A few weeks ago, we’ve argued in a paper in Nature that the Atlantic overturning circulation (sometimes popularly dubbed the Gulf Stream System) has weakened significantly since the late 19th Century, with most of the decline happening since the mid-20th Century. We have since received much praise for our study from colleagues around the world (thanks for that). But there were also some questions and criticisms in the media, so I’d like to present a forum here for discussing these questions and hope that others (particularly those with a different view) will weigh in in the comments section below.
Exhibit #1, and the prime observational finding, is a long-term cooling trend in the subpolar Atlantic – the only region in the world which has cooled while the rest of the planet has warmed. This ‘cold blob’ or ‘warming hole’ has been shown in IPCC reports since the 3rd assessment of 2001; it is shown in Fig. 1 in a version from the last (5th) IPCC report. In fact it is Figure 1 of the Summary for Policy Makers there – you can’t get more prominent than that.
Fig. 1 Observed temperature trends since the beginning of the 20th Century (Figure SPM1 of the last IPCC report).
I think there is a consensus that this is a real phenomenon and can’t be explained away as a data problem. According to NOAA, 2015 was the coldest year in this region since record-keeping began in 1880, while it was the hottest year globally. The key question thus is: what explains this cold blob?
In 2010, my colleagues Dima and Lohmann from Bremen were the first (as far as I know – let me know if you find an earlier source) to suggest, using sea surface temperature (SST) pattern analyses, that the cold blob is a tell-tale sign of a weakening AMOC. They wrote that
“the decreasing trend over the last seven decades is associated to the weakening of the conveyor, possibly in response to increased CO2 concentrations in the atmosphere”
(with ‘conveyor’ they refer to the AMOC). One of several arguments for this was the strong anti-correlation pattern between north and south Atlantic which they found using canonical correlation analysis and which is the well-known see-saw effect of AMOC changes.
I have since become convinced that Dima and Lohman were right. Let me list my main arguments upfront before discussing them further.
- The cold blob is a prediction come true. Climate models have long predicted that such a warming hole would appear in the subpolar Atlantic in response to global warming, due to an AMOC slowdown. This is seen e.g. in the IPCC model projections.
- There is no other convincing explanation for the cold blob. There is strong evidence that it is neither driven by internal atmospheric variability (such as the North Atlantic Oscillation, NAO) nor by aerosol forcing.
- A range of different data sets and analyses suggest a long-term AMOC slowdown.
- Claims that the slowdown is contradicted by current measurements generally turn out to be false. Such claims have presented apples-to-oranges comparisons. To the contrary, what we know from other sources about the AMOC evolution is largely consistent with the AMOC reconstruction we presented in Nature.
Let us look at these four points in turn.
A climate prediction come true
The following graph shows climate projections graph from the last IPCC report.
Fig. 2 Global warming from the late 20th Century to the late 21st Century (average over 32 models, RCP2.6 scenario) – Figure SPM8a of the IPCC AR5.
The IPCC writes that “hatching indicates regions where the multi-model mean is small compared to natural internal variability (i.e., less than one standard deviation of natural internal variability in 20-year means.)” The subpolar North Atlantic stands out as the only region lacking significant predicted warming even by the late 21st Century. The 4th IPCC report included a similar graph (Fig. TS28).
In our paper we have analysed the ‘historic’ runs of the CMIP5 climate models (i.e. those from preindustrial condition to the present) and found that the observed ‘cold blob’ in this region is consistent with what the models predicted, with the amount of cooling in the models depending mainly on how much the AMOC declines (see below). In the mean of the 13 models we examined (Fig. 5 of our paper), the downward trend of the AMOC index is -0.33 °C per century, in the observations we found -0.44 °C per century. (Our AMOC index simply consists of the difference between the surface temperatures of the subpolar Atlantic and the global ocean). The models on average thus predicted three quarters of the decline that the observational data indicate. (In fact most models cluster around the observed decline, but three models with almost zero AMOC decline cause the underestimation in the mean.)
Is there an alternative explanation?
If the ocean temperature in any region changes, this can only be due to a change in heat supply or loss. That can either be a change in heat flow via ocean currents or through the sea surface. Thus the subpolar Atlantic can either have cooled because the ocean currents are bringing less heat into this region, or alternatively because more heat is being lost to the atmosphere. So how do we know which of these two it is?
First, we can analyze the heat flux from ocean to atmosphere, which can be calculated with standard formula from the sea surface temperature and weather data. Halldór Björnsson of the Icelandic weather service has done this and presented the results at the Arctic Circle conference 2016 (they are not published yet). He showed that the short-term temperature fluctuations from year to year correlate with the heat exchange through the sea surface, but that this does not explain the longer-term development of the ‘cold blob’ over decades. His conclusion slide stated:
Surface heat fluxes did not cause the long term changes and are only implicated in the SST variations in the last two decades. Long term variations are likely to be oceanic transport but not due to local atmospheric forcing.
That’s exactly what one expects. Weather dominates the short-term fluctuations, but the ocean currents dominate the long-term development because of the longer response time scale and “memory” of the ocean.
Nevertheless some have suggested that the main mode of atmospheric variability in the north Atlantic, the North Atlantic Oscillation or NAO, might have caused the “cold blob”. In our paper we present a standard lagged correlation analysis of the NAO with the “cold blob” temperature (in form of our AMOC index). The result: there is indeed a significant correlation of the NAO with subpolar Atlantic surface temperatures. But on the longer time scales of interest to us (for 20-year smoothed data), changes in the sea surface temperature lead the NAO changes by three years. We conclude that changes in sea surface temperatures cause the changes in NAO and not vice versa. (And we’re certainly not the first to come to this conclusion.)
And a third point: in summer, the effect of heat flow through the sea surface should dominate, in winter the effect of ocean currents. That is because the well-mixed surface layer of the ocean is thin, so only the uppermost part of the ocean heat transport gets to affect the surface temperature. But the thin surface layer still feels the full brunt of atmospheric changes, and even stronger than in winter, because the thermal inertia of the thin summer surface layer is small. In our paper we analysed the seasonal cycle of the temperature changes in the subpolar Atlantic. The cooling in the “cold blob” is most pronounced in winter – both in the climate model (where we know it’s due to an AMOC slowdown) and in the observations. That yet again suggests the ‘cold blob’ is driven from the ocean and not the atmosphere.
There is another well-known mode of Atlantic temperature variability known as AMO, which correlates strongly with our AMOC index. Its established standard explanation in the scientific literature is… variations in the AMOC. (The NAO and AMO connections are discussed in more detail in the Extended Data section of our paper.)
There may be the possibility that some ocean heat transport change other than an AMOC change could be responsible for the ‘cold blob’ in the subpolar Atlantic, and I wouldn’t argue that we understand the ocean current changes in detail. But if you take a ‘big picture’ view, it is a fact that the AMOC is the dominant mechanism of heat transport into the high-latitude Atlantic, and the region that has cooled is exactly the region that cools in climate models when you slow down the AMOC. We have analysed the ensemble of CMIP5 “historic” model simulations for the past climate change from 1870 to 2016. For each of these model runs, we computed the AMOC slowdown over this time as diagnosed by our AMOC index (i.e. based on subpolar ocean surface temperatures) as well as the actual AMOC slowdown (which we know in the models, unlike in the real world.) The two correlate with a correlation coefficient R=0.95. Thus across the different models, differences in the amount of AMOC slowdown nearly completely explain the differences in subpolar Atlantic temperatures. If you doubt that what the temperatures in the Atlantic are telling us is a story of a slowing AMOC, you doubt not only that the high-resolution CM2.6 climate model is correct, but also the entire CMIP5 model ensemble.
A range of different data sets and analyses suggest a long-term AMOC slowdown
A number of different SST data sets and analyses support the idea of the AMOC slowdown. That is not just the existence of the subpolar cooling trend in the instrumental SST data. It is the cross-correlation with the South Atlantic performed by Dima and Lohmann. It is the fact that land-based proxy data for surface temperature suggest the cold blob is unprecedented for over a millennium. It is the exceptional SST warming off the North American coast, an expected dynamical effect of an AMOC slowdown, and strong warming off the west coast of southern Africa (see Fig. 1 in my previous post).
In addition we have the conclusion by Kanzow et al. from hydrographic sections that the AMOC has weakened by ~ 10% since the 1950s (see below). And the Nitrogen-15 data of Sherwood et al. indicating a water mass change that matches what is predicted by the CM2.6 model for an AMOC slowdown. And the subsurface Atlantic temperature proxy data published recently by Thornalley et al. Plus there is work suggesting a weakening open-ocean convection. And finally, our time evolution of the AMOC that we proposed based on our AMOC index, i.e. based on the temperatures in the cold blob region, for the past decades matches evidence from ocean reanalysis and the RAPID project. Some of these other data are shown together with our AMOC index below (for more discussion of this, see my previous post).
Fig. 3 Time evolution of the Atlantic overturning circulation reconstructed from different data types since 1700. The scales on the left and right indicate the units of the different data types. The lighter blue curve was shifted to the right by 12 years since Thornalley found the best correlation with temperature with this lag. Our index is the dark blue line starting in 1870. Graph: Levke Caesar.
Do measurements contradict our reconstruction?
Measuring the AMOC at a particular latitude in principle requires measuring a cross-section across the entire Atlantic, from surface to bottom. There are only two data sets that aspire to measure AMOC changes in this way. First, the RAPID project which has deployed 226 moored measuring instruments at 26.5 ° North for that purpose since 2004. It shows a downward trend since then, which closely matches what we find with our temperature-based AMOC index. Second is the work by Kanzow et al. (2010) using results of five research expeditions across the Atlantic between 1957 and 2004, correcting an earlier paper by Bryden et al. for seasonal effects and finding a roughly 10% decline over this period (in terms of the linear trend of these five data points).
Some other measurements cover parts of the overturning circulation, and generally for short periods only. For 1994-2013, Rossby et al. (2013) – at the Oleander line between 32° and 40° North – found a decrease in the upper 2000m transport of the Gulf Stream by 0.8 Sverdrup (a Sverdrup is a flow of a million cubic meters per second). It is important to realize that the AMOC is not the same as the Gulf Stream. The latter, as measured by Rossby, has a volume flow of ~90 Sverdrup, while the AMOC has a volume flow of only 15-20 Sverdrup. While the upper northward branch of the AMOC does flow via the Gulf Stream, it thus only contributes about one fifth to the Gulf Stream flow. Any change in Gulf Stream strength could thus be due to a change in the other 80% of Gulf Stream flow, which are wind-driven. The AMOC does however provide the major northward heat transport which affects the northern Atlantic climate, because its return flow is cold and deep. Most of the Gulf Stream flow, in contrast, returns toward the south near the sea surface at a similar temperature as it flowed north, thus leaving little heat behind in the north.
Likewise for 1994-2013, Roessler et al. (2015) found an increase of 1.6 Sv in the transport of the North Atlantic Current between 47° and 53° North. This is a current with a mean transport of ~27 Sverdrup, 60% of which is subtropical waters (i.e., stemming from the south via the Gulf Stream). For this period, our reconstruction yields an AMOC increase by 1.3 Sv.
For 1994-2009, using sea-level data, Willis et al. (2010) reconstructed an increase in the upper AMOC limb at 41°N by 2.8 Sv. For this period, our reconstruction yields an AMOC increase by 2.1 Sv.
Finally, the MOVE project measures the deep southward flow at 15° North. This is a flow of ~20 Sverdrup which can be considered the sum of the north Atlantic overturning circulation plus a small component of returning Antarctic Bottom Water (see Fig. 1 in Send et al. 2011). The following graph shows all these measurements together with our own AMOC index (Caesar et al 2018).
Fig 4. Our AMOC index in black, compared to five different measurement series related more or less strongly to the AMOC. The dashed and dotted linear trends of our index can be directly compared to the linear trends over corresponding data intervals. The solid black line shows our standard smoothed index as shown in our paper and in Fig. 3. Graph by Levke Caesar.
First of all, it is clear that these data contain a lot of year-to-year variability – which doesn’t correlate between the different measurements and for our purposes is just ‘noise’ and not a climate signal. That is why for our index we generally only consider the long-term (multidecadal) changes in SST to reflect changes in the AMOC. Thus, we need to look at the trend lines in Fig. 4.
Given that even these trends cover short periods of noisy data sets and thus are sensitive to the exact start and end years, and that lags between the various parts of the system may be expected, all these trends are surprisingly consistent! At least I don’t see any significant differences or inconsistencies between these various trends. Generally, the earlier trends in the left part of the graph are upward and the later trends going up to the present are downward. That is fully consistent with our reconstruction showing a low around 1990, an AMOC increase up the early 2000s and then a decline up to the present (compare Fig. 3).
Claims that any of these measurements are at odds with our index or even disprove the long-term AMOC decline are thus baseless (and thus rightly fit into Breitbart News where they were raised by the notorious James Delingpole).
One interesting question for further research is how the AMOC in the Atlantic is linked to the exchange with the Nordic Seas across a line between Greenland, Iceland and Scotland. In our 2015 paper we showed a model result suggesting an anti-correlation of these overflows with the AMOC, and our new paper suggests a similar thing: a warm anomaly off Norway coinciding with the cold anomaly in the subpolar Atlantic, both in the high-resolution CM2.6 model and the observations.
So, while there is obviously the need to understand the ocean circulation changes in the North Atlantic in more detail, I personally have no more doubts that the conspicuous ‘cold blob’ in the subpolar Atlantic is indeed due to a long-term decline of the northward heat transport by the AMOC. If you still have doubts, we’d love to hear your arguments!
What explains the cold blob is very simple:
1) Historical global deforestation increases daily thermal amplitude of large land surface.
2) The increase in daily thermal amplitude rises the intensity of atmospheric circulation.
3) As a result, more heat arrives to the North Pole within the polar cell.
4) As a consequence, the arctic ice is melting, thus the water gets colder within that region…
Thus, we can perceive a cold blob near the arctic zone.
[Response: Try getting that theory into the peer-reviewed literature… Stefan]
Good stuff! But I would like your explanation for how you know that the cold blob is not caused by additional melting from the Greenland ice sheet, please.
[Response: You mean by cold meltwater from Greenland flowing in? You can work that out from a simple heat budget calculation. The amount is far too small to matter for the large-scale sea surface temperature, but enough to matter for sea surface salinity. -Stefan]
Does this mean the AMOC is now running amok? Sorry, couldn’t resist!
An excellent review thank you. In bite sized chucks I could parse reasonably enough to feel that I got the essence of the subject matter. You explained quite well how multiple lines of evidence support and confirm each other plus how various model simulations match the observation data over time scales. This all adds greater overall meaning of the individual chunks. (is chunks a reasonable alternative to whatever the proper scientific term should have been? -joking only) Plus good relevant refs with links.
I would make only one suggestion about references – please delete the following sentence:
“Claims that any of these measurements are at odds with our index or even disprove the long-term AMOC decline are thus baseless (and thus rightly fit into Breitbart News where they were raised by the notorious James Delingpole).”Why? Because it adds nothing of value to the article, is irrelevant to the subject matter under discussion, is an unnecessary distraction to readers and is a departure from what was otherwise solid and on point. You do not (and should not) care nor give a moment of your attention to what anything people like a Delingpole or news media site publish about your quality work. Fruit flies do not get a say in your work so neither do they. Seriously edit it out. Toss it in the bin where it belongs. Seemingly a minor adjustment it will instead have a profound unexpected effect making your article even better and more compelling than it already is.
Loved this line of thinking being expressed so succinctly:
“I have since become convinced that Dima and Lohman were right.” 11 very powerful words.
This one too:
“There is no other convincing explanation for the cold blob.”
And particularly your concluding statement made with authority and conviction:
“I personally have no more doubts that the conspicuous ‘cold blob’ in the subpolar Atlantic is indeed due to a long-term decline of the northward heat transport by the AMOC.
Excellent.
Which brings me to the two questions your article’s conclusions naturally raises in my mind which I’d love to hear your own answers to.
1) So what? Knowing this, what is the most important meaning you now take away from this work?
2) Knowing that “the ‘cold blob’ in the subpolar Atlantic is indeed due to a long-term decline of the northward heat transport by the AMOC then what is the most important meaning/s I and the general public should understand about this convincing evidence?
(no need to rush an answer – you’re busy enough as it is – may be the basis of the next article when time permits? Your call. )
Thank you again Stefan. A great read – very thought provoking. I believe you 100%! :-)
[Response: Good question that I often ask myself – should we call out systematic misinformers like Breitbart, or just ignore them? What do other readers think? -Stefan]
Very interesting post.
How do changes in the AMOC affect the amount of heat delivered to the Greenland Ice Sheet? Could changes in the AMOC result in lower (higher) heat transport and slowing (increasing) of melting of the great ice sheet and/or the melting of sea ice in the Arctic? I recall seeing claims that reduction in surface flow has little affect on deeper heat flow.
Can you link an article or two that describe the affect of changes in the AMOC to the cryosphere in the Arctic?
You mention that the warning of the North Atlantic is expected from the AMOC decline. Does the extent and degree of change correlate as well? Following Earth Nullschool, there was a rather sudden jump of the SSTA in May in a very large, already hot area of the North Atlantic from about 2C to 4C. It has since backed off a bit to about 3.3C, but is still quite striking. Climate Reanalyzer shows similar, though the detail is less. It also shows areas of SSTA anomalies in the 4C range.
JRClark asks for meaning/takeaway.
Well. If the AMOC declines by 10%, wouldn’t one expect a similar reduction in the ocean’s CO2 uptake? (And deep ocean oxygen and surface water nutrients (ignoring lag and agricultural runoff and the resulting dead zones)
[Response: You’d expect some decline, but not by as much as the AMOC declines because the Southern Ocean takes up a lot of CO2 as well. -Stefan]
Excellent article, thank you.
Does the North Atlantic cold spot have anything to do with the recent brutal winters in eastern North America? I ask because there are certain very important people who do not understand the difference between the climate and the weather. It might help (or might not) if there were a partial explanation for the weather.
[Response: Don’t think so. Much more likely these are due to the polar vortex becoming unstable in response to the strong Arctic warming, as our PhD student Marlene Kretschmer found out. See e.g. the WaPo article on her work. -Stefan]
Stefan, have you read Kim Stanley Robinson’s climate fiction, which includes the notion of restarting the AMOC by hauling vast quantities of salt — shiploads — to dump into that cold spot?
http://kimstanleyrobinson.info/content/science-capital-trilogy
[Response: Haven’t read it but a couple of decades ago I did the math on how many salt shiploads you’d need each year. Forgot the exact number but it is not feasible. -Stefan]
Everything is cycling. The same part of the US that saw cool weather this winter is now sizzling: http://texasstormwatch.com/2018/05/united-states-sizzling-this-month.html . I guarantee at some point later this year, it will flip.
Another complementary mechanism to @1,
1) Historical global deforestation increases H2O concentration within the atmosphere and decreases land water retention.
2) As a result, sea volume rises.
3) Consequently, sea temperature rises and more heat arrives to the north pole via oceanic currents.
4) Consequently, the arctic ice is melting and the water gets colder within that particular region through ocean circulation.
Thus, we can perceive a cold blob near the arctic zone.
Fred Magyar:
Actually, the problem seems to be that AMOC has lost some of its amokitude 8^}.
I don’t know why this website indulges delusional cranks quite so liberally. One or two posts is ok in the name of freedom of speech, but this ab guy is now spamming your website.
[Response: Try getting that theory into the peer-reviewed literature… Stefan]
Yes, the theory could use a little refining. Maybe start with the fact that the North Pole and the cold blob are about 4,000 km apart?
4 JRClark (I wish there was an edit option, I really stuffed up my questions)
What was trying to ask is now that this aspect of the climate system is nailed down with confidence what could be the most critical implications and logical consequences of that?
Would this knowledge impact the conclusions of previous papers and their level of confidence/doubts too? I’m imagining the logic of dominoes all lined up and when the first one falls so do the rest. In this way what are the most likely consequences and implications of this firm knowledge about the AMOC? What meaning does Stefan take away from this point.
Also how does he feel about this issue and his work now? Must feel really good I suspect. thanks
“ab” — seriously, there are people who might find your ideas publishable.
One journal that comes to mind is http://www.multi-science.co.uk/ee.htm
Just wondering what data exists that excludes a natural cycle? Is it now conclusive that this is the first time a cold blob has appeared in this area?
Preemptive: I’m just asking . . . don’t pound me for doing so. Yeah, I’m talking to you, Ray.
[Response: Well, in our 2015 Nature Climate Change paper we used proxy data to show the recent cold blob is unprecedented in 1100 years. And, of course, as I explain above it is a predicted result of a CO2 increase, across many models. Sure, for every climate prediction that comes true you can ask: can you rule out a natural cycle? Or can you rule out an act of god? Not with science I’m afraid. A more scientific question would be: is there any evidence for a natural cycle? -Stefan]
7 Al Bundy, I don’t know. It doesn’t matter what my ‘take-away meaning’ is what of value is what Stefan believes his is and what mine should be (if I an other knew everything as well as he does).
There is no value in putting words in his mouth or guessing. I believe it is best to learn directly from climate scientists like Stefan and not toothless tigers roaming the internet forests.
I am only interested in the down stream implications and the broader meanings to climate change of Stefan’s accomplishment here after years of him and his partners not really being sure due to ‘doubts’.
I am not interested in anyone elses opinions of it. Nor anyone elses answers to my questions here. If that changes I’ll ask you.
[Response: Implications would be a topic for another blog article. I’ve touched on this briefly in an earlier blog post here. -Stefan]
I’m confused, Stefan, regarding the role of CO2 in this process. You say “the Atlantic overturning circulation . . . has weakened significantly since the late 19th Century, with most of the decline happening since the mid-20th Century.”
However, as I understand it, CO2 emissions via fossil fuels were not sufficient to make much of a difference in warming the atmosphere (and thus the ocean) until roughly 1950. (See for example the following, from Weart’s “The Discovery of Global Warming: “The scientists who brushed aside Callendar’s claims were reasoning well enough. (Subsequent work has shown that the temperature rise up to 1940 was, as his critics thought, mainly caused by some kind of natural cyclical effect, not by the still relatively low CO2 emissions.” (https://history.aip.org/climate/co2.htm)
Also, according to most records, there was little to no land-ocean warming between ca. 1940 and ca. 1979. So if CO2 could not have had more than a minor influence on land and ocean temperatures until the 1980s and 90s, it seems difficult to associate the rise of CO2 emissions with a process that began during “the late 19th Century, with most of the decline happening since the mid-20th Century.” Was there some other aspect of CO2 that produced the effect you’ve been studying? And if not, then I’d appreciate some explanation as to how CO2 could have had a major influence on the development of the AMOC hole you have identified.
[Response: Actually, if you look at our AMOC time series (Fig. 6 of our paper) you see no significant change until about 1970. -Stefan]
Victor:
Can’t get much closer to the middle of the century than that.
Hr.Rahmstorf
I red quite recently at Bjerknessenteret UiB.no that they have found New Things in “Vestisen” the western ices, that gets smaller and withdraws back to Grønland. Thus, the sea there is not so cowered with ice in recent years Winters, and cools off and sinks better they say.
This may add to or Challenge other theories of North Atlantic circulation they also say.
Quite important, Winter measurements of sea water temperatures in or near to those western ices has only been possible in quite recent years, they say.
My comment to this is that Bjerknessenteret in Bergen seldom disappoints us, and thus are Worth following. Further that they are traditional experts on the North Atlantic and sub Arctic situation .
I have also given a comment to Your argument, “Rahmstorfs Blaupunkt” at Uppsalainitiativet.se , where that so called gulf stream and its weakening in recent years is discussed.
Actually, I think I made them aware of Rahmstorfs recent discussion at Real Climate, so they took it up for themselves.
Uppsala is reliable.
My contribution is to remind people that the Beering street is very narrow and shallow, together with the very fameous Moskenesstrømmen Malstrømmen… gives that all that mighty water (=the sea serpent) has to Return somehow. And it Returns through Framstredet west of Svalbard and Danskestredet west of Island, and has to cross under the Gulf stream, because there is no proper way out in the Beering street.
There surely and definitely is a mighty current going from the polar basin and Down the east coast of Grønland. As there also is a mighty current up the Norwegian coast and into the Barets sea.
What matters for me, and further for a lot of People I think, is whether or not Rahmstorfs Blaupunkt will change this and not just “the Gulf Stream”. To my wiew, The Barens sea , the Svalbard waters and the very polar basin seems to warm up solidly and steadily in recent decades, despite of Rahmstofs Blaupunkt.
Then I tell them what Potzdam is fameous for.
Bjerknessenteret has found and demonstrated earlier a quite proper “Gyre” in the North Atlantic Norwegian sea, east of Jan Mayen, by traditional empirical means.
Like in the Sargasso sea, a “gyre” is found and seen traditionally by where rubbish is floating round and round for a long time.
[Response: Sure, I have good contacts with the Bjerknes Center and have visited there a couple of times, and I know the result you mean. It is not directly related to our paper, though, since it concerns the Greenland-Norwegian Sea rather than the subpolar Atlantic. -Stefan]
Another complementary mechanism to @1 and @11 concerning the cold blob formation:
1) Historical global deforestation via the use of fire increases the atmospheric concentration in carbon particles.
2) In the North Pole, those particles mixing with the snow and falling on the ice decrease its albedo.
3) As a result more arctic ice is melting and the water gets colder within that particular region via oceanic circulation.
Thus, we can perceive a cold blob near the arctic zone.
Thank you for a good summary Stefan.
An alternative explanation that has been proposed for the emergence of the cold blob is that stronger wintertime wind forcing has lead to reduced stratification of the upper ocean, and mixed upp cooler water from below. In principle this can be done without changing the heat content of the water column. Marieke de Jong and Laura de Steur wrote a paper about this in 2016 (https://doi.org/10.1002/2016GL069596) and de Jong has a discussion on a blog (https://fmkdejong.wordpress.com/loco/).
Of course, as soon a you reorganize the water column, heat fluxes are liable to change, and the evidence for changes in the AMOC are compelling, so the real change may be a mixture of several inter-linked processes. As often in science the problem is not that we don’t have a clue, but that we have several and need data to constrain them.
On the topic of data, I noticed that ESSD has a recent submission introducing a data set for this region.
https://www.earth-syst-sci-data-discuss.net/essd-2018-22/essd-2018-22.pdf
[Response: Hi Halldor, I know the paper – that is short-term though while we are talking about a long-term trend. Of course you can cause cooling for a couple of years by mixing up colder water from below. But in the long run – and especially for the winter season when we’re talking about deeply mixed water anyway – I’m sure this can’t explain the trend. -Stefan]
V: as I understand it, CO2 emissions via fossil fuels were not sufficient to make much of a difference in warming the atmosphere (and thus the ocean) until roughly 1950.
BPL: You keep saying that, but it’s still not true.
Re #22:
We have a long-term companion dataset of North Atlantic sea surface salinity from 1896: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL072582
There is a non-paywalled version of the article here: https://andrewronaldfriedman.files.wordpress.com/2017/02/friedman_et_al-2017-grl.pdf
Victor, #19–
It’s worth recalling that it’s concentrations, not emissions, that are determinative of forcing. As Callendar (1938) correctly estimated, concentrations then had already been increased by about 6% over pre-Industrial values. That’s too small for reliable detection of resultant warming, but it would have enhanced the natural warming trend seen in the 30s (and incorrectly attributed by Callendar to anthropogenic CO2.)
It would be an interesting side question to know what the calculated ‘enhancement’ should theoretically have been. Even I can do that…
A common approximation is: Forcing = 5.35 x ln C’/C (W/Me2). For a 6% increase, (ln C’/C) is 0.058268908124, so multiplying we get:
5.35 x 0.058268908124 = ~0.31 W/Me2
Pretty small–the current CO2e value is about an order of magnitude greater, and my calculated ‘enhancement’ is only about a thousandth of the direct SW energy–but perhaps that’s not negligible for all purposes; it would correspond to a couple of tenths of a degree C. That would be swamped in the record by natural variability, but nonetheless it would exist as a warm bias relative to a hypothetical ‘no additional GHG’ case.
And–for context–the ‘enhanced forcing’ is close to the forcing associated with observed changes in insolation. So if those solar forcing changes are worth considering now, then CO2 change by 1938 is also worth considering–at least in a scientific sense.
The sst anomaly along the US east coast increases and is expanding year by year, let’s have a look at the recent anomaly in that area:
https://forum.arctic-sea-ice.net/index.php?action=dlattach;topic=2278.0;attach=101437;image
Quite impressive. This will only get worse in an amoc slowdown scenario, if I get that right? While the cold blob is getting bigger, the sst south of it is increasing, right?
@ Ramstorf
Thanks for Your comment 21, ….
We have tried to interprete and to think of it in the Uppsala discussion, and I have mentioned salinity or lack of the same, as the plausible physical explaination of that quite strange and remarkable Blaupunkt on recent maps, southeast of Grønland…..
Because at the same time, the east Arctic and Svalbard & Franz Josefs Land, and Severnaya Zemlya waters just get more and more red hot and ice free according to the same maps.
I tell People to be sceptic and reallize that the colours are not absolute temperatures but rather a display of how the temperatures change, heat up or cool Down.
But I have it hard to explain and thus to Accept Rahmstorfs Blaupunkt, if lower salinity due to increased meltwater freshwater from Grønland, is not the main explaination for why that increasingly cold water does not sink.
And because the further current North of Scotland and up the Norwegian coast into the Barents sea, by many signals (like fisheries and ice situation), still seems to get just warmer and warmer.
The doomesday prediction is that Fimbulwinters will follow when the Gulf Stream stops. I see no Clear signals of that yet, rather the opposite.
#19
Response: Actually, if you look at our AMOC time series (Fig. 6 of our paper) you see no significant change until about 1970. -Stefan
Thanks for your very prompt response, Stefan. Nevertheless, I remain puzzled. In your paper you write:
“Although long-term natural variations cannot be ruled out entirely, the AMOC decline since the 1950s is very likely to be largely anthropogenic, given that it is a feature predicted by climate models in response to rising CO2 levels.”
This strikes me as vague, if not misleading. Yes, CO2 levels were in fact rising since the 1950’s, but what really matters to your theory is not the CO2 levels per se, but the effect they were having on global earth-ocean temperatures. And according to the evidence there seems to have been little to no increase in global temps. until the late 1970’s. Moreover, as we know, there is a decadal time lag between atmospheric temperatures and ocean temperatures, suggesting that any rise in the former would not have influenced the latter until after a period of roughly 20 to 30 years. So how is it then possible to conclude that the anomaly you’ve found is “largely anthropogenic”?
[Response: The global temperature time series is actually largely (two-thirds) a sea-surface temperature time series, and global warming over large parts of land areas is also strongly affected by lag caused by the thermal intertia of the global ocean. So what lag are you talking about? Why would the lack of warming – and in some parts even cooling – of sea surface temperatures in the subpolar Atlantic lag behind the global (largely sea surface) temperature? -Stefan]
RE: Stefan #17
“A more scientific question would be: is there any evidence for a natural cycle?”
Good question. Thank you, for taking the time to reply.
Stefan, thanks for the work you are doing.
From the point of view of a layman, it is counterintuitive that the cold blob is caused not by ice melt from Greenland and other nearby ice repositories, but is both an indicator of a slowdown in the AMOC and a result of such slowdown. The article might be strengthened by addressing this point directly: by laying out the data showing that the ice melt is far to small to account for the temperature anomaly, while being sufficient to disrupt salinity and disrupt overturning; and showing the new AMOC temperature distributions. Comments nos. 2 and 5 reflect the questioning of some readers on this point.
Thanks again.
Contrast Figure 1. with Figure 5 of Rajagopalan and Nihous’s paper An Assessment of Global Ocean Thermal Energy Conversion Resources With a High-Resolution Ocean General Circulation Model
They state, The oceanic surface layer would cool down in tropical OTEC regions with a compensating warming trend elsewhere. Some heat would penetrate the ocean interior until the environment reaches a new steady state. A significant boost of the oceanic thermohaline circulation (THC) would occur.
Figure 5 models the averaged temperature change in degrees C with an upwelling OTEC strategy at an upwelling rate of cold water of 20m/year, which equates to a net global power rate of about 30 terawatts or about 66% higher than the present.
By using a heat pipe design to move the surface heat to a depth of 1000 meters by the phase changes of a working fluid, rather than as the sensible heat of water, you get about the same amount of power without disrupting the thermohaline because the upwelling rate of the sequestered heat is about 4m/year as opposed to 20.
At that rate, a small strengthing of the AMOC occur.
Glad that you enjoy the theory. Let’s look at it… The cold blob anomaly concerns sea surface temperatures… So more cold fresh water is coming into this particular spot.
How is that ? Well, the AMOC is getting stronger and stronger because the sea-levels are rising worldwide, so more heat is produced into the AMOC and thus more heat arrives into the Arctic Ocean.
As a result the East Greenland current increases in intensity, bringing more fresh water from the North Pole into the region of the cold blob.
Victor, #29–
We, qui mo sabe? As it happens, I haven’t heard anything of the sort. Moreover, a search didn’t turn up anything of the sort, either, so do you have a source for this claim?
This study found slightly longer lags for SST than for global 2m temps in response to the annual temperature cycle–but that’s a completely different story, of course:
https://www.sciencedirect.com/science/article/pii/S0921818112001658
The main point of that paper was to suggest, sub rosa, that CO2 levels are driven by temperature, and not vice versa. (Lead author was Ole Humlum, who is a charter member of the denialati.) One of our good hosts here, Rasmus, had a few things to say about it:
https://www.sciencedirect.com/science/article/pii/S0921818113000891#bb0015
But nothing closer to the point, at least in the first couple of search pages.
So I took another tack, and navigated on over to woodfortrees. There I plotted HadSST versus CRU land-only temps, and threw in RSS tropospheric values for good measure. Here’s what it looks like if you start the two former data sets in 1950:
http://woodfortrees.org/plot/hadsst3gl/mean:13/from:1950/plot/crutem4vgl/mean:13/from:1950/plot/rss/mean:13
As you can see, wobble matches wobble very well in terms of phase, if not always amplitude. No sign of the phenomenon we allegedly ‘all know’. (Granted this isn’t the definitive test… but you’d think *something* would be visible if there were a decadal lag.)
Interestingly, if you look at the whole record, you notice something else:
http://woodfortrees.org/plot/hadsst3gl/mean:13/plot/crutem4vgl/mean:13/plot/rss/mean:13
A real divergence between land and ocean values from roughly 1980 on–the so-called ‘modern warming era,’ or darn close to it.
Just to double check, here’s the same data, but filtered with a 10-year mean:
http://woodfortrees.org/plot/hadsst3gl/mean:120/plot/crutem4vgl/mean:120/plot/rss/mean:120
Still no sign of the alleged lag–in fact, the coinciding ‘inflection points’ stand out all the more clearly without the confounding high-frequency variation.
I think this claim of an ocean lag is busted, personally.
Victor @29, I actually think you raised a reasonable point, but you you just don’t seem to get it do you and move on when the facts are pointed out to you. The increased rate of change of the AMOC from 1970 corresponds rather well with the warming period from the late seventies. Its screaming out at you!
Yes the time relationship is is not exact, but remember there could be some residual measuring error in temperatures, or the timing of the AMOC that would easily explain the discrepancy, and other natural factors are in operation that could explain a short discrepancy. There are numerous things to consider before jumping to simplistic sceptical positions.
And since you seem concerned about ocean temperatures, there was actually robust warming from about 1975 as below similarly timed to warming over land. And it is warming over land that counts most because its freshwater from greenlands ice sheets that are driving this AMOC slowdown issue.
https://en.wikipedia.org/wiki/Sea_surface_temperature#/media/File:Annual_Mean_Temperature_Change_for_Land_and_for_Ocean_NASA_GISTEMP_2017_October.png
I mean the timing is almost perfect, and still you don’t seem to get it.
@30, @17,…
“natural cycles”
This meme really troubles me, particularly when the periodicity of the “natural cycle” is presumed large.
The implication is that increasing CO2, with its increase in system energy predicted by physics, has no effect. It just doesn’t make sense– CO2 would interfere with “natural cycles” just as much as it would perturb an equilibrium state or a monotonically changing state, wouldn’t it?
Anyone? Am I missing some sophisticated statistical reasoning here? Are periodic phenomena privileged in some way?
ab, #33–
You outdo yourself, ab… but are you sure AMOC isn’t strengthening due to pending extinction of unicorns?
Main article says:
ab says:
29 May 2018 at 3:38 PM
Hm. So your theory asserts the opposite of what’s been observed has happened. Hm.
http://www.anvari.org/fortune/Miscellaneous_Collections/255425_the-great-tragedy-of-science-the-slaying-of-a-beautiful-theory-by-an-ugly-fact.html
AB #33
“Well, the AMOC is getting stronger and stronger because the sea-levels are rising worldwide, so more heat is produced into the AMOC and thus more heat arrives into the Arctic Ocean.”
You have got to be joking. The amoc is slowing down, not getting stronger. Global sea level rise cannot cause this. The only mechanism found capable of explaining this slowing process is arctic melting at the termination of this current. The blob is one of the results of the process.
Increasing sea levels and consequent increased ocean heat content is dispersed globaly and would be insignificant in terms of transport to the arctic. And the important thing is changing sea surface temperatures caused by the greenhouse effect, and this is mainly what is injecting additional heat energy into the oceans.
In addition the arctic is being warmed by a rapid greenhouse gas warming process that has already caused an approximate 6 degrees change over the land and this is obviously what is melting the ice. Transport of ocean heat energy to the arctic can only affect the periphery slightly, but even then this heat energy is derived largely from greenhouse gas heating anyway.
http://www.cbc.ca/news/politics/arctic-climate-warming-ice-report-1.4083728
[Response: Perhaps ‘AB’ is not joking but just throwing in a bunch of nonsensical theories to confuse this discussion and steer it away from discussing real science. Don’t take the bait. -Stefan]
RE: 4 JRClark says:
28 May 2018 at 9:29 AM
“1) So what? Knowing this, what is the most important meaning you now take away from this work?”
An excellent question.
34 Kevin McKinney says:
Victor, #29–
…as we know, there is a decadal time lag between atmospheric temperatures and ocean temperatures…
KM: We, qui mo sabe? As it happens, I haven’t heard anything of the sort. Moreover, a search didn’t turn up anything of the sort, either, so do you have a source for this claim?
V: Yes. I first came across it in this article, courtesy of Skeptical Science, titled
“Climate Change: The 40 Year Delay Between Cause and Effect,” by one Alan Marshall — https://skepticalscience.com/Climate-Change-The-40-Year-Delay-Between-Cause-and-Effect.html
“The reason the planet takes several decades to respond to increased CO2 is the thermal inertia of the oceans. Consider a saucepan of water placed on a gas stove. Although the flame has a temperature measured in hundreds of degrees C, the water takes a few minutes to reach boiling point. This simple analogy explains climate lag. The mass of the oceans is around 500 times that of the atmosphere. The time that it takes to warm up is measured in decades. Because of the difficulty in quantifying the rate at which the warm upper layers of the ocean mix with the cooler deeper waters, there is significant variation in estimates of climate lag. A paper by James Hansen and others [iii] estimates the time required for 60% of global warming to take place in response to increased emissions to be in the range of 25 to 50 years. The mid-point of this is 37.5 which I have rounded to 40 years.”
Here’s the reference he provides to Hansen’s paper: Science AAAS, ”Earth’s Energy Imbalance: Confirmation and Implications”, available (after free registration) at http://www.scienceonline.org/cgi/reprint/1110252v1.pdf, p.1
Since the explanation he provides seems so reasonable it never occurred to me to doubt it — though obviously there is no way to confirm his 40 year estimate, which does seem a bit excessive.
As far as Humlum is concerned, I agree with his assessment as far as the Antarctic ice core data is concerned, but his claim that temperature drives CO2 levels in recent times strikes me as highly questionable. There’s been plenty of fossil fuel burning of late and it’s hard to deny that this is the principal cause of CO2 rise during the present era.
The graphs you display are interesting, but I imagine it would be really difficult to argue that the thermal inertia of the oceans is some sort of “denialist” fabrication.
Alteration of the Gulf Stream current is related to an increase in bedrock geothermal heat flow from the Mid-Arctic Rift. This divergent seafloor fault system has pulsed massive amounts of heat and chemically charged heated fluid into the overlying ocean which has altered ocean temperatures. Most recently it has been responsible for generating warm blobs south of Greenland that act to slowdown the flow of the Gulf Steam current. Refer to my plateclimatology. com website for details.
[Response: Peer-reviewed reference for these claims, please? -Stefan]
35 nigelj says:
nj: Victor @29, I actually think you raised a reasonable point, but you you just don’t seem to get it do you and move on when the facts are pointed out to you. The increased rate of change of the AMOC from 1970 corresponds rather well with the warming period from the late seventies. Its screaming out at you!
V: Sorry, but I was referring to Stefan’s Nature paper, which dates the weakening in question to some time around 1950, not 1970. From the abstract: “Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century.” As is evident from the following (very typical) graph, land temps declined rather steeply from 1940-1955 and were at a relatively low point as late as 1975. https://jameswight.files.wordpress.com/2010/07/global-land-temperature-reconstructions.png
If you recall, this was the time of the “impending ice-age” scare.
We cannot ignore, moreover, the time it would take for any atmospheric warming to be transferred to the oceans. The Skeptical Science article I quoted above, based on research by James Hansen et al., estimates a lag of 40 years, based on the thermal inertia of the oceans. We need not accept that figure, but it makes little sense to assume that the rise in temps could have been transmitted instantaneously.
Your interpretation of the graph you display assumes a cause and effect relation that may not have existed. Correlation does not imply causation, remember? In any case something has to give. If the graph tells us what you think it does, then it’s necessary to deny the thermal inertia of the oceans, which most climate scientists might find hard to swallow, I imagine.
nj: And it is warming over land that counts most because its freshwater from greenlands ice sheets that are driving this AMOC slowdown issue.
V: I found only one mention of Greenland in Stefan’s paper. Here it is, from the very last paragraph:
“Continued global warming is likely to further weaken the AMOC in the long term, via changes to the hydrological cycle, sea-ice loss and accelerated melting of the Greenland Ice Sheet, causing further freshening of the northern Atlantic.” Nowhere was I able to find any reference to Greenland ice sheet melt as a current or past driver of the AMOC slowdown.
[Response: Greenland melt is not the major driver thus far. In our 2015 Nature Climate Change paper we have some estimates of Greenland’s contribution and conclude it cannot be neglected, but it’s certainly not the major one. It is a contribution that is growing fast, though. -stefan]
In regards to your question if we should call out systematic misinformers like Breitbart, or just ignore them? What do other readers think?
Call them out ! Each and every time they make a claim that is not correct. At the same time, it is important not to fall Into the trenches. The hard core deniers (sorry for the term) that won’t let them be convinced by facts will not otherwise be convinced. They are a lost cause, although thankfully not by far in majority. The vast middle in the ever more polarized political climate is what ultimately will matter.
Victor @40, I think you are mistaken about the timing of the AMOC changes. Stefan has already said at post 17 above the significant slowdown in the AMOC starts about 1970, and you can see it in the graph in the article. This relates closely enough to when global temperatures started warming significantly.
I have already shown you a graph that shows sea surface temperatures and land tempertaures both increasing from the mid 1970’s in tandem, and such things are easily googled. You have missinterpreted Hansen and the link you posted. Sea surface temperatures respond quickly to atmospheric warming, but oceans respond more slowly at full depth, so yes there is a delay there. Its surface temperatures that are most relevant to this issue at hand anyway.
Yes I admit Greenlands influence has been minimal so far. But that just doesn’t really change the points I made.
Nobody claimed correlation means causation. The article has explained causation briefly, and you need to read the research to get the full details and the maths. I’m not going to waste time on this further: DNFTT
This comment really caught my eye
35 nigelj says: ” because its freshwater from greenlands ice sheets that are driving this AMOC slowdown issue.
There are several places a Fact Check could be done but I decided on Stefan’s last post for easy access. https://www.realclimate.org/index.php/archives/2018/04/stronger-evidence-for-a-weaker-atlantic-overturning-circulation/
Where he states 1) “The cooling is simply due to the reduced heat input from the AMOC.” and 2) “I know of no other mechanism that could explain this spatial and temporal pattern than a weakening of the AMOC. Such a slowdown as a result of our greenhouse gas emissions has long been predicted by climate models – these data show that it is already underway.”
And 3) on this page #40 Stefan says again “Greenland melt is not the major driver thus far. In our 2015 Nature Climate Change paper we have some estimates of Greenland’s contribution and conclude it cannot be neglected, but it’s certainly not the major one.”
My layman’s interpretation of what drives the ocean currents like the AMOC is it is primarily driven by temperature differentials between the equator and the polar regions (in both ocean layer temperatures and less so by atmospheric temperatures differentials?) And it’s the reduction in thsoe differentials that is “driving this AMOC slowdown issue.
I defer to Stefan for a more scientifically accurate use of jargon or for corrections to my layman’s understanding.
#38 Victor says:
Since the explanation he provides seems so reasonable it never occurred to me to doubt it — though obviously there is no way to confirm his 40 year estimate, which does seem a bit excessive.
As far as Humlum is concerned, I agree with his assessment as far as the Antarctic ice core data is concerned, but his claim that temperature drives CO2 levels in recent times strikes me as highly questionable. There’s been plenty of fossil fuel burning of late and it’s hard to deny that this is the principal cause of CO2 rise during the present era.
Good job that. (with a broad smile, a wink and a nod)
Victor @40,
This is all very reasoned stuff you present. Four whole comments and no sign of entrenched nonsense from you … so far.
Concerning the timing of the significant dip in AMOC; you say you refer to Caesar et al (2018) but so did Stefan in his Response@19. Both Fig 6 and ‘Exended Data’ Fig 6 show the wobbly AMOC strength dropping out of the previous-wobble-range at roughly 1970. It may have been on the downward slope from the late 1950s but that only became a significant downward slope a decade or so later. This is entirely consistent with the slowdown occuring as described in the precis “since the mid-twentieth century” but not with your interpretation of the precis as you argued @29.
Concerning the “lag of 40 years” in the actions of a climate forcing: you do rather misrepresent this as being ΔF at year0 ⸫ ΔT at year40. Hansen et al (2005) is saying 60% of the warming will have occurred in roughly a forty year period and that this is dependent on ECS.
This is shown more clearly in Hansen et al (2011) that variously graphs the ‘Climate Response Function’ showing that while roughly 60% of the warming would arrive in forty years, a large proportion of that 60% arrives in the first ten years. (See Fig 5 from the paper which shows 45% by year10 and 55% by year40.)
nigelj:
I had the same thought. I had to remind myself I shouldn’t dismiss a worthwhile question just because a crank asked it 8^}!
So: before I go ‘oogling it, does anyone here know when the isotopic signal of elevated fossil carbon emissions became detectable in samples of atmospheric and icecap gas?
As the AMOC continues (?) to slow down, Hansen’s ice-climate feedback mechanism will become increasingly more positive (and I note that this consideration is not addressed in AR5 projections)
Hansen 2016: Ice melt, sea level rise and superstorms: Evidence from paleoclimate data, climate modeling, and modern observations that 2°C global warming could be dangerous.
https://pubs.giss.nasa.gov/abs/ha04710s.html
http://www.atmos-chem-phys.net/16/3761/2016/acp-16-3761-2016.pdf
AR5
It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century but it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century and it is unlikely that the AMOC will collapse beyond the end of the 21st century under the RCP scenarios considered. The best estimate decrease in 2100 is about 20–30% for the RCP4.5 scenario and 36–44% for the RCP8. 5 scenario. [12.4.7, Figure 12.35]
Page 12-6 https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/drafts/fgd/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter12.pdf
WG1AR5_SPM_FINAL
There is no observational evidence of a trend in the Atlantic Meridional Overturning Circulation (AMOC), based on the decade-long record of the complete AMOC and longer records of individual AMOC components. {3.6}
Page 8
https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf
In support of the results in Rahmstorf et al (2018), I would add that another reconstruction of AMOC (Ezer, Glob. Planet. Change, 2015), but from sea level data not SST, shows similar AMOC weakening, about 5sv from ~1960-2012 (a figure of this reconstruction can be found in http://www.ccpo.odu.edu/~tezer/AMOC_1935-2012.png). As for the “cold blob”- evidence that it is a signal of weakening Gulf Stream transport can be seen not only in climate coupled models, but even in early ocean-only models, which indicates that it is not a response to a local air-sea fluxes, but to change in ocean currents (see Fig. 3 in Ezer, 2001, http://www.ccpo.odu.edu/~tezer/PAPERS/2001_AGU_Clim.pdf). Just my 2c.
[Response: Thanks Tal! We’ll cite that in our next article. -Stefan]