Michael Mann and Stefan Rahmstorf claim the Gulf Stream is slowing due to Greenland ice melt, except reality says otherwise

UPDATED – see below

From your “Day after Tomorrow” department (where a slowing Gulf Stream turned NYC into an icebox) comes this claim from the bowels of Mannian Science. Unfortunately, it looks to be of the caliber of Mann’s Hockey Schtick science.

As WUWT reported on a peer reviewed paper last year, H. Thomas Rossby says: URI oceanographer refutes claims that climate change is slowing pace of Gulf Stream saying in a paper published in Geophysical Research Letters:

“The ADCP measures currents at very high accuracy, and so through the repeat measurements we take year after year, we have a very powerful tool by which to monitor the strength of the current,” said Rossby. “There are variations of the current over time that are natural — and yes, we need to understand these better — but we find absolutely no evidence that suggests that the Gulf Stream is slowing down.”

Of course, Rahmstorf and Mann don’t list Rossby’s study in their references, nor seem to use the “highly accurate” ADCP data. Instead they use a model along with [proxies, reconstructions, and] the highly interpolated GISS data to come to the conclusions they want. So, it isn’t surprising they are chasing phantoms in their study. They claim (in Figure 1 from their paper) that this cold spot south of Greenland is caused by meltwater from Greenland and it is evidence of a slowed circulation:

I find it interesting that they note “…that the second cooling patch in central Africa is in a region of poor data coverage and may be an artefact of data inhomogeneities.” Yet somehow that cooling patch south of Greenland is free of such problems in the same GISS dataset. Go figure.

And then there’s this other problem; Greenland’s ice mass seems to be on the increase so far this year and above the 1990-2011 mean:

Source: http://www.dmi.dk/uploads/tx_dmidatastore/webservice/b/m/s/d/e/accumulatedsmb.png

Accessed from http://www.dmi.dk/en/groenland/maalinger/greenland-ice-sheet-surface-mass-budget/

Maybe this is the new Mannian science, wherein global warming causes cooling, and melting causes more ice accumulation. (h/t to Tom McClellan)

Of course, given that Stephan Rahmstorf thinks the “Day After Tomorrow” was just peachy, one wonders if this study isn’t just a embellishment of his movie review:

(Via Wikipedia) However, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, expert for thermohaline ocean circulation and its effects on climate, was impressed how the script writer Jeffrey Nachmanoff was well informed about the science and politics of global climate change after the talk with him at the preview of the film in Berlin. He stated: “Clearly this is a disaster movie and not a scientific documentary, the film makers have taken a lot of artistic license. But the film presents an opportunity to explain that some of the basic background is right: humans are indeed increasingly changing the climate and this is quite a dangerous experiment, including some risk of abrupt and unforeseen changes. After all – our knowledge of the climate system is still rather limited, and we will probably see some surprises as our experiment with the atmosphere unfolds. Luckily it is extremely unlikely that we will see major ocean circulation changes in the next couple of decades (I’d be just as surprised as Jack Hall if they did occur); at least most scientists think this will only become a more serious risk towards the end of the century. And the consequences would certainly not be as dramatic as the ‘super-storm’ depicted in the movie. Nevertheless, a major change in ocean circulation is a risk with serious and partly unpredictable consequences, which we should avoid. And even without events like ocean circulation changes, climate change is serious enough to demand decisive action. I think it would be a mistake and not do the film justice if scientists simply dismiss it as nonsense. For what it is, a blockbuster movie that has to earn back 120 M$ production cost, it is probably as good as you can get. For this type of movie for a very broad audience it is actually quite subversive and manages to slip in many thought-provoking things. I’m sure people will not confuse the film with reality, they are not stupid – they will know it is a work of fiction. But I hope that it will stir their interest for the subject, and that they might take more notice when real climate change and climate policy will be discussed in future.” Source: http://www.pik-potsdam.de/~stefan/tdat_review.html

But…

In 2008, Yahoo! Movies listed The Day After Tomorrow as one of Top 10 Scientifically Inaccurate Movies. The film was criticized for depicting several different meteorological phenomena occurring over the course of hours, instead of the possible time frame of several decades or centuries.

UPDATE: I’ve added figures 5 and 6 from the Mann and Rahmstorf paper below.

When actual Gulf Stream measurement data (the ADCP data cited by Rossby 2014) is available, why would Mann and Rahmstorf use proxies? And why try to say that temperature is the indicator, when you have actual speed data? The obtuseness boggles the mind.

Further, note Figure 6, they claim that discharge exceeds gain, which looks like Mann’s proverbial “hockey stick” and this is based on Box and Colgan (2010) data, which is their citation #33. They claim data all the way back to 1850, which is quite some feat since as far as I know, no actual whole Greenland ice data was measured until the International Geophysical year of 1958, such as:

Bauer, A., Baussart, M., Carbonnell, M., Kasser, P., Perroud, P. and Renaud, A. 1968. Missions aériennes de reconnaissance au Groenland 1957-1958. Observations aériennes et terrestres, exploitation des photographies aeriennes, determination des vitesses des glaciers vělant dans Disko Bugt et Umanak Fjord. Meddelser om Grønland 173(3), 116 pp.

And further, previous papers from Jason Box only start with data at 1958:

It turns out Box and Colgan (2010) is a reconstruction, not actual measurement data.

However, even more puzzling, when you look at the Box and Colgan (2010) Figure 5 from their paper (free PDF here) the mass balance loss and accumulation shows no such “hockey stick” shape.

One wonders if Mann didn’t apply his “special Mannomatic math“, as he did to his “hockey stick” to the Box and Colgan (2010) reconstructed data to get the large divergence between accumulation and loss we see in Mann and Rahmstorf’s Figure 6.

More importantly though, the Box and Colgan (2010) data isn’t actual measurement data, it is a reconstruction based on some data, and some guesses:

According to our reconstruction, TMB has been positive for 39% of the 1840–2010 period (see mass balance surplus areas in Fig. 5). The positive decade-scale mass balance phases correspond with periods of low melting and runoff. For example, from 1970 to 1985 a positive mass balance phase corresponds to a period of enhanced sulfate cooling (Wild et al. 2009) pronounced along west Greenland (Rozanov et al. 2002; Box et al. 2009). Mass budget surpluses can also be produced by high accumulation years, even occasionally despite relatively high runoff (e.g., 1996).

The reconstructed TMB values are compared 1) with those from the surface mass balance in Part II minus the Rignot et al. (2008, 2011)LM for 20 samples spanning 1958–2009 and 2) with the independent GRACE data spanning 2003–10 after Wahr et al. (2006) (Fig. 6). We find RMS errors of 31Gtyr for dataset 1 and 69Gtyr in comparison with dataset 2 (Table 1).

So, clearly, there’s no actual data prior to 1958. The Mann and Rahmstorf paper misleads the reader by not making this clear.

Duke University physicist Robert G. Brown, in comments noted:

Seriously — they found “good evidence” that such a slowing is occurring, only the actual evidence, consisting of the measured speed of the Gulf Stream itself, shows no such thing?

The cognitive dissonance involved is stupifying.

Indeed. Mann and Rahmstorf eschew reality for models and reconstruction. They live in an incestuous climate world of their own making.

Here’s the press release:

Atlantic Ocean overturning found to slow down already today

Potsdam Institute for Climate Impact Research (PIK)

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.

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Article: Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): Evidence for an exceptional 20th-Century slowdown in Atlantic Ocean overturning. Nature Climate Change (online) [DOI:10.1038/nclimate2554]

Weblink to the article: http://dx.doi.org/10.1038/nclimate2554

Abstract