Here at Bitsofscience.org we’ve written quite extensively on why a direct shutdown of the Gulf Stream is unlikely – and that the collapse scenario featured in that one movie we only ever saw the trailer of probably did not even occur during the famous Younger Dryas ice age hick-up and that perhaps the Gulf Stream was just as strong during the ice age cold peak.
But forget about ice ages for as long as you, your grandchildren and their grandchildren will live – if Holocene Mass Extinction even grants us that time. We have a very different planet now, one that is exceedingly breaking all heat records. What we at Bits of Science have thus far largely ignored is that a gradual shutdown is probably equally bad news for anyone anywhere on the planet – and that science says that scenario is perhaps already underway:
The Gulf Stream is already declining in strength. This graph shows current plus paleoclimatic measurements combined. It leads to a ‘reversed hockey stick graph’ – another one that shows the relative stability of the global climate system during the Holocene is now quickly changing and switching to a dynamic state that will likely be far beyond anything mankind has witnessed.
If you live anywhere on planet Earth a gradual shutdown of the Gulf Stream is probably about equally bad news, to realise global climate warming leads to full-scale disruption of our planet’s connected system of ocean currents (that much of marine and terrestrial life depends on!), and considering the time scale at which this is happening, as new measurements and climate model runs by the Potsdam Institute for Climate Impact Research published in Nature Climate Change show AMOC is already on a weakening trend.
AMOC stands for ‘Atlantic Meridional Overturning Circulation’, known best for its North Atlantic subsurface branch the ‘Gulf Stream’, that transports warm water (first carried from West Africa to the Caribbean by (possibly also declining!) Atlantic trade winds) from the Gulf of Mexico and the South-Eastern US to Europe’s West coast. This Potsdam Institute climate model run projects the current declining AMOC trend is set to continue and speed up over decades to come – the ‘gradually halted Gulf Stream’ scenario.
Connection to the global climate system: AMOC slowdown is a positive feedback on climate warming – NOT a negative one!
A slowdown of AMOC does not imply ‘winter is coming’. Firstly the slowdown is caused by global warming, which leads to massive run-off of freshwater from the melting Greenland icesheet, disturbing North Atlantic Ocean salinity, therefore deep water formation, the driving engine to AMOC – we’re sure you know the story. Secondly the influence of the Gulf Stream is very local – and will be partially masked by overlying climate trends. For instance European winters are warming, while AMOC has been decreasing(!) (Over all seasons combined climate warming in Europe is about twice the global average.) Thirdly and most importantly, a slowdown/shutdown of AMOC leads to a slowdown of the entire thermohaline circulation or ‘ocean conveyor belt’. This means the tropics will warm faster, less atmospheric heat is taken to the deep ocean and -most worryingly on the longer time scale- ocean fertilisation (plankton!) and carbon sequestration might be dramatically lowered, leading to further rise of atmospheric CO2 concentrations and further climate warming.
This is a very interesting image, also from the same Potsdam study in Nature Climate Change. Shown is that one area of ‘global cooling’ over the 20th century – the North Atlantic Subpolar Gyre, indicative of a Gulf Stream slowdown. Meanwhile coastal waters off the US South East warm faster.
Now compare to the current SST anomaly map for the North Atlantic, in this NOAA forecast for October. Indeed – AMOC is currently in a weak state.
That however may change once the 2015-2016 Super El Niño is gone, and the Western Hemisphere Warm Pool disperses across the Atlantic. Not sure when that is going to happen. The current state will be of influence to both North America and Europe during the 2015-2016 winter. We should not forget AMOC is higly variable and also seems to follow a multi-year cycle with some limited predictability.
© Rolf Schuttenhelm | www.bitsofscience.org