The Northern Hemisphere winter of 2014-15 was the warmest on record globally, according to the National Oceanic and Atmospheric Administration. That region was, in fact, the coldest it has been since the dawn of instrument records, at up to 3.6 degrees Fahrenheit colder than average. Intergovernmental Panel on Climate Change had judged that there is up to a 10% likelihood of a Gulf Stream shutdown before year 2100, though many climate scientists estimate this likelihood is even higher.
“Evidence is mounting that the long-feared circulation decline is already well underway,” says co-author Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany, in a blog post for RealClimate.
The slowdown in this current, the study finds, is unprecedented in hundreds to perhaps as long as 1,000 years, and is most likely related to another tipping point, which is the melting of the Greenland ice sheet. The influx of freshwater from the ice sheet is one of the main sources of freshwater inflow into the North Atlantic Ocean.
As it pours into the Atlantic, the freshwater is lighter and colder than heavier, salty water that typically occupies that area. It therefore tends to sit on top of the water column, accumulating over the years and interfering with the formation and sinking of dense, cold and salt-enriched waters. This chokes off the northward flowing Gulf Stream, slowing it down, and affecting ocean circulation downstream as well.
While it’s not anywhere close to the apocalypse that a rapid Gulf Stream shutdown was shown to be in the 2004 blockbuster disaster film The Day After Tomorrow, a rapid slowdown in this current would boost sea level rise rates along the highly populated Mid-Atlantic and Northeast coasts of the U.S. It could also bring much cooler conditions than is currently the norm to parts of northern Europe.
Once again, we are learning that the climate model projections may be too conservative. In this case, the fact that the Greenland Ice Sheet is loosing mass and contributing to freshwater runoff into the North Atlantic decades ahead of schedule may be the reason that the Atlantic Meridional Overturning Circulation (AMOC) is weakening decades ahead of schedule as well.
There is no reliable observational evidence of the strength of the Gulf Stream over time, since even modern measurements are relatively scarce. To get around this problem, the study’s authors created an index based on sea surface temperatures to infer the strength of the current over time. Specifically, they took into account the temperature difference between the area most influenced by changes in the strength of the circulation, which is that telltale cold patch in the North Atlantic, and the rest of the Northern Hemisphere.
Furthermore, the study found that the index they devised to track the current’s strength over time closely matched modeled trends, which lends some confidence to the findings.
However, many uncertainties remain about what is going on with the Gulf Stream, and any changes in the broader Global Conveyer Belt as a whole. Direct measurements will help reduce these mysteries, and efforts are underway to fill some data gaps in the North Atlantic and the Southern Ocean. These are the two most important regions where salty, dense bottom water forms, powering ocean currents thousands of miles up and downstream. Read more…