Freshwater Runoff Into Arctic on the Rise, Scientists Say
for National Geographic News
|December 13, 2002|
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.
"Use of the combined discharge rather than the single river discharge makes the test for continental scale more robust and comprehensive," said Peterson.
According to the researchers' analysis, the average annual discharge from these six rivers is currently about 31 cubic miles (128 cubic kilometers) greater than it was when routine measurements of discharge began in the 1930s.
Scientists are particularly interested in the potential impacts of this increase in freshwater on thermohaline circulation. The circulation is driven by differences in density of sea water, which is controlled by temperature and salinity.
Historically, thermohaline circulation brings warm, salty water to the North Atlantic, where it is cooled and sinks. "Cool and salty water is dense, it can sink to great depths," Peterson explained.
This newly formed deep water is subsequently exported southward. This ocean circulation, which brings warm waters northward and ships cooler water south, is thought to be responsible for the warming of northern Europe by several degrees.
"One thing that can slow this circulation down," said Peterson, "is, if you add freshwater to that area of the North Atlantic, it lowers the density. It counteracts the process that is increasing density."
According to several ocean circulation models, if enough freshwater is added to the Arctic Ocean, over time it will shut down the NADW. "If you stop the process, you stop the conveyor that brings warm water north," said Peterson.
The paradoxical result, he added, would be a cooling of northern Europe.
However, most of the rest of the world gets warmer when NADW is shut down, said Rahmstorf, who is an expert in ocean current modeling. Ocean currents don't generate heat, they just move it around, he said.
In fact, a shut down of the NADW "would probably enhance global warming, since shutting down NADW will also reduce the ocean's uptake of carbon dioxide, so that more of our emissions remain in the air," said Rahmstorf.
According to the Intergovernmental Panel on Climate Change, which was established in 1988 to assess the risk of human-induced climate change, global surface air temperature is expected to rise between 2.5 to 10.4 degrees Fahrenheit (1.4 and 5.8 degrees Celsius) by 2100.
"This would represent an 18 to 70 percent increase in Eurasian Arctic river discharge over present conditions," the researchers write in Science. That would make for what Peterson calls an "interesting amount of water relative to what modelers predict."
While the researchers say their extrapolation of Arctic river discharge data should not be taken as a prediction, they suggest that such increases "are potentially important with respect to NADW formation."
Going forward, the researchers will work to better understand the links between the atmospheric, continental, and oceanic components of the Arctic hydrologic cycle.
Peterson and his colleagues plan to measure chemical tracers in major Arctic rivers, so that, in collaboration with oceanographers, they can find out where the river water is going in the ocean. "If we can do that, we can make a more rigorous test of ocean circulation simulation models," said Peterson. Such tests will make predictions more robust.
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