Many of the earlier predictions of climate research have now become reality. The world is getting warmer, sea levels are rising faster and faster, and more frequent heat waves, extreme rainfall, devastating wildfires and more severe tropical storms are affecting many millions of people. Now there is growing evidence that another climate forecast is already coming true: the Gulf Stream system in the Atlantic is apparently weakening, with consequences for Europe too.
The gigantic overturning circulation of the Atlantic water (dubbed AMOC) moves almost 20 million cubic meters of water per second – almost a hundred times the Amazon flow. Warm surface water flows to the north and returns to the south as a cold deep current. This means an enormous heat transport – more than a million gigawatts, almost one hundred times the energy consumption of mankind. This heat is released into the air in the northern Atlantic and has a lasting effect on our climate.
But since the 1980s, climate researchers have been warning of a weakening or even a cessation of this flow as a result of global warming. In 1987, the famous US oceanographer Wally Broecker titled an article in the scientific journal Nature “Unpleasant surprises in the greenhouse”. Even Hollywood took up the subject in 2004 in the film “The Day After Tomorrow” by the German director Roland Emmerich. However, there were no measurement data that could prove an ongoing slowdown.
Only since 2004 has there been continuous monitoring at 26°N in the Atlantic (RAPID project). Although the data show a weakening of the current system, the measurement series is still too short to distinguish a possible climate trend from decadal variability. For the longer-term development of the Gulf Stream system, we must therefore rely on indirect evidence.
A long-term AMOC weakening should lead to a cooling in the northern Atlantic. Such a regional cooling in the middle of global warming has been predicted by climate models for a long time. And indeed, the evaluation of data on sea surface temperatures shows that the northern Atlantic is the only region of the world that has escaped global warming and has even cooled down since the 19th century (see graph). In addition, one can see a particularly strong warming off the North American coast, which according to model simulations is part of the characteristic “fingerprint” of a weakening of the Gulf Stream circulation.
This fingerprint is regarded as important evidence, and not least because of this, the Intergovernmental Panel on Climate Change (IPCC) stated for the first time a year ago in the Summary for Policy Makers of its Special Report on the Oceans:
“Observations, both in situ (2004–2017) and based on sea surface temperature reconstructions, indicate that the Atlantic Meridional Overturning Circulation (AMOC) has weakened relative to 1850–1900.”
New studies support long-term weakening
Two new studies now provide further independent evidence of this weakening. In August a paper by Christopher Piecuch of the Woods Hole Oceanographic Institution on the Florida Current – the part of the Gulf Stream system along the Florida coast – was published. Although continuous measurements of the current have only been available since 1982, Piecuch was able to reconstruct the strength of the Florida Current over the last 110 years from measurements of the sea level difference between the two sides of the current. To do so, he used 46 tide gauge stations in Florida and the Caribbean as well as a simple physical principle: the Coriolis force deflects currents in the northern hemisphere to the right, so that the water on the right side of a current stands higher than on the left. The stronger the current, the greater the difference in sea level. Comparison with measurements since 1982 shows that the method works reliably.
The result: the Florida current has weakened significantly since 1909 and in the last twenty years has probably been as weak as never before. Piecuch’s calculations also show that the resulting reduction of heat transport is sufficient to explain the ‘cold blob’ in the northern Atlantic.
This Monday, in Nature Climate Change a further study appeared, of researchers of Peking University and Ohio State University (Chenyu Zhu and Zhengyu Liu). For the first time, their paper provides evidence for an AMOC slowdown based on data from outside the North Atlantic. Model simulations show that a weakening of the AMOC leads to an accumulation of salt in the subtropical South Atlantic. This is due to the fact that strong evaporation in this region constantly increases the salinity, while the upper branch of the ocean circulation drains the salty water northwards, continually bringing in less salty water from the south. When this current weakens, the water in this region becomes saltier. This is exactly what the measured data show, in accordance with computer simulations. The authors speak of a “salinity fingerprint” of the weakening Atlantic circulation.
Video animation of ocean currents in the CM2.6 climate model of the Geophysical Fluid Dynamics Lab in Princeton:
In addition to these oceanographic measurements, a number of studies with sediment data indicate that the Gulf Stream circulation is now weaker than it has been for at least a millennium.
These current changes also affect Europe, because the ‘cold blob’ out in the Atlantic also influences the weather. It sounds paradoxical when you think of the shock frost scenario of the Hollywood blockbuster The Day After Tomorrow: but British researchers found that in summer the jet stream in the atmosphere likes to take a route around the south side of the cold blob – this then brings warm winds from the southwest into Europe, leading to heat waves there, as in the summer of 2015. Another study found a decrease in summer precipitation in northern Europe and stronger winter storms. What exactly the further consequences will be is the subject of current research.
However, the latest generation (CMIP6) of climate models shows one thing: if we continue to heat up our planet, the AMOC will weaken further – by 34 to 45% by 2100. This could bring us dangerously close to the tipping point at which the flow becomes unstable.
This article appeared originally in German in Der Spiegel: Das Golfstromsystem macht schlapp