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Last year, twenty thousand peer reviewed studies on ‘climate change’ were published. No single person can keep track of all those – you’d have to read 55 papers every single day. (And, by the way, that huge mass of publications is why climate deniers will always find something to cherry-pick that suits their agenda.) That is why climate assessments are so important, where a lot of scientists pool their expertise and discuss and assess and summarize the state of the art.

So let us have a quick look what last year’s climate assessments say about the much-discussed topic of whether the Atlantic Meridional Overturning Circulation (AMOC, a.k.a. Gulf Stream System) has already slowed down, as predicted by climate models in response to global warming.

First, there is the IPCC 1.5 °C report (SR15) prepared for the Paris Climate Agreement and published in September 2018. It doesn’t say all that much about the AMOC, given that it is not a full IPCC assessment, but it does say this:

It is more likely than not that the Atlantic Meridional Overturning Circulation (AMOC) has been weakening  in recent decades, given the detection of the cooling of surface waters in the north Atlantic and evidence that  the Gulf Stream has slowed by 30% since the late 1950s (Srokosz and Bryden, 2015; Caesar et al., 2018).  There is only limited evidence linking the current anomalously weak state of AMOC to anthropogenic warming (Caesar et al., 2018). It is very likely that the AMOC will weaken over the 21 st century. […]

Weakening of the Atlantic Meridional Overturning Circulation (AMOC) is projected to be highly disruptive to natural and human systems as the delivery of heat to higher latitudes via this current system is reduced.

[Note: those “30% since the late 1950s” are probably in error; they are not supported by either of the two references provided.]

Then, in November, the 4th US National Climate Assessment was published that had been two years in the making. It says:

The primary concern related to ocean circu­lation is the potential slowing of the Atlantic Ocean Meridional Overturning Circulation (AMOC). An AMOC slowdown would affect poleward heat transport, regional climate, sea level rise along the East Coast of the United States, and the overall response of the Earth’s climate system to human-induced change. […]

As the atmosphere warms, surface waters entering the North Atlantic may release less heat and become diluted by increased freshwater melt from Greenland and Northern Hemisphere glaciers. Both of these factors would slow the rate of sinking and weaken the entire AMOC.

Though observational data have been insuffi­cient to determine if a long-term slowdown in the AMOC began during the 20th century, one recent study quantifies a 15% weakening since the mid-20th century and another, a weakening over the last 150 years. Over the next few decades, however, it is very likely that the AMOC will weaken.

Finally, Future Earth (a global Earth science research programme) and the Earth League (a grouping of leading institutions and individuals in the Earth sciences) have issued a climate science update for the Katowice climate summit in early December, called 10 New Insights in Climate Science 2018. It says:

 A weakening of the Atlantic overturning circulation, often referred to as the Gulf Stream system, has been expected from model simulations. Recent studies confirm that it has slowed down by 15% since the middle of the 20th century and is at its weakest in over a thousand years. This is already having observed effects, such as extreme weather in Europe, and further weakening is expected to strongly affect European weather as well as exacerbating sea-level rise at the east coast of North America.

In December also a new study – too late to be included in the assessments – was published by Thibodeau et al in Geophysical Research Letters, which further supports an unprecedented AMOC weakening during the past decades. The authors write:

In this study, we used geochemical evidence to highlight a slowdown in the North Atlantic Ocean circulation over the last century. This change appears to be unique over the last 1,500 years and could be related to global warming and freshwater input from ice sheet melt.

My view

Our regular readers know that one of my topics of interest is the stability of the Gulf Stream System – I’ve worked on this on and off for over 25 years, ever since finishing my PhD in physical oceanography. So let me add my own comments on the findings cited above.

First of all, while we don’t have regular direct measurements of the AMOC going back throughout the 20th Century, indirect evidence for an AMOC slowdown is not new. Dima and Lohmannalready concluded in 2010 that “the conveyor has been slowing down over the last seven decades” (where ‘conveyor’ refers to the AMOC).

Strangely, this finding was not discussed at all in the fifth IPCC report published in 2013. Therefore, the IPCC now finding that an ongoing slowdown is “more likely than not” is progress, yet still a very cautious statement. Likewise the statement about the “limited evidence” for the slowdown being human-caused is also very cautious. Why do I find this overly cautious?

The main points there are that an AMOC slowdown leads to a particular fingerprint pattern in sea surface temperature change – which is basically what Dima and Lohmann already identified, and this pattern is predicted by high-resolution climate models in response to rising greenhouse gases, and it is also found in the observations. There is no known alternative explanation for what might cause this fingerprint. That fingerprint is not subtle: it is so strong that the subpolar Atlantic is the world’s only region which has resisted global warming over the past hundred years and even has cooled down, reaching record low temperatures in 2015 when the globe as a whole was record-hot.

Although the AMOC slowdown fingerprint is most clearly seen in long-term sea surface temperature trends, it is also apparent in the 2018 temperature anomaly, despite a single year including a lot of short-term variability noise. No place on Earth had a larger cold anomaly than the subpolar Atlantic. Image: Berkeley Earth project.

In fact, the strength of this pattern and the conclusion that it corresponds to a 15% AMOC slowdown just matches the median slowdown found in the historic climate runs of the CMIP5 climate models – in other words, it is exactly what the models predict as a response to human-caused climate change. The physical mechanism is understood – how warming and ice melt weakens the AMOC (and that these factors are human-caused), and how an AMOC weakening causes the observed surface temperature fingerprint. In addition, there are several independent data sets that show this slowdown to be unprecedented for at least a millennium.

In IPCC jargon, personally I would therefore give the statement that the AMOC has slowed down since the early-mid 20th , and that this is at least partly human-caused, a “very likely” rating.

Links

If you doubt that the AMOC has weakened, read this

Stronger evidence for a weaker Atlantic overturning circulation

The underestimated danger of a breakdown of the Gulf Stream System

AMOC slowdown: Connecting the dots

What’s going on in the North Atlantic?

Stefan Rahmstorf is a physicist and oceanographer by training, Stefan Rahmstorf has moved from early work in general relativity theory to working on climate issues.

He has done research at the New Zealand Oceanographic Institute, at the Institute of Marine Science in Kiel and since 1996 at the Potsdam Institute for Climate Impact Research in Germany (in Potsdam near Berlin).

His work focuses on the role of ocean currents in climate change, past and present.

In 1999 Rahmstorf was awarded the $ 1 million Centennial Fellowship Award of the US-based James S. McDonnell foundation.

Since 2000 he teaches physics of the oceans as a professor at Potsdam University.

Rahmstorf is a member of the Academia Europaea and served from 2004-2013 in the GermanAdvisory Council on Global Change (WBGU). He was also one of the lead authors of the 4th Assessment Report of the IPCC. In 2007 he became an Honorary Fellow of the University of Walesand in 2010 a Fellow of the American Geophysical Union.

More information about his research and publication record can be found here.

First published in Real Climate

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