Arguments
The North Atlantic ocean current, which warms northern Europe, may be slowing
Posted on 22 August 2019 by greenman3610
This is a re-post from Yale Climate Connections.
A stubborn blue spot of cool ocean temperatures stands out like the proverbial sore thumb in a recent NASA image of the warming world – a circle of cool blue on a planet increasingly shaded in hot red.
A region of the North Atlantic south of Greenland has experienced some of its coldest temperatures on record in recent years, a cooling unprecedented in the past thousand years. What explains that anomaly?
Climatologist Michael Mann of Penn State University, in this month’s “This is Not Cool” video, explains that this phenomenon may be an indication that the North Atlantic current, part of a larger global ocean circulation, is slowing down.
This current played a role in the 2004 science fiction movie The Day After Tomorrow, a film that was “based on science, but greatly overblown” and that therefore “frustrated a lot of climatologists,” Jason Box of the Geological Survey of Denmark and Greenland points out.
Stefan Rahmstorf of the University of Potsdam, Germany, says this circulation – called the thermohaline circulation, but popularly known to many in the U.S. as “the Gulf Stream” – keeps northern Europe several degrees warmer than it would otherwise be at that latitude.
Although the movie’s disaster plot was pure science fiction, the consequences of a shutdown would be serious for agriculture – and for temperate weather – in northern Europe.
The current depends on the saltiness of the North Atlantic to create the sinking motion of water, that is, it’s the “pump” driving the current. Saltier water is heavier than fresh water.
As the Greenland ice sheet melts, large volumes of fresh water enter the North Atlantic and freshen the very salty sea water, slowing the “pump,” Jorgen Peder Steffensen of the University of Copenhagen explains in the video, produced for Yale Climate Connections by independent videographer Peter Sinclair of Midland, Michigan.
“That would make it terribly cold in Denmark, where I come from,” Steffensen says. “In principle, there’s no reason why Earth could not get warmer but still northern Europe and North America could get cold. Still, that area is not large compared to the global area.”
Melting ice from Greenland largely explains the freshening North Atlantic, Box agrees.
“We are 50 to a hundred years ahead of schedule with the slowdown of this ocean circulation pattern, relative to the models,” according to Mann. “The more observations we get, the more sophisticated our models become, the more we’re learning that things can happen faster, and with a greater magnitude, than we predicted just years ago.”
Related article and a related research study by J Hanson.
And here I was, under the impression, that the Gulf Stream was the result of the Coriolis force of the rotating Earth, that water diverted off the African continental shelf north to the Poles, where the water cooled, and returned again to the Tropics.
Now who would have guessed that Global Warming could change this?
Brian G Valentine @2
www.theguardian.com/environment/2018/apr/11/critical-gulf-stream-current-weakest-for-1600-years-research-finds
"The (gulf stream) current, known as the Atlantic Meridional Overturning Circulation (Amoc), carries warm water northwards towards the north pole. There it cools, becomes denser and sinks, and then flows back southwards. But global warming hampers the cooling of the water, while melting ice in the Arctic, particularly from Greenland, floods the area with less dense freshwater, weakening the Amoc current."
Salt doesn't freeze!
Yes, Brian, it is largely due to coriolis effect ... Salt and heat are simply other factors!
Hi, what about this ?
http://ocp.ldeo.columbia.edu/res/div/ocp/gs/
It says,
"The Gulf Stream and future climate change
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."
Is this true, and does that mean AMOC slowdown isn't a worry ?
thanks in advance.
Human 2932847 @6
Your link is one persons view. I dont know the answer, but periods of rapid cooling in the past in Europe coincided with the gulf stream slowing or stopping here,
nigelj @7
Well, being on his own doesn't make him wrong, and he draws on several other's work so I assume there is a body of work about this.
thanks for link
The sciencemag article describes conditions during glacial times, not the interglacial in which we are now. Presumably, conditions where quite different, and Europe was already ice-bound. The climate change referenced in the article wasn't the change from an interglacial into a glacial - day after tomorrow style - but variations in an already glacial kind of environment. So I don't know how much that work tells us about our current situation, I'd like to know more.
Human 2932847 @9, maybe there is other reseach I dont know, but plenty of research suggests the slowing gulf strean brings cool conditions to Europe, see my comment at 1 and the links.
I dont see why it matters that the gulf stream slowdown happened during an ice age which is just a cold period. The gulf stream slowed down and the ice age deepened further, thats the point apparent in the article. If slowing cools things then, why wouldn't it do the same now? I can't see a reason.
In addition, its non controversial that the gulf stream originates in the warm oceans off africa where the trade winds blow this water north ultimately, influenced by the coriolis effect and the thermohaline circulation. Its hard for me to see why slowing this down wouldn't cool the northern atlantic.
I guess its a question of how much and what other influnces there are, and I agree its all not 100% certain. The research you quote has to be right to the extent that warm air does travel north due to the basic global circulation system and global warming is affecting this pattern as well. But it looks like the conventional view is quite strong that a slowing gulf stream could cause a cooler climate in the north to some extent.
Hmm, well, that's Columbia university Earth Institute saying the effect of the Gulf Stream is way overestimated. If you disagree you could email and chew it over with them.
I would have thought that ice caps over Europe (and America) affect a lot of things like wind, ocean currents, albedo etc. There would be no North Sea or Baltic. Your link says that there was huge amounts of fresh water released from the American ice cap, more than from Greenland, but that wouldn't be available now. There would be different solar insolation - what else ? It just seems like a very different context to the current one.
They say
The determinants of North Atlantic regional climates
We showed that there are three processes that need to be evaluated:
I don't know anything about the middle one - but the first and third points must be affected by ice cap conditions over sea and land - of which there was more in the past.
Human 2932547 @11,
You are mistaken by the authorship you attribute to your citation which appears to be the same article that you linked-to @6 - 'Climate mythology:The Gulf Stream, European climate and Abrupt Change' This has no multiple authorship, just Richard Seager of the Lamont-Doherty Earth Observatory, Columbia University. nigelj @7 rightly describes it as a "one person's view."
Note the link on the webpage to a presentation by Seager which gives a better understanding of his argument and the modelling it is based on. (Both this presentation & the article appear to be over a decade old.) The presentation sets out the following conclusion:-
Your comment @9 (& repeated @11) concerning the article linked by nigelj @7 is perhaps overhasty. You brand the article as irrelevant because we are concerned with interglacial condiditons and the article concerened glacial conditions which could be fundamentally different. But do note the final lines of that article which runs with an 'on the other hand' rider, the ending:-
The only work Seager has had published directly** concerning the AMOC appears to be Delworth et al (2008) 'The Potential for Abrupt Change in the Atlantic Meridional Overturning Circulation'. Interestingly this skates over the warmth brought to northern latitudes by the AMOC.
Delworth et al (2008) does concur that there is measurable cooling resulting from a weaker AMOC but that the reduced strength of the AMOC will be small enough through the 21st century that this cooling will probably do no more than reduce the rate of AGW warming. "Even with the projected moderate AMOC weakening, it is still very likely that on multidecadal to century time scales a warming trend will occur over most of the European region downstream of the North Atlantic Current in response to increasing greenhouse gases, as well as over North America."
So it is a little strange that later in considering the impact of a collapsed AMOC, they write:-
So not one mention of temperature effects, or importantly, no mention of there being minimal temperature effects.
** Seager & Battist (2007) considers abrupt climate change and its causes which includes the AMOC.
@MA Roger
Thanks for your comments and points of order. I'm still not really seeing why the slowdown of the AMOC would be a cooling problem as everything I've read from Seager (granting the oddity you noted) indicates a continuity of warming due to AGW, doesn't it ?
What's the crux of disagreement between people who think cooling would be a problem, and people who don't ? Is it just factoring in Greenland melt ?
Human 2934527 @13,
The understanding that a weaker AMOC cools lands bordering the N Atlantic is pretty-much accepted by all. There are exceptions. one being Richard Seager from a decade ago. I would suggest his position back then was fuelled by the work from a few years earlier enumerating the poleward energy fluxes. From his presentation slide 12 (linked @12) “First hint that this may all be myth comes from using observations to estimate atmosphere and ocean heat transports.“ This basically shows that above 40N only a tenth of the north-bound energy is via the oceans, a finding that has pretty-much stood the test of time. Thus from Schmitt (2018) 'The Ocean's Role in Climate' its Fig 3:-
Being an ocean-based account, |Schmitt (2018) splits the atmosphere fluxes into dry and wet, but the proportion of the total within the ocean is small beyond 40N, not much more than 10 percent.
And that 'not much more than 10 percent' is almost all the AMOC which operates on a rather small bit of the planet so its influence over that bit of planet is quite large. (The number often given to the AMOC energy flux is ~0.4PW. Today's climate forcing due to AGW is [3.1Wm^-2 x 510M sq km=] 1.6PW acting over the entire world.) So I would suggest that Seager was barking up the wrong tree in his 2007 presentation & his modelling was presumably somehow flawed in concept.
What has been of interest to the science of recent years is getting the strength of the AMOC & its present rate of decline, of interest because AGW models suggest it will decline under AGW and this will cool the N Atlantic borderlands. The science is now starting to get results on the decline. (See for instance this RealClimate post from this January.)
As for the future the jury is still out as to how much the AMOC will slow, with some evidence still in play that it could actually collapse under AGW. Thus Lui et al (2017) 'Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate'. which modelled a 2xCO2 world.
And the overwhelming view is that slowing of the AMOC does lead to cooling of the European Atlantic coastal regions. A figure obtained for a RealClimate post from the researches of Lui et al is captioned:-
Temperature change in the winter months (DJF), 300 years after CO2 doubling in the experiment. Due to the almost completely extinct Atlantic flow, the northern Atlantic region has cooled significantly. Source: Wei Liu, with permission.
Of course, the dangers of relying on a single paper can be demonstrated by the paper Chen & Tung (2018) 'Global surface warming enhanced by weak Atlantic overturning circulation'. One of its two authors is Ka-Kat Tung who is happy to beat the denialist drum and, as I can testify, a very unreliable source of AGW research. So folk should take what Chen & Tung (2018) says with an oceanful of salt.
Thank you again for taking the time to help me. If I may say, though, I don't think it matters whether Seager is alone in his views or his papers are ten years old, he seems a perfectly sane and competent scientist, not a fringe flake or paid shill. So his academic minority status doesn't seem a problem to me. He also seems to draw his conclusions from other's work and has several co-authors. So I don't know why you say it's only his opinion.
Anyway, thanks for showing me the plurality of views, which may give an updated refutation of Seager's points, and you probably know what the predominant views are in the field of things AMOC so I'll stay open minded.
If it's out of date, maybe Columbia should look at updating it's website ?
Looking at Ka-Kat Tung's stuff online, he doesn't seem to be denying AGW, and it's not obvious why he's unreliable - certainly not to a layman such as I. He might be, he might not, I don't know.
Human 2932847 @15,
"I don't think it matters whether Seager is alone in his views or his papers are ten years old, he seems a perfectly sane and competent scientist, not a fringe flake or paid shill."
I agree Seager doesn't come across as a fringe flake or some sort of closet climate denialist, and his article you linked to seems ok, but it does matter that he is alone in his view. This doesn't mean hes wrong, but it does mean we need to be rather cautious, a point made by MAR rather clearly I would say with his example of Chen and Tung.
The little detail that stood out to me is western europe is much milder in climate than canada on the same latitude line, and this not well explained by the atmospheric heat transport, or oceans giving up heat in winter, but is easily explained by the gulf stream, so a slowing gulf stream would seem fairly large implications for Europe.
I don't know anything about why Chen and Tung are supposed to be bad - I'd never heard of them. And Seager has co-authors signing off on his papers, I count five co-authors on, "Is the Gulf Stream responsible for Europe's mild winters?", why don't these count ?
It may be a personal quirk, but if I'm told that someone should be ignored because they're on their own I just start to root for the underdog, which may be counterproductive to the resonable aim of eliminating unreliable outliers from consideration.
I don't want to get bogged down in lonesomeness and reputations of the scientists, please.
Human 2932847,
You ask that you "don't want to get bogged down in lonesomeness and reputations of the scientists" so let us ignore the denier K-K Tung.
You have done well tracking down Seager et al (2002) 'Is the Gulf Stream responsible for Europe's mild winters?'. Note that multiple authors is not a mark of good findings. A more normal gauge of the importance of a paper (but again not always a mark of good findings) is the reported number of citations and Seager et al (2002) has gained a healthy 313.
The findings of the paper are accepted in part (the cause of the E-W Atlantic temperature differential & the AMOC's impact on Arctic ice edge) but the controversial finding (the AMOC is not significant in warming W Europe) has had a remarkably long life (eg Buckley & Marsall 2015, Palter 2015) given the only up-date is Seager (2006) 'The Source of Europe’s Mild Climate:- The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth' which is more an opinion piece that a research paper.
Generally, as Seager (2006) sets out, most ignore this controversial finding (eg Wood et al 2003, Rahmstorf 2003), is last paper saying:-
Generally, the controversial finding has been left behind, including by Richard Seager. But as it is a part of the science, it should not be purged from the records (perhaps the suggestion @15).
Seager et al (2002) was easy enough to find, being in the reference list. I agree that having multiple authors isn't a guarantee of correctness, but it is a guarantee of multiple authorship, for sure, which was my point. Is it really so controversial if 313 people cite it ?
My point about changing the Columbia website is more a question - if it's so bad, and it's purpose is probably educational, then why is it still there ? I thought this might indicate it's merit in the eyes of academics, and widespread dissemination of the idea to students.
If it's really been supeceded then that's what I really want to know.
Human 2932847 @20,
Regarding Seager et al (2002), I think you were looking in the very short list within the 2007 weblog you linked-to @6 (& its associated presentation slide show). I failed to consider this list as I was not expecting a publication to precede the weblog/presentation. I was expecting such publication afterwards and was thus faced with a list over 200 papers long to trawl through.
Regarding a high number of citations, do note that the last publication I can see with Seager as co-author addressing directly the AMOC is Delworth et al (2008) mentioned @12 and that does not cite Seager et al (2002), that being a rather strong message. (And ditto Hurrell et al 2010 which also considers the Atlantic MOC with Seager as co-author.)
And of the four papers citating Seager et al (2002) which I mentioned @19, two of them did not accept the controversial aspect of Seager et al (2002) and the other two did make mention of the controversy but were in truth far from supportive. So can it be "really so controversial if 313 people cite it?" Yes it can indeed!
"If it's really been supeceded then that's what I really want to know." Even as no-more-than a controversial alternative, there is no indication within post-2015 literature that it now hasn't been.
Thank you for going to this effort for me, I'll follow through and read up on what has been presented, and hope that it is useful for others too.
I don't see much sign of controversy , though, apart from here.
This Scientific American summary goes into the debate over Seager's stuff. It's from 2013 and so has it been superceded ? It says -
"recent modeling studies with higher resolution of ocean currents suggest that fresh Arctic meltwater may pour mostly into currents that are more restricted to the coastlines and there-fore have less influence on the open ocean, where downwelling primarily occurs. Even if freshwater significantly affected the amount of waters downwelled in the North Atlantic, it turns out to be highly unlikely that this change would effectively shut down the Gulf Stream. A shutdown is unlikely because the path and the strength of the Gulf Stream depend largely on the speed and direction of the large-scale midlatitude winds."
Which doesn't sound like much of a threat.
What would be a good source for the latest theories about the Gulf Stream, AMOC etc where these questions are more settled ?
Neglected to link the Sci Am article - here http://fds.duke.edu/db/attachment/2372
Human 2932847 @23-25,
The article you cite appears a bit odd. Riser & Lozier (2013) 'Rethinking the Gulf Sream' describes itself presenting "Three new climate studies [that] indicate that our long-held belief about the Gulf Stream’s role in tempering Europe’s winters may not be correct. Yet the studies themselves do not agree." Yet these three are hardily "new" dating from 2002, 2009 & 2011. (the 2002 paper being our old friend Seager et al).
And such a finding wouldn't show "much sign of controversy"?
Riser & Losier (2013) does set out the two sides of the Seager controversy before pointing to that recent detailed modelling suggests it unlikely that meltwaters will "shut down" the AMOC.
I have tried to stress that the research is more interested in the fate of the AMOC and measuring the trends so far, rather than the effects of slowdown on Europe (such effects bring the issue we discuss here). There is detailed modelling (more recent that Riser & Lozier 2013) desribed in this RealClimate OP by Rahmstorf. This work is all about identifiying a fingerprint of AMOC strength in SST data. It does demonstrate the AMOC fingerprint caused by slowing. And regarding the cooling of Europe, note the cooling ocean temperatures up-wind of Europe.
And "where these questions are more settled"? This is an area of active research. To keep up with it, the "questions" of interest need some defining.
No I agree, there's some controversy there. I would call it disagreement, which has different social connotations, but you could use controversy too.
So, would it be fair to say that we've moved on from the simplistic idea of the Gulf Stream alone keeping Europe warm - which I believed until reading Seager's debunk - and air currents have a larger role to play, whether as much as Seager says or not ?
Because the picture I'm getting here is that a Gulf Stream slow down or stop would not be as catastrophic as once thought, but still problematic.
Human 2932847 @27,
We have been going round & round on this for some time now.
The idea of a "debunk" being delivered by Seager et al (2002) suggests there is no disagreement today, yet that is surely not correct. The results of Seager et al (2002) are at variance with other more recent work, for instance Lui et al (2017).
Consider the quantative results of Seager et al (2002). These were that the removal of the AMOC shows (his Fig 12) would drop "at most 2 degC ... south of 60ºN and about 3–6 degC ... between 60ºN and 70ºN." The major AMOC warming is seemingly pushed north to impact only Scandanavia. And if that is compared with the graphic from the Lui et al research up-thread @14, the UK is presented as 2ºC cooler and Scandanavia 3ºC cooler.
This however is not Lui et al concuring with Seager et al. Far from it.
The difference is that the values in the graphic @14 are in a 2xCO2 world on top of being without the AMOC. The northern hemisphere should therefore have warmed perhaps 3ºC and exhibit even more warming at the poles due to Arctic Amplification. Further, the temperature difference between the eastern coasts of the Pacific & the Atlantic are not "maintained" as in Seager eta al Table 1, but vastly increased.
And do note that both these works Seager et al (2002) & Lui et al (2017) pre-date Lozier et al (2019) 'A sea change in our view of overturning in the subpolar North Atlantic'. (There is coverage of the paper by CarbonBrief.) If the AMOC shutdown were shifted eastward as the measurements set out by Lozier et al (2019) suggest, it would surely make a bit of a difference to the resulting change in European winter temperature. But this is all an area of active research so definitive results should not be expected right away.
Finally, do note when proclaiming that an AMOC shutdown would "not be as catastrophic as once thought, but still problematic" - its impacts extend beyond a cooler Europe. The 1 metre SLR along the eastern US coast would be a bit more than "problematic" as would the drying of the Sahel.
Thank you for taking the time to help again.
Point taken about about the global effects of an AMOC shutdown. I was just thinking about Europe, or rather the bit where I live, just for simplicity.
Would a slowdown/shutdown of AMOC directly affect sea level on UK coasts ?
(BTW I wouldn't say a debunk means the final and definitive answer or end of disagreement. eg. Flat Earth has been debunked but the controversy and disagreement continue).
The AMOC is slowing is disputed by new research. Seems that the AMOC slowdown has reversed, and that it has incredibly large variability.
Reported in paper "Atlantic Meridional Overturning Circulation: Observed Transport and Variability"
Eleanor Frajka-Williams, et al. 2019. Open source, link below.
"From first transbasin measurements retrieved at 26◦N by the RAPID array, a number of startling results have emerged (summarized in Srokosz and Bryden, 2015): that the AMOC ranged from 4 to 35 Sv over a single year, had a seasonal cycle with amplitude over 5 Sv, and that the dip in 2009/10 of 30% exceeded the range of interannual variability found in climate models. The international efforts to measure the AMOC in the Atlantic at a range of latitudes have delivered new understanding of AMOC variability, its structure and meridional coherence. In situ mooring arrays form the primary measurements of the large-scale meridional circulation,[/b]."
AMOC has been above its historic mean for the last 5 years or so, see attached graph.
Figure caption: FIGURE 6 | A time series of AMOC transport (MOCρ ) at the OVIDE section (eastern subpolar gyre: Portugal to Cape Farewell) for 1993–2017, constructed from altimetry and hydrography. The gray line is from altimetry combined with a time-mean of Argo velocities; the green curve is low-pass filtered using a 2-year running mean. The black curve is from altimetry and Argo. Red circles are estimates from OVIDE hydrography with associated errors given by the red lines. The mean of the gray curve is given by the black dashed line (Updated from Mercier et al., 2015).
https://www.frontiersin.org/articles/10.3389/fmars.2019.00260/full
[DB] Please limit image widths to 450 to avoid breaking the formatting of the page. Hyperlinked URL.
Hefaistos, thank you for pointing out the Frajka-Williams et al. 2019 Review Article.
As a layman, I had last encountered the subject of AMOC speed, in the Bryden et al. 2005 (and later) paper . . . suggesting a 30% slowing over the period 1957~2004 (but they emphasized the uncertainties).
It is reassuring to see more extensive data, showing a high level of natural variability, with little or no trend over the past 24 years.
Hefaistos and Eclectic,
Realclimate often reviews articles on the AMOC. The author's list of Hefaistos link was very long and of competent people.