24/6/11

Gulf Stream could be threatened by Arctic flush

Newscientist Enviroment
18:00 05 April 2011 by Fred Pearce
For similar stories, visit the Mysteries of the Deep Sea and Climate Change 

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Rapid warming in the Arctic is creating a new and fast-growing pool of fresh water in the Arctic Ocean. Measuring at least 7500 cubic kilometres, it could flush into the Atlantic Ocean and slow the Gulf Stream, bringing colder winters to Europe.
The water is mostly coming from melting permafrost and rising rainfall, which is increasing flows in Siberian rivers that drain into the Arctic, such as the Ob and Yenisei. More comes from melting sea ice, says Laura de Steur of the Royal Netherlands Institute of Sea Research in 't Horntje, who is tracking the build-up.
Salinity anomalies like this are a regular feature of the Arctic. The last major event occurred in the 1960s. They happen when strong winds circling the Arctic restrict southward water movement. Eventually, the winds falter and the water flushes into the Atlantic through the Fram strait, between Greenland and Europe.
Recent Arctic melting runs the risk of increasing the freshwater build-up, potentially making the consequences of the eventual breakout more extreme, says de Steur. This is the first time that scientists have measured a salinity anomaly in the Arctic in detail, and in time to analyse how the freshwater pool breaks out into the North Atlantic.
De Steur believes the consequences could be more dramatic than in the past, because of how global warming is changing the dynamics of the region. "Sea ice is melting quicker. It is thinner and more mobile, and could exit the Arctic faster. Also more of it will enter the Atlantic as liquid water rather than ice."
Conveyor breakdown
A dramatic freshening of the North Atlantic could disrupt the engine of a global ocean circulation system called the thermohaline circulation, or ocean conveyor. This system, of which the Gulf Stream forms a part, is driven by dense, salty water in the North Atlantic plunging to the ocean bottom near Greenland.
"In the worst case, these Arctic surges can significantly change the densities of marine surface waters in the far North Atlantic," says de Steur.
Some 13,000 years ago, a major freshening of the North Atlantic shut down the circulation and plunged the Earth into a cold snap, known as the Younger Dryas era, which lasted for 1300 years. That was the result of an influx of fresh water much larger than is building up now, but some climate models do predict the circulation could weaken in coming decades, says Detlef Quadfasel of the climate centre at Hamburg University in Germany. The discovery of pooling fresh water in the Arctic suggests how this could happen.
The monitoring is being carried out as part of Project Clamer, a 10-nation European project into the impact of climate change on the waters around Europe.

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