Some of the biggest rivers in the world--those that flow into the Arctic Ocean--are dumping 7% more water than they did in the 1930s. If the trend continues, the influx of fresh water could disrupt ocean circulation and the northern climate. Global warming is the apparent culprit, but many experts caution that too little is known about the Arctic and North Atlantic Oceans to make any solid predictions about the effects of extra runoff.
Increasing river runoff, climatologists worry, might put the brakes on an important current in the North Atlantic called the thermohaline circulation (THC). Under present conditions, cold, salty surface waters sink to great depths and then move south, while warmer water on the surface moves northward. Any freshening of the surface waters in the North Atlantic would reduce the seawater density and slow the THC.
To track arctic water flow, ecosystem scientist Bruce Peterson and colleagues at the Marine Biological Laboratory in Woods Hole, Massachusetts, teamed with hydrologists from the United States and Russia to analyze discharge records for six major Eurasian rivers. On average, the total annual runoff has increased by 2 cubic kilometers every year for the past 64 years, the team reports in the 13 December issue of Science.
Global warming is likely to be causing the increase, climatologists say. Warmer air can hold more moisture, which then precipitates as air masses move to high latitudes, leading to an increase in river discharge. The researchers estimate that for each degree of global warming, these Eurasian rivers would pour an extra 212 km3 per year into the Arctic Ocean. If global temperature rises by 5.8 degrees Celsius by 2100--the upper end of estimates from the Intergovernmental Panel on Climate Change--the rivers might increase freshwater flow to the Arctic Ocean by 1260 km3 per year.
"It's a worrying number," says co-author Stefan Rahmstorf, a climatologist at the Potsdam Institute for Climate Impact Research in Germany. But experts are cautious about interpreting the finding. "I would call this intriguingly important," says Bert Semtner, an oceanographer at the Naval Postgraduate School in Monterey, California. He and others stress that much remains to be learned about how freshwater discharge impacts ocean circulation.