Goodbye Gulf Stream by 2200

07 Jul 2005 - Articles, Water & Oceans, Arctic

North Atlantic Ocean and Nordic Seas circulation - surface (red, orange, yellow) and deep (violet, blue, green) currents.

In a paper published in Science in June 2005, Ruth Curry of the Woods Hole Oceanographic Institution (WHOI) and Cecilie Mauritzen of the Norwegian Meteorological Institute report that patterns of fresh water accumulation in the Nordic Seas (in the Arctic) in recent decades suggest that the Gulf Stream would cease functioning in around two centuries if accumulation continued at the rate observed in their study.

Significant slowing of the Gulf Stream could be under way even by the end of this century - and possibly sooner, depending on the rate of global warming and consequent impacts in the Arctic. This new quantitative analysis over the period 1965-1995 provides a good benchmark for monitoring the extent to which global warming threatens the operation of the Gulf Stream.

The Gulf Stream is the ocean current which runs past the west coast of Europe and which helps keep temperatures in Lisbon far warmer on average than in New York (around the same latitude). Serious disruption to the Gulf Stream's flow has occurred in the past with significant consequences for the Earth's climate and so scientists are monitoring developments closely.

Scientists know that the key mechanism for keeping thermohaline circulation (of which the Gulf Stream is a part) functioning is the extent to which the Nordic Seas remain saltier, and hence more dense, than waters immediately further south in the deeper parts of the northern Atlantic Ocean. Seawater in the Nordic Seas generally becomes even saltier and denser than usual as part of the process of sea-ice formation in the far north - thus creating the conditions under which it sinks more than a kilometre below the surface to flow southward, displacing less dense water in the Atlantic Ocean's depths.

"Mapping" where the fresh water goes

Curry's and Mauritzen's research has quantified and "mapped" increased accumulation of fresh water in the Nordic Seas - with consequent salinity changes - over the period 1965-1995. In large part this stems from global warming in the second half of the 20th century that has increased quantities of rain in the Arctic, brought about melting of ice from Greenland glaciers and reduced sea-ice formation.

In an average year, around 5,000 cubic kilometres of fresh water (roughly equivalent to the annual output of the Amazon, the world's largest river) dilutes the salinity of seawater flows into the Atlantic Ocean from the Arctic. There were, however, a number of significant additional freshwater injections from the Arctic into the Atlantic - around 19,000 cubic kilometres in the thirty year period studied.

Some 2,500 cubic kilometres of the additional freshwater, however, accumulated in the critical upper layer of the Nordic Seas (where the Gulf Stream begins to "overturn"). This rate of accumulation in the Nordic Seas, according to Curry and Mauritzen, has not yet brought about sustained changes to the overturning rate but, if it continues, would be likely to do so over the next century. By 2200 it would bring the Gulf Stream to a halt.

Further global warming would halt the Gulf Stream sooner

If, as a result of global warming, significant parts of the massive Greenland ice-sheet were to melt or break off, and if sea-ice formation were to shrink as fast or even faster than the rapid rate witnessed over the period 2002-05 then the timetable mooted in this study would likely have to be brought forward considerably, as noted by the authors.

By: Gauthier Chapelle