Update: The Rahmstorf paper is getting more important by the minute. Now there are not one, not two, but three protest articles by Anthony Watts in the space of a few hours. You'll be surprised (probably not) that what Anthony thinks refutes the study actually lends support to it. See below.
Added by Sou 6:30 am 26 March 2015
There is a new paper in Nature Climate Change by Stefan Rahmstorf and others, which is getting a lot of protest from deniers. This signifies that it is potentially an important paper. Stefan has written about it at realclimate.org.
What the research suggests is that the Atlantic Meridional Overturning Circulation (AMOC) may have slowed a lot in the late twentieth century (particularly between 1970 and 1990). This is attributed to the influx of fresh cold water, primarily from melting of the Greenland ice sheet.
The research draws upon other research. In particular (see references below):
- several years of work described in papers by Jason Box and colleagues. They have put together reconstructions of the Greenland ice going back decades - from 1840 to 2010.
- reconstructions of sea surface temperature based on work by Michael E Mann and co (2009).
The paper doesn't include measurements of the speed of the ocean circulation system. It is about the cold region and how there are indications that it is the coldest it's been in the past thousand years. From this it is inferred that the ocean circulation system itself is slowing. Here is a map from NOAA, which I've annotated to show the region that this research is focused upon.
That cold region near Greenland has been shown to be colder in recent years than it was in the Little Ice Age. (I don't hear deniers trumpeting this fact, oddly enough.)
The paper also refers to other published papers pointing to a decline in the AMOC in recent years. One way in which this research differs from other papers is the length of time that it considers. The method the scientists used was to construct what they called an AMOC index, "by subtracting the Northern Hemisphere mean surface temperature from that of the subpolar gyre". The subpolar gyre is the region where the cold patch is, near Greenland (it's just a region, not an oceanic circulation feature). As stated in the paper:
We thus assume that differences in surface temperature evolution between the subpolar gyre and the whole Northern Hemisphere are largely due to changes in the AMOC.
It's not really that simple. There is more to the paper than that. The authors list other lines of evidence, too. I've put some links to further reading below, if you can't access the paper itself.
The Gulf Stream and AMOC
The gulf stream system is the surface ocean current that carries warm water northward in the Atlantic, from around Florida in the eastern USA and meanders way up north. As I understand it, if the conveyor belt slows down, then this affects the gulf stream, so the warm water wouldn't travel as far.
This has been a concern for some time. For example, Wallace Broecker wrote about it in Science back in 1997. He said, referring to climate models and their inability at the time to properly simulate the ocean circulations at the proper locations:
Hence, it must be concluded that predictions made with these models regarding the response of the ocean's thermohaline circulation to the buildup of the atmosphere's greenhouse blanket could be in error. Even so, there is no reason to believe that these models are supersensitive to perturbations. It may well be just the opposite. So, until the next generation of models comes along, we must take seriously the predictions by the existing ones that a large buildup of greenhouse gases might cause the ocean's thermohaline circulation to collapse. We also must take seriously the possibility that such a collapse would have profound consequences to atmospheric operation and hence global climate. Of course, those who challenge the validity of the sizable greenhouse warmings predicted by these models will fault this result on the grounds that the warming and freshening of polar waters necessary for a thermohaline shutdown will not occur.
The commonly held view that if the circulation collapsed it would freeze up Europe is said to be incorrect, however. See this article by Richard Seager from Lamont-Doherty Earth Observatory of Columbia University. He writes how atmospheric circulation plays a greater role than the temperature of the oceans. At best it would offset some of the greenhouse warming in that region. He says:
A slowdown of the Gulf Stream and ocean circulation in the future, induced by freshening of the waters caused by anthropogenic climate change (via melting glaciers and increased water vapor transport into high latitudes) or simply by warming, would thus introduce a modest cooling tendency. This would leave the temperature contrast across the Atlantic unchanged and not plunge Europe back into the ice age or anything like it. In fact the cooling tendency would probably be overwhelmed by the direct radiatively-driven warming by rising greenhouse gases.
Impact in the southern hemisphere
If the AMOC weakens, then the warm water from the tropics would bunch up somewhere I expect. I'll have to do some more digging to see what the impact would be elsewhere. In Rahmstorf15 there was one throwaway line that I noticed, in a section discussion the Little Ice Age (my emphasis):
There is no sign in our index that a weak AMOC caused the LIA in the Northern Hemisphere; rather the data are consistent with previous findings that the LIA reflects a response to natural volcanic and solar forcing, and if anything this surface cooling strengthened the AMOC at least during the first part of the LIA. The fact that LIA coldness seems to have been even more pronounced in South America than in Europe further argues against a weak AMOC, as the latter would have warmed the Southern Hemisphere.
Anthony Watts protests and blames Michael Mann
Anthony Watts has already written two articles protesting the study (archived here and here). Two protest articles isn't a lot by denier standards. (Remember Marcott13 and the 30 plus WUWT protest articles!) However the fact that Anthony returned from his months-long vacation to write them, and that he wrote them back to back, suggests that the paper worries him rather a lot. It's important to deniers.
It also shows that he realises that climate science deniers typically only recognise the names of a very small number of climate scientists. Anthony's protests are focused on the lead author, Stefan Rahmstorf and one of the co-authors, Michael Mann. He ignores the Greenland ice expert Jason Box and all the other co-authors.
You can ignore Anthony's second protest. He reckons that all it takes to refute a paper in Nature Climate Change is two lines in a newspaper. No need for any research, let alone a published peer-reviewed paper or comment. Anthony relies on an article by one of his science deniers, who trumpets one short quote from another oceanographer, Martin Visbeck, at the tail end of a German newspaper article on the subject, with no supporting evidence, as proof that the research has been "debunked". The quote being (courtesy of Google translate):
The climate scientist Martin Visbeck from GEOMAR Helmholtz Centre for Ocean Research Kiel looks Rahmstorf's interpretation of the results critically: "The concentration of study on the part of the subpolar Atlantic and the spectral analysis are interesting," he says. But there were also other estimates of the AMOC, pointing to a very different course. The work does not provide strong evidence for the development of the AMOC during the past fifty years.
The gist of Anthony's first protest (which he also grandiosely calls a "debunking") is a recent paper by Tom Rossby and colleagues, which indicated there has not been a slowdown in the gulf stream over the past twenty years. However that research doesn't necessarily conflict with the Rahmstorf paper. For one thing it was using measurements from the gulf stream in the mid-north Atlantic, far from the sub-polar gyre. More particularly the observations were taken between 1992 and 2012. From the Rossby paper:
Between 1992 and 2012, we completed 20 years of operations measuring currents between the U.S. East Coast and Bermuda, spanning the outer shelf, the Slope Sea (between the continental shelf and the Gulf Stream), the Gulf Stream, and the Sargasso Sea.
Remember that the Rahmstorf paper was about a twentieth century slowdown, particularly between 1970 and 1990, before Thomas Rossby started taking measurements. The Rahmstorf paper states in the abstract:
Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered.
Tom Rossby doesn't preclude any future slowdown. In fact it's probably concern about the possibility of a slowdown in the ocean circulation that prompted his research. In the introduction he wrote, referring to earlier research including Broecker's 1997 paper (my emphasis):
In more recent years, as knowledge of ocean dynamics and modeling skills have improved, it is more widely thought that the MOC might weaken to some degree, but neither shut down nor increase in strength [Intergovernmental Panel on Climate Change Fourth Assessment Report, 2007; Yin et al., 2009]. While comforting, considerable uncertainty remains, especially given indications of increased ice melt on Greenland [e.g., Jungclaus et al., 2006], hence the need for continued and improved observation and further research [e.g., Srokosz et al., 2012].
I fully expect Rahmstorf15 will generate some discussion and controversy in scientific circles. I'd take notice of that and ignore the idiocy of deniers, who reject everything and anything to do with climate as a matter of course. I also expect that there will continue to be a lot of research on the subject. The big worry will be the impact of the melting of Greenland. That will need to be closely monitored.
Update - A third protest from WUWT
Anthony Watts has written a third article (archived here) protesting the Rahmstorf paper. This latest one is about a NASA study, which, like the Rossby paper, found that the AMOC showed no significant slowing between 2002 and 2009, and that it "sped up about 20 percent from 1993 to 2009".
In fact, this lends support for the Rahmstorf study, which also found that there was a recovery post-1990. Here is a chart of the "AMOC index" from Rahmstorf15 (click to enlarge):
Furthermore, Rahmstorf cites the NASA research that deluded Anthony thinks refutes the work (the 19 at the end is a reference to the NASA study) - my emphasis and h/t David Sanger:
The twentieth century shows a gradual decline in the AMOC index, followed by a sharp drop starting around 1970 with a partial recovery after 1990 (discussed further below). This recovery is consistent with the finding of an AMOC increase since 1993 based on floats and satellite altimeter data 19.
BTW -- Anthony got so excited that he thought the NASA press release was new. He was much chagrined to be told by Andrew Freedman that it was from 2010!
[Added by Sou 6:30 am 26 March 2015]
From the WUWT comments
Best to ignore most of the comments. They are mostly scientist-bashing, particularly Mann-bashing (a favourite sport of the denialati), with almost no comments on the science itself. I did come across a gem that has to be shared, and not just because this paper was about ocean circulation not rising sea levels:
Curious George thinks seas cannot rise because there is a limited water supply on our planet :)
March 24, 2015 at 2:13 pm
Dr. Rahmstorf developed a model of a sea level rise, ignoring an inconvenient fact of a limited water supply on our planet.
You can read more about this research at realclimate.org and on Jason Box's blog. Both articles are full of links to further reading. Chris Mooney has written an article about it, too.
There's a nice blog article by Kaitlin Alexander at ClimateSight about AMOC and the gulf stream. She wrote it a couple of years ago.
There's also an easy-to-follow article about the gulf stream system at the Max Planck Institute for Meteorology website.
Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford and E.J. Schaffernicht, "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation", Nature Climate Change, 23 MARCH 2015 | DOI: 10.1038/NCLIMATE2554
Box, Jason E., and William Colgan. "Greenland ice sheet mass balance reconstruction. Part III: Marine ice loss and total mass balance (1840–2010)." Journal of Climate 26, no. 18 (2013): 6990-7002. doi: http://dx.doi.org/10.1175/JCLI-D-12-00546.1 (pdf here)
Mann, Michael E., Zhihua Zhang, Scott Rutherford, Raymond S. Bradley, Malcolm K. Hughes, Drew Shindell, Caspar Ammann, Greg Faluvegi, and Fenbiao Ni. "Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly." Science 326, no. 5957 (2009): 1256-1260. DOI: 10.1126/science.1177303
Sherwood, Owen A., Moritz F. Lehmann, Carsten J. Schubert, David B. Scott, and Matthew D. McCarthy. "Nutrient regime shift in the western North Atlantic indicated by compound-specific δ15N of deep-sea gorgonian corals." Proceedings of the National Academy of Sciences 108, no. 3 (2011): 1011-1015. doi: 10.1073/pnas.1004904108 (open access)
Broecker, Wallace S. "Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance?." Science 278, no. 5343 (1997): 1582-1588. DOI: 10.1126/science.278.5343.1582 (pdf here)
Rossby, T., C. N. Flagg, K. Donohue, A. Sanchez‐Franks, and J. Lillibridge. "On the long‐term stability of Gulf Stream transport based on 20 years of direct measurements." Geophysical Research Letters 41, no. 1 (2014): 114-120. DOI: 10.1002/2013GL058636