Sudden Climate Change: Just a movie plot?

 

1. Just another sci-fi flick?

2. Climatic roller-coaster

3. Furious feedback

4. Warming Southern Ice

Here's the ice-core stash at the National Ice Core Laboratory. Photo: NICL
Data from ice cores have overturned the illusion that climate always changes gradually.

At the height of the ice age, 18,000 years ago, ice covered much of the northern hemisphere. The global thermometer has risen about 8 to 12 degrees C since then. Some predictions say global warming will cause an equivalent amount of warming in the next few centuries. Original model from NOAA
Levels of snowfall, oxygen-18, methane and nitrogen-15 all changed drastically when Greenland's temperature jumped 8 to 12 degrees C 11,600 years ago. Scientists consi der Greenland ice cores the best evidence of abrupt climatic change. The gray band shows a crude picture of the warming period that ended the "Younger Dryas" cold snap. Data from "Abrupt Climate Change ..." (see bibliography).

A simplified view of how ocean currents move heat from the equator toward the poles. Image: NASA

Core of the argument
Shelves of steel canisters storing ice cores.Day After, the movie, opens in Antarctica, with ice drillers sampling a long, frozen record of climate. In the movie, as in reality, the cores contain what scientists, slaking their thirst for turbid terminology, call "proxy" measurements of environmental conditions. Many of these proxies rely on isotopes trapped in ice. Isotopes are chemically identical atoms with different masses. That difference causes their behavior to vary with temperature and other environmental conditions.

And that's why the frozen ice-o-topes can record past climates.

If you can date the layers of ice in a core, you can match dates with temperatures to get a running tab of air temperatures over the eons. That kind of data has overturned the notion that climate must change gradually.

The idea of abrupt change has been around for a long while, says Reid Bryson, a pioneering climatologist who is now a senior scientist at Center for Climatic Research at the University of Wisconsin-Madison. "We'd been talking in the 1960s about abrupt change," he says, based in part on records of plant pollen trapped in lake sediment. Knowing which plant species were present allows you to infer the growing conditions. But counting pollen is tedious, so the samples were usually spaced far apart, and that masked rapid changes. "When they looked at close intervals, they could see that the change was very fast," says Bryson. "Using the right analytical technique, the end of the ice age came pretty damn fast."

Past and present view of N. Hemisphere from space.

Icing on the cake
Pollen was popular, but ice was nice: Deep ice in Greenland and Antarctica is a thousand centuries old. And ice can contain a number of valuable signals. Researchers originally divined air temperature from oxygen isotopes. Here's why: In cold conditions, the heavy, rare isotope oxygen-18 precipitates more quickly from water vapor in the air. Thus low levels of O-18 in ice correspond to low air temperatures.

Ratios of nitrogen and argon isotopes showed the temperature when snow was consolidating into ice. Even dust helps: Lower levels of precipitation reduce vegetation, causing the wind to pick up more dust. Thus more dust indicates lower regional precipitation.

These and other analytic techniques have been applied to ice from three deep-ice drilling projects in Greenland, and it was there that abrupt climate change gained credence among climatologists.

Four climate indicators with even fluctuation. Much of the research focused on a cold period called the Younger Dryas, which ended 11,640 years ago. In 1989, Willi Dansgaard of the University of Copenhagen reported that Greenland had warmed out of the Younger Dryas in just 20 years. Based on a study of oxygen isotopes, he calculated that the air had warmed at least 7 degrees C. in that instant of geological time.

That was surprising. And still, ice cores can be misleading, says Jeffrey Severinghaus, an associate professor of geosciences at the University of California-San Diego. "There are always worries when you see something strange in an ice core." If ice layers for some years are absent, a gradual change may seem abrupt.

Yet cores from multiple drilling efforts, and myriad analytical techniques have confirmed the surprising conclusions about abrupt warming. Abrupt warming at the end of the Younger Dryas is "a closed case," says Severinghaus. "The jury would convict, there is plenty of evidence that it did happen, and that there were big changes."

Ice core corps
By "big changes," Severinghaus means a warming of 8 to 12 degrees C, in less than 50 years, and "probably in less than 10 years." Furthermore, at the same time that oxygen-18 indicated warming, methane concentrations jumped, and dust levels fell by half. All signs, in other words, of a drastic change in the regional, if not global, climate.

Heat moves toward poles from equator. "One reason I think it's pretty reliable, is that at the same time you get the inferred temperature change, you also can detect a rapid change in the accumulation of snowfall," says Stephen Vavrus, an associate scientist at the Center for Climatic Research at the University of Wisconsin-Madison. Vavrus, who examines past and future climates with computer models, explains that a warmer climate usually has more precipitation, because heat is what causes water to evaporate from the ocean.

Ice cores still contain mysteries. For one thing, as Bryson indicates, climate changes differently in different locations. For example, a study of Antarctic Ice (see "Abrupt Climate Change ..." in the bibliography) found that the local temperature had jumped about 5 degrees C in a few decades, but no rise appeared in ice from 500 kilometers away.

Why such a rapid change?

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