Gulf Stream probed for early warnings of system failure

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Climate researchers set sail from the Canary Islands today to begin an ambitious, four-year programme that will assess the behaviour of currents, such as the Gulf Stream, in the north Atlantic Ocean. The US$20-million programme is part of a wider investigation into rapid climate change, known as Rapid. It will take the most detailed look yet at the strength, structure and variability of the currents that carry warmth northwards in the Atlantic.

Climatologists worry that global warming could disrupt these currents, which make Western Europe’s climate far warmer than other parts of the world at the same latitude. Without the Gulf Stream, for example, the south of England would be as cold as Iceland.

Researchers think the currents are caused by a combination of wind, differences in water density, and the special geometry of Atlantic Ocean basins and the surrounding continents. They think the system may have broken down before, driving large, abrupt changes in climate.

Given the extent of global climate change at the moment, some suspect this could happen again. Models suggest that currents are already affected by increased freshwater flow from precipitation, river runoff and ice-sheet melting.

The Rapid team on board the British research vessel RRS Discovery will try to establish if this is really happening. “Complete collapse of the north Atlantic circulation is a worst-case scenario,” says Stefan Rahmstorf, a climate modeller at the Potsdam Institute of Climate Impact Research in Germany. “But no one really has any firm idea of what is going on out there, and that is why this project is so important.”

The team will use 22 moorings across the subtropical Atlantic: near the Canary Islands, on the Mid-Atlantic Ridge and off the Bahamas. Sensors will travel up and down wires from buoys to the moorings on the sea floor. Differences in water density will be calculated from the temperature and salinity measured throughout these water columns. With US measurements from the Florida Strait and satellite observation of wind-driven surface currents, these will help researchers understand water flow in the north Atlantic.

“It is unlikely that we will detect dramatic changes within the next four years,” says Jochem Marotzke, an oceanographer at the Max Planck Institute of Meteorology in Hamburg, and one of the principal investigators in Rapid. “But we will learn lots of exciting things about ocean circulation, and work out how to design a stable monitoring system for the next few decades.” Rapid is being jointly sponsored by Britain’s Natural Environment Research Council and the US National Science Foundation.

One long-term goal, says Marotzke, is an early-warning programme that would raise the alarm if the present system was close to failure. Even if nothing could be done, society could prepare for the results, he says. A German-Norwegian project called Integration is already assessing the impact of a failure on climate, fisheries and agriculture.