The Gulf Stream carries masses of warm Atlantic water from the Caribbean to Europe. Therefore, winters here are not as cold as in North America at the same latitudes. Palm trees on Britain's south coast would be unthinkable without the Gulf Stream.
At the end of the last millennium, Professor Stefan Rahmstorf of the Potsdam Institute for Climate Research predicted that the Gulf Stream as a result of global climate change could weaken or even dry out altogether.
Millennium Prize
As a result, on the European continent, contrary to the global trend, the climate should become colder, not warmer. For this discovery, Ramstorf was awarded the $ 1 million Millennium Prize of the American James McDonnell Foundation in 1999.
Almost twenty years later, there is no new information about the actual future of the Gulf Stream. However, until now, the weakening of the Gulf Stream, or, scientifically speaking, the Atlantic meridional circulation (AMC), has not found indisputable evidence.
This is explained, among other things, by the fact that the corresponding measurements have been made only since 2004 and the changes observed since then can be explained simply by natural variability. However, Ramstorf is convinced that the Gulf Stream will weaken markedly in the long term, at least 50 years from now.
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Salt as a circulation motor
His hypothesis is of a fundamental nature: the melting of glaciers in Greenland continues, as a result, the salt content in the waters of the North Atlantic decreases, and therefore the mixing mechanism is weakened, which in the northern seas forces warm surface water to sink to a depth and thereby maintains the global circulation of waters in the oceans.
But what happens if the Gulf Stream does weaken? Do we Europeans really need to prepare for a cooling climate? Science does not yet have a consensus on this issue.
But recently, scientists from the group of C. Chen (X. Chen) published in the journal Nature an article with the results of a study, according to which the weakening of the AMC will allegedly lead to a strong increase in the temperature of the earth's surface around the world.
Can the Gulf Stream cool down?
The author of the article argues this as follows: in the past, the AMC partially compensated for the global temperature rise caused by greenhouse (carbon dioxide) gas, transferring heat from the surface of the world's oceans to its depths. If this mechanism weakens, then less heat from the atmosphere will accumulate in the world's oceans, as a result of which the temperature on Earth will rise, including in Europe.
“This hypothesis is questionable,” Stefan Ramstorf criticizes the article. "The authors argue that during periods of strong AMC, as a result of convection, heat is transferred to deeper layers of water, and therefore the earth's surface heats up less."
This is pure speculation
“However, convection in the subpolar Atlantic occurs because surface water gets colder than deep water on cold winter days, and therefore water layers begin to mix at depth - despite the stable distribution of salt water layers. Fresh water lies in convection zones in the center of Labrador above saline water. Convection always directs heat from bottom to top, not top to bottom”. Even if we assume that the mechanism described in the article is, in principle, correct, Ramstorf concludes, the forecast of scientists for the next twenty years is "pure speculation."
Also sharply criticized by this hypothesis Johann Jungclaus (Johann Jungclaus), a Hamburg scientist from the Max Planck Society for Scientific Research. Jungklaus, research team leader in the Division of Terrestrial Oceanology at the Max Planck Institute for Meteorology, says: “The authors describe many coincidences and build causal relationships, although they cannot truly prove them. They assume that all temperature changes in the North Atlantic and most of the global oceanic warm budget are associated with changes in the AMC. But they, in particular, do not take into account the horizontal redistribution of waters, caused, for example, by an intense subpolar whirlpool."
Interesting because it is provocative
But Professor Mojib Latif from the Helmholtz Center for Marine Research (Geomar / Kiel) is rather conciliatory: "The hypothesis formulated by the authors is still interesting because it is provocative."
But, proceeding from the essence of the question, he believes that the methodology of the researchers is questionable: "We do not have any information about the development of the AMC in the second half of the XX century, and we also do not know how the AMC will develop in the future decades."
In his opinion, even if we assume that the Gulf Stream or the AMC will really weaken, the conclusions of this study are questionable.
Global warming continues
“Most climate models do not support the hypothesis that the Earth will be warming up faster if the AMC weaken in the future,” Latif says.
But new thoughts revive scientific activity. In the coming years, researchers will be even more active in studying the processes in the Gulf Stream and the consequences of a possible weakening of circulation.
But one thing is already clear for Stefan Ramstorf: “Global warming will continue until we stop the emission of greenhouse gases into the atmosphere. However, it will not increase due to the weakening of the AMC”.
Norbert Lossau