, ice caps, and other factors that could speed up or stifle a warming or cooling trend.
Researchers are inching closer to knowing what drives earth's climate, Sloan and Zachos believe. However, they emphasize that past climates aren't directly relevant to the threat of climate change today. The reason is clear: We're tampering with earth's thermostat to an amazing degree. Humans are pumping greenhouse gases into the air at a rate that may be unprecedented. So far, computer models and fossil records have yielded no clues about how the planet will react.
Even so, paleoclimatology makes one lesson quite clear. "The climate appears capable of changing rapidly, as if there are sudden jumps between threshold levels," says Zachos. "That's a bit worrisome. When climate change does come, and it almost certainly will, it may come very quickly."
Robert Irion