10 September 2010
Earth’s climate is jumpy. Tens of thousands of years ago, the planet suffered more than two dozen episodes of abrupt warming, with global temperatures soaring by as much as a few degrees Celsius in as little as a decade.
Many scientists are worried that an abrupt climate change could happen again, and that by continuing to emit increasing amounts of greenhouse gases, people may be increasing the risk of the planet crossing a tipping point, in which a small extra push would create a large amount of warming.
Knowing that, researchers have been looking for early signs that might give us advance warning about another such abrupt change. The thinking was that if we had warning, we might decide to try to head off the sudden change by using geoengineering (planetary-scale projects to cool the planet), or, at the least, we might be able to prepare better to adapt to the changing climactic conditions.
However, a new study pours cold water on that notion. In the paper to be published in Geophysical Research Letters, climate scientist Peter Ditlevsen and Sigfus Johnsen of the University of Copenhagen in Denmark looked at the best records of abrupt climate change that exist, and suggest it’s unlikely that we’d ever be able to spot early warning signs.
They examined 3-kilometer-long ice cores drilled from Greenland’s ice sheet, looking specifically at oxygen atoms trapped in the ice. Changes in ratios of different isotopes of oxygen (lighter and heavier versions of the same element) reveal the fluctuations in the ancient climate, with a resolution down to periods as short as a few years.
In this high-resolution record, there were more than two dozen times over the past 140,000 years when the planet abruptly heated up. During these periods of sudden warming, known as Dansgaard-Oeschger events, Greenland’s temperature climbed by 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit) in about a decade. (In comparison, Greenland’s temperature rose about 20 degrees C (36 degrees F) when the planet emerged from the last ice age.)
Because this record is of such high resolution, “it should be possible to detect if the jumps were preceded by early warnings,” the researchers write in their new study.
Earlier studies by other scientists had suggested the types of things researchers could look for to detect an impending abrupt shift. They focused on a system’s variance—a measure of how much the temperature changes from year to year, and its autocorrelation – its tendency to stay the same. (If the autocorrelation of the temperature is high, then the temperature varies slowly from year to year.)
Scientists have proposed that if both of these properties—the variance and the autocorrelation—change according to a particular pattern, it could serve as a warning of an impending tip into a new state of the climate. However, when Ditlevsen looked for such a warning sign in the Greenland ice core records before the Dansgaard-Oeschger events, the telltale pattern wasn’t there. Instead, it seems that what triggered the events was random variation in the climate.
“That’s the scary thing,” Ditlevsen says. “You’re not warned.”
– Mason Inman, contributing science writer
A University of Copenhagen press release on this paper is online here.