Watching for the Warming
While November temperatures in interior Alaska began slinking down into double digits below freezing, I heard a lot of complaints about global warming: Where is it when we need it? Somehow, when one's chilly car shows about as many signs of life as a mummified mammoth, it's hard to worry that the seas are growing too hot to keep coral reefs thriving in the Caribbean.
Closer to home, some scientists have identified a warming trend at one place in Canada. They have documented some of its effects, and---no comfort here---they believe that the most marked ones can be seen in the warmer months.
For 20 years, researchers from the Freshwater Institute of the Canadian Department of Fisheries and Oceans have gathered continuous records of the weather, hydrology, water chemistry, and biology of lakes and streams in the Experimental Lakes Area in northwestern Ontario. To guarantee comparable records, they used consistent sampling and analytical methods throughout the study's duration.
In a recent issue of Science magazine, they presented findings from Lake 239, a small boreal lake similar to many between the Coast and Brooks ranges in Alaska. Between the late 1960s and the mid-1980s, the mean annual air temperature at Lake 239 increased by about two degrees centigrade. At the start of the study, the annual mean ranged from 1 to 3°C (34 to 37°F); by the mid-1980s, it was between 2 and 5°C (36 to 41°F).
That doesn't sound like much, but it's had some pretty impressive effects on the lake and its surroundings. For one thing, the ice-free season is now about 20 days longer. Most of that change comes from earlier breakup dates in the spring---the ice melts away sooner.
Along with the warmer temperatures has come less precipitation, as the present global-change models predict for that area. Furthermore, total evaporation increases with higher temperatures; more water is lost from water bodies, soil, and plants. The drier, warmer weather near Lake 239 means less water reaches and stays in the lake. It has also meant more forest fires in the area.
Decreased water flow and burnt-away vegetation in the watershed changed the lake's physical and chemical properties. Once fire ash settled out, for example, the water in Lake 239 became clearer. It no longer contained the colored organic matter that once drained from the surrounding vegetation. No longer slowed by neighboring forest, stronger winds swept the lake surface. The combination of clearer water and more wind changed the lake's mixing depth--the point down to which sun- and air-warmed surface water mixes into colder bottom water.
Such changes naturally change the environment for all the organisms living in the lake. So far, the scientists report, there seem to be few signs of trouble in Lake 239. The warmer water and increased flow of nutrients from the burned-over land has led to a more abundant and diverse community of phytoplankton. Since these microscopic plants form the base of most aquatic food chains, that means there's plenty for larger creatures to eat.
The researchers are more concerned about eventual effects on some temperature-sensitive animals living in the lakes. Opossum shrimp are one of these animals. Little crustaceans much favored by carnivorous lake fishes, they like cold water and the high oxygen content that cold water can maintain. Another cold-loving animal is the lake trout. Summer water temperatures in Lake 239 now come perilously close---within a degree---of what lake trout can tolerate.
The Canadian team carefully notes that they don't take their local warming as proving that the greenhouse effect has taken hold. They do think they have documented changes that the North can expect to see if such an effect does come to pass. Those changes won't all be pleasant---and won't make it any easier to start cars in November.
