for National Geographic News
The six largest Eurasian rivers are dumping a lot more freshwater into the Arctic Ocean now than they were several decades ago, according to an international team of scientists.
Their finding gives teeth to the long-held prediction that freshwater runoff into the ocean would increase in the Arctic as a result of global warming.
"The mechanism is most likely due to increased precipitation as forecast by global climate models," said Bruce Peterson, a researcher at the Marine Biological Laboratory in Woods Hole, Massachusetts.
Other factors for the observed 7 percent increase in runoff from 1936 to 1999 could be changes in ice and permafrost melt or changes in the seasonality of precipitation and runoff, he said.
If this current rate of freshwater influx into the oceans continues to rise, it could have a large-scale impact on ocean circulation patterns in the North Atlantic, the team reports in the December 13 issue of the journal Science.
The influx could slow down or shut off the North Atlantic Deep Water (NADW) formation, the driving factor behind the conveyor belt current known as thermohaline circulation, which brings large amounts of warm water to the North Atlantic region.
Co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany said stoppage of the NADW could cause temperatures in continental Europe to drop by 3.6 to 9 degrees Fahrenheit (2 to 5 degrees Celsius).
"However, you have to take into account that the Earth will have warmed a lot by the time this happens, so depending on how much that is, most of Europe may not get colder than it is now," he said.
Climate models have long predicted that as global temperatures warm, evaporation of surface water will increase and more moisture will be held in the atmosphere. This moisture will lead to more precipitation at high latitudes, such as the Arctic, and subsequently more river runoff.
If these models are correct, the researchers suggest, the 1 degree Fahrenheit (0.6 degree Celsius) rise in temperature over the last century should have caused a corresponding increase in Arctic river discharge. Until now, however, researchers have not been able to obtain good observational data on regional trends.
Previous observational studies have focused on specific areas and time periods. Peterson and his colleagues obtained their data by looking at discharge trends of the six largest Arctic rivers over a 64-year period.
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