Gulf Stream in Trouble

Atlantic Conveyor - graph by Rahmstorf from PIK 20150317

Press Release by the Potsdam Institute for Climate Impact Research


Atlantic Ocean overturning found to slow down already today

The Atlantic overturning is one of Earth’s most important heat transport systems, pumping warm water northwards and cold water southwards.  Also known as the Gulf Stream system, it is responsible for the mild climate in northwestern Europe. Scientists now found evidence for a slowdown of the overturning – multiple lines of observation suggest that in recent decades, the current system has been weaker than ever before in the last century, or even in the last millennium. The gradual but accelerating melting of the Greenland ice-sheet, caused by man-made global warming, is a possible major contributor to the slowdown. Further weakening could impact marine ecosystems and sea level as well as weather systems in the US and Europe.

“It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up,” says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, lead author of the study to be published in Nature Climate Change. Previous research had already indicated that a slowdown of the so-called Atlantic meridional overturning circulation might be to blame for this. “Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” says Rahmstorf.

Because long-term direct ocean current measurements are lacking, the scientists mainly used sea-surface and atmospheric temperature data to derive information about the ocean currents, exploiting the fact that ocean currents are the leading cause of temperature variations in the subpolar north Atlantic. From so-called proxy data – gathered from ice-cores, tree-rings, coral, and ocean and lake sediments – temperatures can be reconstructed for more than a millennium back in time. The recent changes found by the team are unprecedented since the year 900 AD, strongly suggesting they are caused by man-made global warming.

“The melting Greenland ice sheet is likely disturbing the circulation”

The Atlantic overturning is driven by differences in the density of the ocean water. From the south, the warm and hence lighter water flows northwards, where the cold and thus heavier water sinks to deeper ocean layers and flows southwards. “Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation,” says Jason Box of the Geological Survey of Denmark and Greenland. The freshwater is diluting the ocean water. Less saline water is less dense and has therefore less tendency to sink into the deep. “So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further,” explains Box.

The observed cooling in the North Atlantic, just south of Greenland, is stronger than what most computer simulations of the climate have predicted so far. “Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both,” says Michael Mann of Pennsylvania State University in the US. “That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”

No new ice-age – but major negative effects are possible

The cooling above the Northern Atlantic would only slightly reduce the continued warming of the continents. The scientists certainly do not expect a new ice age, thus the imagery of the ten-year-old Hollywood blockbuster ‘The Day After Tomorrow’ is far from reality. However, it is well established that a large, even gradual change in Atlantic ocean circulation could have major negative effects.

“If the slowdown of the Atlantic overturning continues, the impacts might be substantial,” says Rahmstorf. “Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”

If the circulation weakens too much it can even break down completely – the Atlantic overturning has for long been considered a possible tipping element in the Earth System. This would mean a relatively rapid and hard-to-reverse change. The latest report by the Intergovernmental Panel on Climate Change (IPCC) estimates there to be an up to one-in-ten chance that this could happen as early as within this century. However, expert surveys indicate that many researchers assess the risk to be higher. The study now published by the international team of researchers around Rahmstorf provides information on which to base a new and better risk assessment.

Article: Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change (online) [DOI:10.1038/nclimate2554]

Weblink to the article:

Attached material:

– Figure 1 of the article by Rahmstorf et al (2015), showing linear trends of surface temperature for 1901–2013, based on the temperature data of NASA GISS (white indicates insufficient data)

– Atlantic Conveyor – Graph of the Atlantic Meridional Overturning Circulation by Stefan Rahmstorf from PIK

Further information:

– Weblink to NASA animation “The Great Ocean Conveyor Belt” (downloadable video that shows the current system that now is found to slow down in the North Atlantic): here

– Weblink to a study on possible impacts of a shutdown of the thermohaline circulation: here

– Weblink to the expert assessment of an AMOC tipping: here.  

Map based on NASA GISS data of warming 1901-2013 in Rahmstorf AMOC NCC study 2015

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Comment on “Gulf Stream in Trouble”

  1. Leave it to Potsdam to recycle this stale item that “ocean currents are about to halt.” Quick, run and tell the King. The sky is falling too!

    The well-known comedy duo of Mann and Rutherford assure us that Atlantic Ocean currents are slowing down. This lies in the face of an earlier observational paper by Rossby et al that found no evidence of any slowing at all. Oddly, in spite of their “highly accurate” ADCP data, the Martin and Lewis of the Climate Circuit neither list nor use Rossby’s study in their references. Why bother with Rossby’s 20 years of weekly data across the NYC/Bermuda transect? Why, because it shows no slowing, inter-annual variability yes, but no slowing. And that is over 10 years! In 2010, NASA JPL published a satellite/Argo method that suggested there might even have been a slight speed up. Oh dear.

    As is usual amongst the climate-warriors, they “model” along with data to come to a conclusion that suits their purpose. Not real science but science as the alarmists like to present, more P.R. than science. Of course, it wouldn’t be the first time that the ever popular Dr. Mann has been found to have “doctored” his data. Not so surprising is that Mann and Rutherford have added Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research to the act as a back-up singer. He is a known performer when the lights go on.

    The magic words appear in all eco-warrior writing and modelling: “possible”, “could”, “measurements are lacking”, “proxy data”, “strongly suggesting”, “likely”, “appears”, and so on. My personal favorite is “proxy data.”

    Mann and Rutherford’s latest routine is a reminder that we have to look out for the cold that is caused by ‘CAGW’ (catastrophic anthropogenic global warming) now that the heat caused by ‘CAGW’ (catastrophic anthropogenic global warming) cannot be found. To have any of this “work” taken seriously, climate warriors need to eliminate, or drastically reduce, the great number of alarmist papers based on poor data, unreliable models and blatant appeals for grant money. The warping of science has led us, not to follow the idea but to follow the money. One can only hope that comedy routines such as this will bring real scientists closer to the conclusion that models are not an accurate representation of the climate system. The math doesn’t add up because the math simply does not exist.

    The area Mann, Rutherford and Rahmstorf claim the cooling is essentially where the NADW (North Atlantic Deep Water) is created. The NADW is the most important driver of the AMOC (Atlantic Meridional Overturning Circulation and is also important in the global thermohaline circulation). The NADW sinks not only because it is denser than surrounding water masses but because it is salty and cold. Colder water would tend to strengthen the AMOC. While trying to scare us, Mann and company do not use colder but fresher water in the North Atlantic which would be lighter and not sink. They don’t even mention salinity. Conclusion, the AMOC is not about to stop. The Mann, Rutherford and Rahmstorf “proxy study” is nonsense, a ‘triumph’ of models over reality.

    There are some who believe the paper also contains several instances of academic misconduct misrepresenting, as it does, the conclusions of its source paper with regard to the Greenland ice mass balance. That sources balance was itself proxies rather than observational. Real science requires the Observational. For the record, such estimates exist from 1958 on so were readily available. The NOAA/NASA estimate is almost no ice loss in the 1990’s, perhaps 200 gt/year from about 2000-2010 and probable ice mass gain the last few years. Funny how our comic trio missed that.

    Of course, with publication comes the re-quoting by a compliant press. Hell, it’s easier than looking it up, isn’t it? How Nature’s own peer review “process” failed to deal with the most recent research on the topic is mind boggling.
    The Tyndall Centre for Climate Change Research made the point that it is an extremely unlikely event:
    “ elicitation on abrupt climate change undertaken by Arnell et al. (2004) found that several experts declined to respond because they felt the science was too uncertain and that subjective judgements would not be appropriate. Thus, there exists no globally accepted consensus on the likelihood or extent of rapid climate change and agreement among scientists and policy makers over the ‘danger’ posed by abrupt changes in the climate system appears unlikely. Little information exists on lay belief and understanding of the subject.”

    In a feature article (“The Source of Europe’s Mild Climate”) for American Scientist in 2006, Richard Seager wrote:

    “The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth. Like many other myths, this one rests on a strand of truth. The Gulf Stream carries with it considerable heat when it flows out from the Gulf of Mexico and then north along the East Coast before departing U.S. waters at Cape Hatteras and heading northeast toward Europe. All along the way, it warms the overlying atmosphere. In the seas between Norway and Newfoundland, the current has lost so much of its heat, and the water has become so salty (through evaporation), that it is dense enough to sink.

    The return flow occurs at the bottom of the North Atlantic, also along the eastern flank of North America. This overturning is frequently referred to as the North Atlantic thermohaline circulation, or simply the “Atlantic conveyor.” It is part of the global pattern of ocean circulation, which is driven by winds and the exchange of heat and water vapor at the sea surface.

    The Gulf Stream indeed contributes to Europe’s warmth, but it is wrong to conflate the climate difference across the North Atlantic with the northward flow of warm water in the Gulf Stream. This erroneous logic leads to such statements as (from The Times of London): ‘The British Isles lie on the same latitude as Labrador on the East Coast of Canada, and are protected from a similarly icy climate by the Atlantic conveyor belt.’”

    The Trio has been booed off the stage once again. I can hardly await their next offering. I like a good laugh.

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