Arctic sea ice melting one warm river at a time
A new NASA study finds that warmer than normal waters from rivers draining into the Arctic Ocean each summer are eating away at the sea ice in the Arctic Ocean. Led by Son Nghiem of NASA's Jet Propulsion Laboratory in Pasadena, Calif., the research team used satellite data to measure the surface temperature of the waters discharging from Canada's McKenzie River into the Beaufort Sea during the summer of 2012 and noticed surface waters being warmed suddenly due to the sudden influx of warm river water This warmed the surface layers of the ocean, which in turn increased the melting of sea ice.
This Arctic process contrasts starkly with those that occur in Antarctica, a frozen continent without any large rivers. The sea ice cover in the Southern Ocean surrounding Antarctica has been relatively stable, while Arctic sea ice has been declining rapidly over the past decade.
"River discharge is a key factor contributing to the high sensitivity of Arctic sea ice to climate change," said Nghiem. "We found that rivers are effective conveyers of heat across immense watersheds in the Northern Hemisphere. These watersheds undergo continental warming in summertime, unleashing an enormous amount of energy into the Arctic Ocean, and enhancing sea ice melt. You don't have this in Antarctica."
The team said the impacts of these warm river waters are increasing due to three factors. First, the overall volume of water discharged from rivers into the Arctic Ocean has increased. Second, rivers are getting warmer as their watersheds (drainage basins) heat up. And third, Arctic sea ice cover is becoming thinner and more fragmented, making it more vulnerable to rapid melt. In addition, as river heating contributes to earlier and greater loss of the Arctic's reflective sea ice cover in summer, the amount of solar heat absorbed into the ocean increases, causing even more sea ice to melt.
To demonstrate the extensive intrusion of warm Arctic river waters onto the Arctic sea surface, the team selected the Mackenzie River in western Canada. They chose the summer of 2012 because that year holds the record for the smallest total extent of sea ice measured across the Arctic in the more than 30 years that satellites have been making observations.
The researchers used data from satellite microwave sensors to examine the extent of sea ice in the study area from 1979 to 2012 and compared it to reports of Mackenzie River discharge. "Within this period, we found the record largest extent of open water in the Beaufort Sea occurred in 1998, which corresponds to the year of record high discharge from the river," noted co-author Ignatius Rigor of the University of Washington in Seattle.
Aerial view of mouth of McKenzie River into Beaufort Sea image via Shutterstock.