Some 20,000 musk oxen died on Canada's far-northern Banks Island because of such conditions during the winter several years ago. Yet, their deaths went unnoticed until the next spring. The new satellite-detection method could provide an early warning to native people, giving them a realistic chance of getting food to herds to prevent mass starvation.
"We are talking about Banks Island, but this applies to the whole Arctic - Alaska, northern Canada, Siberia, Scandinavia - wherever there is permafrost," says Jaakko Putkonen of the University of Washington in Seattle, who participated in the satellite study.
Banks Island is at the edge of the Beaufort Sea inside the Arctic Circle. In October 2003, rain fell for several days there on top of a 6-inch snow cover. The rain seeped through the snow to the soil surface. The temperature then plunged, and the water became a thick layer of ice that lasted the winter. It prevented browsing animals from reaching their food supply of lichens and mosses at the soil's surface.
"Starvation happened over a period of many months, and no one knew until they went up to do the population count the next spring," says the University of Washington's Thomas Grenfell, who traced satellite clues of the Banks Island event with Putkonen.
Rain falling on snow can mean lingering death for musk oxen, reindeer, and other animals that root through the snow to graze on the Arctic tundra. Grenfell and Putkonen found evidence for the 2003 rain-on-snow occurrence in passive satellite microwave imagery, which they believe could provide a signature to help detect such events anywhere.
In the new study, the scientists examined data from 10 different satellite microwave channels, each providing slightly different information on the qualities of the snowpack. "The subtleties in the microwave levels mean there can be high error margins on this information, but the Banks Island event stood out like a sore thumb," Grenfell says.
He and Putkonen detail their work in a paper now in press with Water Resources Research, a journal of the American Geophysical Union.
Grenfell has conducted more than 40 field experiments in polar regions and has become quite familiar with precipitation characteristics there. Much of the previous work he did was with researchers who were interested in the nature of the snowpack, but he found that the presence of water interfered with interpreting satellite microwave readings. For the new research, the signal from water was key, he says.
The researchers hope to search other satellite microwave records for evidence of rain-on-snow events of the last 30 years that are known from anecdotal information.
The 2003 rain-on-snow event affected the northern part of the 43,000-square-mile Banks Island. The musk oxen population of 70,000 was cut by nearly 30 percent, but a caribou herd on the southern part of the island was unaffected. The closest weather station, about 60 miles from the musk oxen range, didn't record any rainfall at the time of the event that resulted in the massive die off, so few people recognized that the oxen were in distress.
Currently, there is no way to know exactly where or how often these potentially devastating rain-on-snow events occur, the researchers say, but using satellite data to locate them could make up for a scarcity of weather stations in the sparsely populated Arctic.
Rain-on-snow events historically have occurred mostly in coastal areas. However, in earlier research, Putkonen found that models predict that climate change will push winter rainfall much farther into northern continents and large islands.
While food shortages can trigger a large die off, there also can be severe consequences from milder events that force animals to exert more energy to get food. That reduces body weight and limits reproduction, which in turn can cause long-term damage to herds.
Peter Weiss | American Geophysical Union
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