Polar regions have long been recognized as useful sentinels of global climate change. Now, researchers have uncovered dramatic new evidence of a long-term warming trend in Arctic lakes. The timing of the shifts argues strongly that industrial greenhouse gases have profoundly affected even these remote ecosystems.
Extensive climate monitoring data for polar areas are generally sparse. However, the microfossils of aquatic organisms, preserved in the chronologically layered muds of the Arctic's ubiquitous lakes, provide a good climate history book.
Building on a previous limited study of climate warming on three Arctic lakes (Science, 21 October 1994, 416), ecologist John Smol of Queens University, Kingston, Canada, and colleagues extracted sediment cores from 46 lakes in Canada, Finland, Norway, and Russia. The team then used standard methods to date the layers and identify the species of algae and invertebrates, such as water fleas and midges, that inhabited the cold Arctic lakes.
The results showed that species assemblages have changed markedly over the past 150 years. For example, at some subarctic lakes, the number of planktonic algae increased 25%, while some high-Arctic sites showed a whopping 100% change in species composition. In contrast, there was little evidence of biological change at more southerly lakes in Labrador--where climate has warmed only minimally.
From known associations of individual aquatic species with various environmental conditions, the researchers conclude that summers have lengthened and lake ice cover has diminished across much of the Arctic since the mid-1800s. This has prolonged the growing season and opened up new habitats for species adapted to warmer conditions, Smol says. "We're crossing ecological thresholds here, and once you pass them it's hard to go back," says Smol. Other fossil evidence dating back to the ice-age origins of some of the lakes shows the changes of the last century-and-a-half are unprecedented, he adds. Smol's team reports its findings online this week in the Proceedings of the National Academy of Sciences.
John Hobbie, director of the Arctic Long-Term Ecological Research Project at the Marine Biological Lab, Woods Hole, Massachusetts, notes that the widespread changes identified by the researchers began after millennia of biotic constancy. "The conclusion is undeniable," Hobbie says. "Not only are ecosystems around the Arctic affected by human influences today, but the influence began more than a century ago."