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Does a slow AMOC increase the rate of global warming?

Filed under: — stefan @ 18 July 2018

Established understanding of the AMOC (sometimes popularly called Gulf Stream System) says that a weaker AMOC leads to a slightly cooler global mean surface temperature due to changes in ocean heat storage. But now, a new paper in Nature claims the opposite and even predicts a phase of rapid global warming. What’s the story?

By Stefan Rahmstorf and Michael Mann

In 1751, the captain of an English slave-trading ship made a historic discovery. While sailing at latitude 25°N in the subtropical North Atlantic Ocean, Captain Henry Ellis lowered a “bucket sea-gauge” down through the warm surface waters into the deep. By means of a long rope and a system of valves, water from various depths could be brought up to the deck, where its temperature was read from a built-in thermometer. To his surprise Captain Ellis found that the deep water was icy cold.

These were the first ever recorded temperature measurements of the deep ocean. And they revealed what is now known to be a fundamental feature of all the world oceans: deep water is always cold. The warm waters of the tropics and subtropics are confined to a thin layer at the surface; the heat of the sun does not slowly warm up the depths as might be expected. Ellis wrote:

“This experiment, which seem’d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate.”

In 1797 another Englishman, Count Rumford, published a correct explanation for Ellis’ “useful” discovery:

“It appears to be extremely difficult, if not quite impossible, to account for this degree of cold at the bottom of the sea in the torrid zone, on any other supposition than that of cold currents from the poles.”

Thus the thermohaline overturning circulation was discovered. The deep sea is kept cold by what we now call “deep water formation”: the sinking of cold, dense water in the subpolar Atlantic and Antarctic oceans down into the deep, where it spreads around the globe. In equilibrium, the temperatures of the deep ocean are kept constant by the balance of two opposing trends: a warming tendency by turbulent diffusion of heat from above, and a cooling tendency by the inflow of cold water from the poles, i.e. deep water formation.

The global thermohaline overturning circulation (from Rahmstorf, Nature 2002)

If the rate of thermohaline overturning slows down, then heat diffusion gains the upper hand and the deep ocean warms. If it speeds up, the opposite happens and the deep ocean cools. Model simulations show that this is true for decadal variability (e.g. Knight et al. 2005) as well during global warming (e.g. Liu et al. 2017). Knight et al. found that decadal variability of the AMOC can cause  small variations in global mean surface temperature, with a strong AMOC linked to high global surface temperatures. Liu et al. found that their climate model warms less under the same greenhouse gas scenario when the AMOC is weakened more.

It is surprising, then, that today a paper was published in Nature that claims the exact opposite: namely that a strong overturning circulation warms rather than cools the deep ocean. The idea is that the increasing heat accumulating at the surface through the increasing greenhouse effect is brought down into the ocean depths by deep water formation, thus reducing warming at the surface. But is this true, and what is the evidence for it?

The evidence

The great Carl Sagan once said that extraordinary claims require extraordinary evidence, so we approached the paper by Chen and Tung with considerable curiosity. We were soon disappointed, however. The only evidence presented is the putative coincidence of two phases of slow global surface warming (1942-1975 and 1998-2014) with strong AMOC (i.e. Atlantic Meridional Overturning Circulation), and a phase of more rapid warming (1975-1998) with weak AMOC (their Fig. 3). There is no statistical examination of this supposed correlation. A number of further graphs showing various spatial patterns do nothing to support the new hypothesis: these graphs just show well-known patterns like the interhemispheric see-saw effect of AMOC variations and the Zhang fingerprint (also shown by Caesar et al. in Nature earlier this year), which are fully consistent with the established view of the effect of AMOC variability on ocean temperatures. Indeed one of us (Mike) has co-authored the paper by Knight et al. showing that natural variability in the AMOC sometimes called the “Atlantic Multidecadal Oscillation” or “AMO” (a term originally coined by Mike) demonstrates an in-phase relationship between large-scale (i.e. northern hemisphere and global) surface warming and the strength of the AMO, i.e. the opposite of what the present authors claim. The peak impact on global mean temperature, incidentally, is found to be only ~0.1C, calling into question any claim that trends in global mean temperature will be substantially influenced by the phase of the AMO.

And as our regular Realclimate readers know very well, the distinction of phases of fast global warming up to 1998 and slow warming from 1998 is highly questionable. First of all, 1998 is the year with the strongest upward spike in global temperature and the strongest El Niño event on record. Second, the supposed “warming slowdown” after 1998 is known to be largely an artifact of the HadCRUT temperature data, because these do not cover the Arctic which has warmed the most in this period. Other data – HadCRUT with infilled Arctic temperatures (Cowtan and Way), GISTEMP, NOAA, Berkeley – do not show this. (Guess which global temperature data set Chen and Tung exclusively used.) And the little variation in the global warming rate since 1975 which remains in these other data sets is strongly correlated with El Niño. Model simulations with the correct El Niño phase reproduce the observed global temperature evolution (Kosaka and Xie 2013), and if the El Niño effect is removed from the global temperature time series the rate of warming is steady (see next graph). There is thus much stronger evidence for an El Niño effect than for the AMOC hypothesis. The well-established explanations for the variations in global warming trend are unfortunately not discussed in the paper.

The mechanism

On the mechanism for why a strong AMOC would heat rather than cool the deep ocean, Chen and Tung write: “Deep convections can now carry more heat downward.” (Deep convection is the vertical mixing process at the beginning of deep water formation.) That should make anyone familiar with the conditions in the subpolar Atlantic stop. Isn’t deep convection thermally driven there, by surface water becoming colder and thereby denser than the deep water? After all, this is a region of net surface freshwater input, from precipitation, river runoff and ice melt, so in the convection areas the surface water is fresher than the deep water, which inhibits convection. Thermally driven convection moves heat upwards, not downwards.

To be sure, Realclimate asked veteran sea-going oceanographer Igor Yashayaev of the Bedford Institute of Oceanography in Canada, who has decades of experience studying convection in the subpolar Atlantic, whether there is any evidence for salinity-driven convection there which could carry heat downward? Yashayaev’s reply:

“In the polar and subpolar regions, under no circumstances we saw convection bringing warmer water down deep.”

Chen and Tung do not show any models simulations either to provide evidence that their mechanism can actually work, neither do they discuss the various published model results that have come to the opposite conclusion. Fig. 2 of their paper shows coherent subpolar temperature anomalies over the top 900 meters of water column – but that corresponds to the depth of the Gulf Stream, rather than that of deep convection, so in our view is more likely simply a result of the established fact that a stronger AMOC transports more warm water into the subpolar North Atlantic. That graph also shows that the surface Atlantic is in that case anomalously warm, going right against their hypothesis that the surface is made cooler by a stronger AMOC.

Anthropogenic AMOC slowdown

The abstract of the paper states that

“Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability, rather than an anthropogenically forced trend.”

The phrasing of this as an either-or question is odd, given that most modern climate time series show a mixture of both: a long-term climate trend plus variability on different time scales. This is also what models predict for the AMOC, and there is considerable evidence in observational data that this is indeed the case (see Rahmstorf et al. 2015, Caesar et al. 2018 and this Realclimate article). So we were curious what analysis this statement in the abstract is based on. Unfortunately, no analysis about the presence or absence of a climatic or anthropogenic trend in the AMOC is found in the entire paper.

The paper does show the AMOC index attributed to Caesar et al. 2018 in its Fig. 3a, and Caesar et al. reported an anthropogenic AMOC slowdown. However, for reasons that we were unable to clarify in correspondence with the authors, the AMOC index they show is quite different from the one reported by Caesar et al., as a comparison with Fig. 6 of the latter paper immediately shows. In the version by Chen and Tung, the index is lacking the downward trend which was the main result of Caesar et al.

The forecast

The one feature of the paper that is likely to raise most media interest is a forecast for the coming decades. The authors predict “a prolonged AMOC minimum, probably lasting about two decades” which “will manifest as a period of rapid global surface warming”.

The prediction of an AMOC minimum lasting two decades is merely based on their assessment that a previous AMOC minimum lasted two decades, thus the next one should do the same. Neither Caesar’s AMOC index starting in 1870 nor the long proxy-based AMOC time series we presented in Rahmstorf et al. (2015) nor model simulations support the idea of such regularly repeating AMOC cycles. In addition there is forcing, e.g. from the increasing meltwater from Greenland, which will affect the future AMOC. And as discussed above, the idea that a weak AMOC promotes rapid global warming is in itself not supported by any convincing evidence.

It is difficult not to think of the prediction by Keenlyside et al. in Nature in 2008. These authors made headlines around the world by predicting a phase of global cooling, ironically also largely based on a prediction of weak AMOC but, based on model simulations, finding the opposite effect on global temperature as Chen and Tung claim. Back then the Realclimate team had solid reasons to predict that the forecast would turn out to be wrong – which indeed it did. This time, we once again do not doubt that rapid global warming will continue until we strongly reduce greenhouse gas emissions – but for reasons that have nothing to with the AMOC.

Clarification (6 Sept 2018): We added “northern hemisphere and global” in brackets to clarify what is meant by “large-scale” in the reference to Knight et al. above. This is shown in Fig. 4 of Knight et al. 2005.

62 Responses to “Does a slow AMOC increase the rate of global warming?”

  1. 1
    prokaryotes says:

    It appears to me that any slow down would result in less water pumped through the conveyor belt. Subsequently this would mean less warm water is transported, but it also means it flows slower, and general Ocean surface layers heat faster, unless disrupted by more pole ward moving storms. Additional, increased meltwater could also provide some sort of lid for warmer surface water to prevail, as has been observed in Antarctica in a slightly different configuration – again altering deep water formation. Just in the Arctic it keeps the warmer waters longer afloat, or for a longer duration in a zone where it acts as an agent of deglaciation.

    Hansen: And what we discovered in our modeling was that fresh water that is put on to the surface of the southern ocean makes it more stable. Normally you have deep water formation Antarctic bottom water formation around the coast of Antarctica and that process allows heat from the ocean to be dispelled to the atmosphere into space but if you put more fresh water on the surface it it prevents or reduces that overturning because the water is less dense and so it doesn’t sink as easily and that then causes the heat in the ocean to instead of coming out it’s used to melt the ice shelves so it’s an amplifying feedback. https://youtu.be/HG599TJpF_0?t=4m39s

  2. 2
    spencer says:

    Unfortunately there are enough different forces acting on the AMOC to make it quite complicated, and I’m having trouble thinking up a short-form common-sense description for pubic use. A crucial but too-seldom emphasized feature of our situation is that noticeable atmospheric global warming is delayed because so much of the increased greenhouse-effect energy input goes into warming the oceans (Charney Panel 1979). Chen & Tung emphasize this in their abstract. Following them, naively one would think that a stronger AMOC would remove more heat from the atmosphere and thus further delay global warming. Is there a short intuitive counter-argument or are we stuck with “models show…” ?

    (Also impeding public discussion, by the way: most publicity about a possible AMOC slowdown has centered on the strong regional effect on climate around the N. Atlantic– where so much of the media resides– rather than global means.)

    [Response: That makes sense because the regional response is large, the effect on global mean temperature very small. -Stefan]

  3. 3
    Hartmut Heinrich says:

    Imagine there is a collapse of the West Antarctic Ice sheet that leads to an expansion of a much cooler and less saline Southern Ocean till the southern tip of Africa. What would be the influence on the AMOC in the Atlantic Ocean? And on the deep water production(s). ???

  4. 4

    So, should these issues have been raised by pre-publication reviewers? Nature of course enjoys a strong reputation generally. Should they have done better in reviewing this effort? Or are there ‘redeeming features’ to the paper that should be discussed or acknowledged here?

    [Response: In my understanding it certainly is the reviewers job to make sure that the evidence presented is sound and the published relevant literature – including contrary evidence – is properly discussed. -Stefan]

  5. 5

    Got looking at the temperature record in the context of this post, using the woodfortrees tool. Interestingly, if you analyze the 30-year record–I used HADCRUT 4 in this case–in terms of decadal trends, you find that the first decade trend is insignificantly different from the trend in the ‘slowdown’ decade beginning in 1998, with both rounding to 0.12 C/decade. By contrast, the most recent decade clocks in at 0.33 C/decade.

    http://woodfortrees.org/plot/hadcrut4gl/from:1988/plot/hadcrut4gl/from:1988/trend/plot/hadcrut4gl/from:1988/to:1998/trend/plot/hadcrut4gl/from:1998/to:2008/trend/plot/hadcrut4gl/from:2008/trend

  6. 6
    Michael S Sublette says:

    Nice writeup, gents. Cheers. -Sean

  7. 7
    Jim Baird says:

    Stefan, you have argued that ocean heat content is an extremely unresponsive policy in comparison with the goal of limiting global warming to below 2°C, because while the increase in global temperature could be stopped within decades by reducing emissions, ocean heat content will continue to increase for at least a thousand years after we have reached zero emissions.

    Victor, Kennel and this writer, have argued the opposite.

    You base your argument on physics. The average depth of the ocean is 3700 meters deep and is heated at a power level on the order of ~1 Watt per square meter of the surface. Also, it is heated from above. Warm water floats and thus hinders the penetration of heat into the ocean. Therefore, it will take the ocean thousands of years to fully catch up with the surface warming we have already caused.

    Ocean heat, however, can be converted to work and moved with heat pipes that counter your concern about the physics of ocean heat penetration. From a depth of 1,000 meters, it takes 250 years for the residual heat not converted to work by ocean thermal energy conversion to resurface, given Munk’s estimation of the diffusion rate of 4 meters/year.

    Heat penetration of this magnitude extends the warming lead time from as little 32 years to 250 years and extends the surface cooling by the same amount, with a commensurate warming of the 1,000-meter-long water column. Once back at the surface the initial residual heat can be recycled another 13 more times (the thermodynamic efficiency of heat pipe OTEC is 7.6%) thus deferring the impact of global warming to the year 5,300.

    The attenuation of sea level rise, storm frequency and intensity and impact on coastal infrastructure and the abatement of global warming, desertification and ocean acidity accompanied by the transfer of trapped warming into the deep is climate and energy policy worthy of pursuing.

    Just as the previous AMOC minimum lasted two decades, the “hiatus” lasted about 15 years. The depth of heat penetration, in that case, was about 250 meters and was accompanied by a thickening of the thermocline.

    Movement of heat four times deeper, below the thermocline, extends the cooling benefit over 10 times longer.

    Levitus et al. estimated, “if all the heat the oceans have accumulated was instantly transferred to the lower 10 km of the global atmosphere it would result in a volume mean warming of this atmospheric layer by approximately 36°C.”

    Why would we not want to prolong this transfer as long as possible, and allow for the gradual transfer of this heat back to space, in order that the planet can return to the pre-industrial temperature?

  8. 8
    fed up palaeo-scientist says:

    Standard stuff that Nature likes to publish. Impressive findings that will make the news. Actual scientific rigour not important.

    I stopped reading that journal. It’s a waste of my time. If we all stop reading it, then it will go away.

  9. 9
    prokaryotes says:

    The headlines have said that the Gulf Stream is collapsing and the Ice Age is coming sooner than scientists think,” Prof Ka-Kit Tung from the University of Washington told BBC News.

    “The air temperatures globally will be warming and there’s no barrier for that so there won’t be much cooling in the UK, you will probably still see the normal global warming,” said Prof Tung.

    bbc .com /news/science-environment-44875508

  10. 10
    MarkR says:

    Zhou, Zelinka & Klein (2017) doi: 10.1038/ngeo2828
    Cloud pattern changes caused ~0.35 W m-2 net cooling *increase* during the 2000s.

    Chen & Tung give 89 ZJ heat uptake below 200 m for their period, similar to 0.35 W m-2. But if isn’t all an increase relative to pre-2000, then there’s no net flux change and how does this explain surface T variability?

    Yuan et al. (2018) doi: 10.1029/2018GL077904

    Say the Pacific explains about 150 % more variance than the Atlantic with regards to cloud changes.

    Looks to me like the evidence suggests Pacific-driven changes in cloudiness would explain more of the difference between higher/lower surface temperature sensitivity periods than the Chen & Tung AMO argument.

  11. 11
    MA Rodger says:

    Chen & Tung (2018) has been swallowed by the BBC uncritically and also CarbonBrief despite them having a sight of this OP here at RealClimate. Yet the print-press seem less susceptable as I can only see the Scottish Sun running with the story, so far.
    Perhaps one problem is that the deniosphere is having problems with KK Tung’s latest wheeze (eg at Wattsupia) which tries to provide a natural driver for the majority of AGW. (His previous wheeze was that it was AMO rather than AMOC, this argued through use of a rather dodgy use of Multiple Regression Analysis,)

  12. 12
    Nemesis says:

    This article is exactly what I’ve been waiting for. Thanks a lot.

  13. 13
    H Pace says:

    The BBC website reports the Chen and Tung paper as pretty much proven fact. https://www.bbc.co.uk/news/science-environment-44875508

  14. 14
    Bernd Eggen says:

    The BBC missed again a trick in getting a more rounded article by gathering more information from credible sources, eg from the above excellent discussion. Was also surprised by the amount of comments on the BBC article – already over 500 !

    At least CarbonBrief has updated their article to reference the RealClimate one, see
    URL https://www.carbonbrief.org/slowdown-atlantic-conveyor-belt-could-trigger-two-decades-rapid-global-warming

  15. 15
    Karsten Vedel Johansen says:

    This excellent article of yours does confirm many of my thoughts when I read the abstract of the discussed paper. My first suspicion was raised by the oversimplification of the problem, the second by the obvious holes in the knowledge about the historical discussions about THC and AMOC, and the third by the uncritical use of the myth about the “hiatus”.

    [Response: Exactly. In essence this paper is an attempt to explain the so-called ‘hiatus’, but that never existed (I could list a whole collection of peer-reviewed references here which show that, e.g. Rahmstorf et al. 2017 and references therein). And whilst all previous evidence shows that to explain a cool anomaly in global temperature with the AMOC you need a weak AMOC, the authors just do the opposite because that appears to fit the data. Of course, given the hiatus never existed it doesn’t actually fit those data, and while the pre-1970’s interval of constant global temperature is real, that one has an established explanation by the increasing cooling effect of aerosol pollution. That is why climate models reproduce that earlier plateau, as is shown in the IPCC reports. -Stefan]

  16. 16
    Susan Anderson says:

    Thank you! Really nothing else to say, except as a human being affected by all this, I deeply appreciate the continuing focus on this problem, and the way it is exploited to pretend that things are not as they are. My personal opinion, as someone who lives near the boundary of effective sea level rise in the new few years (few being less than two decades) is that knowledge is power

  17. 17
    prokaryotes says:

    Bernd Eggen said “Was also surprised by the amount of comments on the BBC article – already over 500”

    Ofc, the deniers use that article to make their pitch. Evidence that the BBC there really doesn’t care, otherwise they would moderate more. Reading the BBC article is almost like diving back in a time machine from 2010.

  18. 18
    mike says:

    “This time, we once again do not doubt that rapid global warming will continue until we strongly reduce greenhouse gas emissions – but for reasons that have nothing to with the AMOC.”

    might be better to go with “this time, we once again state that we expect” that rapid global warming will continue…

    The “do not doubt” language is kind of a double negative and some folks have trouble with trouble with double negatives. It’s kind of like speaking in passive voice. I think humans absorb info better and respond more appropriately if a message is carefully crafted to be clear and unequivocal. I don’t like to blame scientists for not conveying the science well, but when I read a closer like this one, I think… oops, that won’t get it done.

    Cheers to you sciency folks

    Mike

  19. 19
    SellaTheChemist says:

    Great article. Very helpful. But if i can quibble, Benjamin Rumford was not “English” but American – he was born in Massachusetts and educated in part at Harvard. He sided with the British in the War of Independence and fled to Europe. He’s the same guy who showed the equivalence of work and heat and later married the widow Lavoisier…

  20. 20
    Ka-Kit Tung says:

    Note: We have prepared a multi-part rebuttal to the long critique posted by Drs. Rahmstorf and Mann. Dr. Rahmstorf suggested that we post our document as a Reply here. But we found that figures and formats do not appear when we paste our document here. We contacted Realclimate and Dr. Rahmstorf to allow us post our rebuttal in the same format as in original post (it is only fair). While waiting for their reply an abridged version is posted below without figures and without the text that rely on the figures.
    ______________________________________________________________________

    Part 1. Anthropogenic forced decline of AMOC or multidecadal variability?
    By Ka-Kit Tung and Xianyao Chen

    Dr. Stefan Rahmstorf, co-author of this Realclimate blog, is also a co-author of a paper (Caesar et al, 2018) that appeared in April of this year in Nature. They used a subpolar SST gyre proxy to show the variation of AMOC back in time. They found a 15% decline (3 plus minus 1 Sverdrups) of the AMOC strength since 1950s, and concluded “the AMOC decline since 1950s is very likely to be largely anthropogenic, given that it is a feature predicted by climate models in response to rising CO2 levels”.

    The evidence provided by Caesar et al (2018) is that it is “predicted by climate models in response to rising CO2 levels”. This evidence was also laid out by Dr. Rahmstorf in a previous Realclimate blog in support of the interpretation of anthropogenically forced decline of AMOC. In the model experiment, CO2 is increased, and the SST pattern associated with a declining AMOC is found, which is called a “fingerprint”. What is it a “fingerprint” of? The title of their paper correctly states that it is a “fingerprint of a weakening Atlantic ocean overturning circulation”, but later it is used to imply that it is a fingerprint of CO2-induced weakening. That “fingerprint” is not unique to CO2-forced weakening AMOC. Spatial pattern similar to theirs (or opposite in sign when AMOC strengthens) have been found in model simulations of internal variability where CO2 concentration was kept constant (the so-called Preindustrial runs), and in AMOC perturbation experiments where the depth was deepened in a small region was changed (see the GFDL model runs in Zhang 2008, GRL, Zhang et al. 2011, JGR). When AMOC weakens, whether forced by CO2 or unforced, the reduced transport of heat by its upper, northward branch from subtropical to subpolar Atlantic leads to a cooling of the SST in the subpolar region and warming in the subtropical region, a dipole pattern identified by Caesar et al as the “fingerprint”. It is not justified to conclude that the observed recent decline in AMOC is CO2 forced just because its spatial pattern is similar to that “fingerprint of weakening Atlantic Ocean overturning circulation”. That fingerprint does not tell us about the cause of the weakening; just that AMOC weakened.

    Our paper (Chen and Tung, 2018) published on July 18, used several proxies and measurements. These proxies, with minor differences, are consistent with each other, and reveal a multidecadal variability since 1950s. Our subpolar salinity proxy, calibrated with the SSH-deduced AMOC strength, shows that the recent decline of around 3-4 Sverdrup (around15%) is from a recent peak in 2005. But from 1993 to 2005 there was a speeding up of AMOC of comparable magnitude, 4 Sverdrups. This speeding up was first reported by Willis (2010, GRL) using satellite altimetry measurements of sea-surface height (SSH), from which he could calculate the geostrophic northward volume flow. From the earlier peak in 1960s, AMOC first declined, by 3-4 Sverdrups, leveled off at a flat trough and then sped up since 1993 before declining again. This behavior is not consistent with a long-term trend due to CO2 increase; the latter should be largely monotonic. We did not base our conclusion on Caesar et al.’s subpolar gyre SST index, for reasons that will be discussed later in this piece. It may help improve the tone of this exchange to point out that despite our concerns about that proxy, we did not criticize Caesar et al. (2018). It was included for completeness late in the review process, after our conclusion had already been reached. The appearance of their paper in April (our paper was submitted in January) did not alter our conclusion.

    In the Abstract, we stated that this behavior, with ups and downs on multidecadal time scales, is “best explained” by multidecadal variability.

    “Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability, rather than an anthropogenically forced trend.”
    Drs. Rahmstorf and Mann criticized the phrasing as “odd”:

    “The phrasing of this as an either-or question is odd, given that most modern climate time series show a mixture of both: a long-term climate trend plus variability on different time scales.”

    We both know and agree that the climate time series in question contain an anthropogenically forced trend and a multidecadal variability. But the question was not one that demands an “either or” answer. The question was what “best” explains the observed variation. Similar mixed (not either or) phrasing was used in Caesar et al. (2018): “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 declining trend is superimposed by shorter-term (interdecadal) natural variability.”

    While we both agree that there is an anthropogenically forced trend, the argument presented in Chen and Tung (2018) suggested that this trend is small, as predicted by most IPCC models. The observed “trend” since the peak in 2005 measured by RAPID is 10 times larger because it contains a large multidecadal variability.

    The subpolar gyre SST index used as a proxy for AMOC was defined by Caesar et al (2018) as the mean SST in a specified region of the subpolar gyre minus the global mean SST. We were skeptical about this definition, and that was the reason we did not use it to draw any conclusion. We are supposed to define a regional index about AMOC in the Atlantic, but the global mean SST includes contributions from other ocean basins. And so this index includes negative of the variations in other ocean basins and hinders interpretation, such as the negative of the ENSO variation, and a negative of IPO (Interdecadal Pacific Oscillation).

    There is a deeper concern. The mean SST in the subpolar gyre has no trend since 1945. The global SST has an upward trend due to global warming. (The trend is larger in the November-May average used by Caesar et al. than annual or two-year mean.) The proxy used in Caesar et al. (2018) is a difference of the two, giving a downward trend. It is so by construction. Their conclusion that the downward trend in AMOC is caused by global warming is now seen to be a circular argument. The same circular argument was used in an earlier paper by Rahmstorf et al. (2015) Nature Climate Change. A millennium long AMOC proxy was constructed using the same procedure, except the global mean SST was replaced by reconstructed Northern Hemisphere mean temperature using paleoclimate multi-proxy records (the so-called Hockey Stick). The subtraction of the Hockey Stick gives a negative Hockey Stick in their AMOC index and the conclusion of “exceptional twentieth slowdown in Atlantic overturning circulation” from global warming. The argument may not be wrong, but it is circular.

    We have been in contact with Dr. Rahmstorf since July 15, 2018, when he wrote that he read an advance copy of our paper and wonder why our figure looks different than the Figure 6 in Caesar et al. We produced a comparison of the two curves and showed to him. We were still trying to pinpoint the minor differences near the peak around 2005 and requested more information from him (so far not received), when their blog appeared. Nevertheless the difference around 2005 does not affect the conclusions of our paper. The 2005 peak was not involved in either our or their argument. For our argument it is important to note simply the ups and downs that are inconsistent with a monotonic anthropgenically forced trend. The large 15% decline can be measured from either the 1950s peak (1960s in other proxies) or the 2005 peak.

    The Caesar et al.’s subpolar gyre SST proxy used seven-month average (November to May). Our reproduction following their definition but in monthly time scales (as requested by a reviewer, who wanted to see the “unfiltered time serious”) was shown as Extended Data Figure 1 in Chen and Tung (2018) for 1945 to the end of 2016. A two-year running mean using all months was shown in our Figure 3a. Although Caesar et al. (2018) provided to Nature the digitized data for the blue curve, they did not provide enough detail for others to reproduce it.

    The monthly time series shown in our Extended Data Figure 1 covering the period 1945 to the end of 2016 does not have a trend, but the November-May average has a small downward trend. Our monthly, seven-month and two-year averaged time series all show that the index turned up since January 2016. The upturn is rather large in the monthly and 2-year running smoothed time series. It is smaller in the November to May seven-month average because of the months in 2016 that are not used in that average, but the upturn should still be there. This, combined with other indicators noted in our paper, suggests the AMOC’s decline is ending. Curiously, for some reason AMOC proxy’s latest speeding up is missing in their time series.

    Some of the rest of the difference may be due to different treatment of polar data with ice cover part of the year. This may affect the global mean SST, which was subtracted from the gyre SST to arrive at that index.

    That we could not reproduce Caesar et al.’s proxy exactly (ours having a lower linear trend since 1950s) did not impact our conclusion that the recent variation is best explained as a multidecadal variability with downs as well as ups. We agree with Dr. Rahmstorf when he told us that if we could not reproduce their proxy we should not have included it in our paper. We regret it, because now it becomes a distraction in this Realclimate exchange. We hope this does not distract from our argument and conclusion presented in our paper based on our own proxies.

    We will rebut the other comments in the next blog.

    [Response: Dear Ka-Kit, thank you very much for stopping by and presenting your arguments! There is not much that I disagree with here. I am glad that you acknowledge that there is a human-caused weakening trend in the AMOC, so we indeed fully agree on this issue – including on the fact that this is much less steep than the short-term weakening since 2004 seen in the RAPID data, as also mentioned in our paper (Caesar et al. 2018), but rather only as large as predicted (on average) by the IPCC models. So this point seems to have been mostly an issue of unfortunate wording in your abstract which gave the impression that you disagree there is a human-caused downward trend.

    We did nowhere imply that the Zhang fingerprint pattern points to an anthropogenic cause, but rather only say that it is a sign of weakening since the beginning of the 20th Century. This pattern shows clearly in the linear temperature trends since the beginning of the 20th Century. That this trend is likely anthropogenic is what we concluded from the fact that it is of the magnitude predicted by a suite of climate models in response to anthropogenic forcing (as you cite above). In addition, in Rahmstorf et al. 2015 we found from proxy data that this modern weakening is unique in at least a millennium, which argues against it being part of normal, natural variability.

    And of course there is nothing circular about our arguments. The subpolar Atlantic is the only region of the globe which has not warmed despite rising greenhouse gases, and the most likely explanation for this is that the AMOC has weakened, thus bringing less heat into this region and thereby counteracting the greenhouse warming there. This is exactly what has been predicted by climate models. What is circular about that argument?

    Regarding the curve of Caesar et al: if you present the index of Caesar et al. and cite us as the source of the curve, it would have been good scientific practice to present it as we published it. As you say, the data for that curve are publically available at Nature and if you had any problems with those data, you could have contacted us and we would have been happy to help. Publishing a different curve but presenting it as if that is the curve from Caesar et al. is not OK. Of course you can do your own recalculation of our AMOC index and publish that, but then you have to say that this is what you have done, and you should have first found out why you got different results. Since all authors involved from our side in this index calculation are in their summer holidays now (including myself), we can’t resolve this on the quick now. The time for that would have been before you publish your paper with the misleading claim that this is our curve. Regards, Stefan ]

  21. 21
    Ka-Kit Tung says:

    Dear Stefan, Yes, there are many aspects of this problem that we are in agreement with. That was why I was surprised with the mocking tone of your original post with Mike. I was also surprised since you and I have been in constant contact and our exchanges have been friendly. As you can tell, my disagreement was with that post and not your paper, which we never criticized. I would appreciate an opportunity to post my rebuttal with figures and formats. My rebuttal will not be unfriendly, and I would focus on giving the readers scientific information, explained in laymen’s terms (as you have been able to do so well), such as on deep convection in the subpolar Atlantic, the relationship between AMO and AMOC etc. I think your arguments in the original post were misleading to the general readers. If it is not possible to post figures in the Reply I would appreciate you invite me as a guest contributor.

    There may be some misunderstanding: As we explained in our Methods section of our paper, what we showed was reproduced following your prescription of your proxy, not a copy of your proxy. We showed the unsmoothed version because of a reviewer’s suggestion; this cannot be done unless we reproduce instead of copying your index. Other proxies shown in our paper were also reproduced by us instead of copied. So it was consistently done. We had given full credit to the original authors. It is perhaps because of the way we gave credit that you thought we were saying your proxy was copied. We did not mention the small differences between our reproduced result and your original because we did not use that difference to arrive at a different conclusion. So let us agree not to let this point to further distract us in our debate. KK

    [Response: Dear Ka-Kit, thanks again for joining the discussion here, we greatly appreciate it. Unfortunately I cannot edit and publish a guest article by you as I am on holiday, but please post your views as comments and you can link to any images that you put somewhere on the web. I am glad we agree on the presence of an anthropogenic trend and it would be good if you emphasise this point to any media contacts. I can tell you that all the media that contacted me had misunderstood that point – they assumed that your intention was to contradict our published papers by suggesting there is no anthropogenic weakening. A number of articles have appeared with claims and headlines like “The slowing of Atlantic overturning is not part of global warming but a natural cycle”. You cannot expect that the public or many journalists read the full paper or understand the difference between a long-term climatic trend and short-term decadal variability, and that both are present at the same time and your paper was only looking at the latter. I look forward to your further explanations, in particular how the alternative heat budget works that you are proposing. -Stefan]

  22. 22
    Ka-Kit Tung says:

    Dear Stefan, Thank you for your advice on how to deal with the media. I now understanding why you reacted so negatively to our paper. I cannot control the media but I can be more careful. I will take that to heart.

    Since the publication of two papers earlier in Nature, Thornalley et al (2018) and Caesar et al (2018), the headlines in the media have been in the other extreme, and these are not justified by the conclusions of the papers themselves. I think both papers were scientifically rigorous and rigorously reviewed, but in speaking with the media, somehow an over simplistic message that the recent AMOC decline is anthropogenically caused was conveyed. Undoubtedly, anthropogenic forcing played a role, but neither paper can quantify that role.

    So the lesson is, we all need to be careful with our message.

  23. 23
    Ka-Kit Tung says:

    Dear Stefan, I will wait till you return from vacation to submit our guest article on deep convection and AMO-AMOC. It will be easier for the readers to have the figures in the article and the formatting with headings. I will hold off rebutting the critique of you original post with Mike on these issues. KK

    [Response: Dear Ka-Kit, you are very welcome to submit a guest article. I can’t promise anything since the publication decisions are taken by the Realclimate team. The key is that your article fits the Realclimate style: interesting and readable to our diverse audience and to the point (no longer than our article above), with references to the scientific literature. Look forward to seeing it after the holidays, Stefan]

  24. 24
    Carrie says:

    21 Ka-Kit Tung

    I took notice of the last words in the paper:
    “We note, however, that we have discussed here only one component of a complex system: global heat balance is maintained by the combined ocean and atmosphere systems and a change in the transport of one regional component may affect the partitioning of change between other parts of the ocean or of the atmosphere, depending on the timescales involved.”

    “That was why I was surprised with the mocking tone of your original post with Mike.” Me too. Happens way too much. Aggressiveness is the norm these days, sadly. I’m sorry you have been ambushed like this.

    About the Response: Would an apology hurt you that much? Can you not understand what he has just said to you in his reply and have no empathy of what’s been done to them ‘publicly’ here?

    In almost every other situation where the “media” have got the headlines screwed up and misunderstood the details and nuances of one of your or your friend’s papers it’s always their fault not yours. This time it’s automatically KK’s by default. Oh the poor media! being misled like this – seriously?

    “and your paper was only looking at the latter.

    Mmmm, did you read the Paper properly and in full and in Context?

    See last sentence Above.

  25. 25
    Carrie says:

    “We have been in contact with Dr. Rahmstorf since July 15, 2018, when he wrote that he read an advance copy of our paper and wonder why our figure looks different than the Figure 6 in Caesar et al. We produced a comparison of the two curves and showed to him. We were still trying to pinpoint the minor differences near the peak around 2005 and requested more information from him (so far not received), when their blog appeared.”

    and

    “That was why I was surprised with the mocking tone of your original post with Mike. I was also surprised since you and I have been in constant contact and our exchanges have been friendly.”

    By not being personally offended and therefore not overly defensive, by not leaping to assumptions, by not hastily drawing conclusions, by not prejudging others intentions, while possessing a modicum of genuine respect for Chen and Tung may have led to at least a draft copy of this RC blog article being emailed to Chen and Tung for their feedback, clarifications and possible corrections before rushing to publish it.

    [Response: The article is critical but factual. The only sentence that could be viewed as “mocking” concerns the authors cherrypicking the only data set that shows a “warming slowdown” due to a known data coverage problem. Other colleagues have used much harsher words about this. As bloggers we try to be timely so we published as soon as our piece was written. Just like science journalists we don’t require approval by anyone on what we write. We take full responsibility for any errors. -Stefan]

  26. 26
    Jim Baird says:

    Dear Drs. Rahmstorf and Tung, few here dispute the long-term climatic trend.

    Isn’t the objective to reverse this?

    Stefan has argued, the increase in global temperature could be stopped within decades. By reducing emissions, ocean heat content will continue to increase for at least a thousand years after we have reached zero emissions. If this heat were evenly distributed over the entire global ocean, water temperatures would have warmed on average by less than 0.05 °C (global ocean mass 1.4 × 1021 kg, heat capacity 4 J/gK).

    The problem is this heat isn’t evenly distributed. It accumulates at the equator and is eliminated at the poles with a net accumulation according to Lyman of 335 terawatts annually.

    How can global temperature be stopped within decades when this heat has to come out of the ocean in order for it to come into equilibrium with the atmosphere?

    The thermohaline is initiated by a phase change from water to ice. The sinking salty water is replaced by warmer waters from the tropics.

    The SST of tropical water can be reduced for at least 250 years by the phase change of a low-boiling-point working fluid to a gas and then back to a vapor at 1,000 meters.


    As illustrated here,
    this can be accomplished at less cost than fossil fuel production and produce more energy.

    The effective policy option is ocean heat management IMHO.

  27. 27
    Mal Adapted says:

    Ka-Kit Tung:

    That was why I was surprised with the mocking tone of your original post with Mike.

    It seems to me the central issue here is ‘framing’, i.e. how to present scientific information in the fraught political context of anthropogenic global warming.

    When trained scientists debate verifiable physical phenomena in refereed venues, their purpose is to add to the global store of justified knowledge about ‘reality’, i.e. “that which, when you stop believing in it, doesn’t go away” (PK Dick). Disciplined by their competitively skeptical peers, scientists learn (whoo boy) early in their training that a hostile tone adds little if any epistemic value to peer-reviewed debate. In that context, what matters is which view is better justified epistemically.

    In the highly charged social context of anthropogenic global warming, OTOH, the goal of public AGW deniers is to win the public policy debate on behalf of their paying clients. Their clients’ goal is to forestall collective action to decarbonize the global economy, for as long as it’s profitable to pay for professional disinformation services. Tone is evidently an effective weapon for mercenary AGW deniers, who are trained in rhetorical warfare and disciplined by nothing but the profit motive. What matters is what their target audience believes is true, but does go away when they stop believing it: namely that material prosperity carries no hidden or deferred costs.

    In my unredeemably mediocre judgment, the ceaseless spate of climate disinformation in the public sphere ensues principally from deliberate investment in disinformation, by interests whose vast personal and family wealth stems from socializing a global cost of their business. I’m as confident as I need to be that truculent rhetoric issuing from the likes of James Inhofe and Donald Trump is more or less carefully crafted to obfuscate reality as scientists perceive it. There appears to be a swamping strategy at work, too 8^(. Be that as it may, the tragic failure of western liberal democracies (not going to unpack that phrase here) to take collective action to mitigate AGW is the cultural reality climate scientists confront.

    AFAICT, this blog is wholly about verifiable climate reality, and its moderators welcome courteous, disciplined scientific debate. However, its audience seems to be mostly educated laypeople (not going there either), and comments are refereed with broader tolerance for rhetorical tactics such as more or less subtle mockery (heh), presumably with recognition they may be effective in the political context of forestalling global climate catastrophe.

    FWIW, Ka-Kit, as a 10-year RC regular I’m reasonably confident any mockery from Stefan isn’t personally motivated. Of course, as a purely disinterested virtual observer, I’m willing to be set straight, but that appears contrary to the rule of parsimony 8^}!

  28. 28
    Mal Adapted says:

    moi:

    the ceaseless spate of climate disinformation in the public sphere ensues principally from deliberate investment in disinformation

    Brought to you by the department of redundancy department. Beware of over-editing.

  29. 29
    Alastair B. McDonald says:

    Stefan and KaKit,

    Aren’t you both missing the real point of your science, that global warming caused by anthropogenic emissions of CO2 is not causing Western Europe to cool.

    This runs counter to the prevailing paradigm that abrupt climate change in the northern hemisphere was caused by changes to the flow of the THC. That idea is supported by the fact that the THC reduced during the Younger Dryas but correlation is no causation.

    It seems to me that the cause of the three abrupt climate changes that happened during the Last Termination, entry into B-O, entry into and exit from the YD, were caused by advance and retreat of sea ice in the Arctic, driven by the ice albedo positive feedback. This caused the changes to the THC, (which added a positive feedback.) The THC was not the driver of climate change, it was a symptom.

    The bottom line is that the Arctic sea ice could suddenly retreat again, driven by the effects of CO2 from fossil fuels. Wally Broecker’s angry beast is not going to cause cooling, but rather an abrupt warming.

    [Response: This discussion is about the effect of small changes in the AMOC. The currently weak AMOC is enough to cool the subpolar Atlantic, but not enough to cool a much larger area that would include north-western Europe. A full breakdown of the AMOC – which can’t be ruled out for the future – would cool northwestern Europe. But that is a different discussion which we’ve had e.g. here: http://www.realclimate.org/index.php/archives/2017/01/the-underestimated-danger-of-a-breakdown-of-the-gulf-stream-system/ -Stefan]

  30. 30
    Ka-Kit Tung says:

    Re:#29: Yes, you are right that we are missing the real point of our science in some of the debates, being distracted by a certain time series. I wanted to give a more detailed rebuttal with figures to the statements made in the original blog: that since AMOC and AMO are in phase in preindustrial times that it will continue to be so during the industrial period. The original blog seemed to conclude that the answer is NO to the title question: “Does a slow AMOC increase the rate of global warming?” The in-phase correlation between AMOC and AMO, and between AMOC and Northern Hemisphere mean surface temperature CHANGED when there is a persistent top-of-atmosphere forcing due to CO2. There is some interesting, and perhaps counterintuitive, science there.

    [Response: Another strong claim – I look forward to seeing statistical evidence for the alleged change in relationship between AMOC and AMO. Were you at the North Atlantic variability session at AGU last year? There were several statistical analyses presented there of the AMO which consistently concluded that (a) the subpolar and (sub)tropical SST variability (both included in the AMO, which is just a north Atlantic SST index) are uncorrelated and (b) the subpolar variability is driven by ocean heat transport (i.e. the AMOC). We mention that in Caesar et al. Point (a) incidentally speaks against your claim in your paper that the AMOC affects Atlantic hurricane activity, because that is affected by subtropical SST, i.e. uncorrelated to subpolar SST and therefore AMOC. -Stefan]

  31. 31
    MA Rodger says:

    Perhaps one of the (many) difficulties in proposing that AMOC waggles NT mean surface temperatures is that the most recent wiggle in the AMOC 1993-2015 is as big as any of the wiggles within the reconstructions of AMOC. Yet the so-called hiatus 1999-2015 provides a NH mean surface temperature waggle of such miniscule size. It is so small, it is even a problem describing it as a ‘slowdown’. Chen & Tung (2018) (which for some reason plots HadCRUT global mean surface temperature in its Fig3b, which is not truly global) only manages to create the impression of ‘slowdown’ visually by introducing a “non-linear secular trend” of unknown origin. Without that underived “non-linear secular trend” there is no difference between the GMST waggles of 1999-2015 and , say, 1865-75 which is a period not labeled ‘slowdown’. And if the AMOC wiggle is waggling NT Mean surface temperatures, the large ‘slowdowns’ of 1880-1910 and 1945-75 shown in Fig3b must be the result of truly massive wiggles in AMOC, wiggles of a size not present in any AMOC reconstruction.

  32. 32
    zebra says:

    #30

    This is a fascinating discussion, and I look forward to seeing it develop. I would like to suggest how both parties could make it more accessible to “the public” and move it away from the “climate wars” context.

    How about clarifying, in some kind of bullet-point or chart-like form, what the options are, in terms of cause and effect. For me, the headline use of “increase the rate of global warming” was really confusing until this final interchange.

    What I seem to be understanding is that you are talking about a local, relatively short-term effect.

    If A, there will be a local increase in surface temps, if B, there will be a local decrease. That change would obviously be incorporated into the calculation of the global mean.

    But, in either case, there would be no significant effect on the current forcing from CO2, correct? So, we don’t have a “feedback”, or a “forcing”, but internal variation, whichever side is correct about the various correlations. The global energy (im)balance is still the same; it is the distribution of energy that is in question. Is this a fair interpretation?

    If so, again, some kind of mutually agreed logic-chart with what the specific disagreements are would be helpful.

  33. 33
    Carrie says:

    25 Carrie [Response: The article is critical but factual. The only sentence that could be viewed as “mocking” concerns the authors cherrypicking the only data set that shows a “warming slowdown” due to a known data coverage problem. Other colleagues have used much harsher words about this. As bloggers we try to be timely so we published as soon as our piece was written. Just like science journalists we don’t require approval by anyone on what we write. We take full responsibility for any errors. -Stefan]

    Stefan, I have seen other articles by you and Mann that have been ‘critical and factual’ before about other’s Papers. They were also polite and respectful in several ways not present in this article and your followup responses.

    “Mocking” is a subjective personal judgement. Unlike you I am willing to take Dr Tung’s opinion and feedback about his/her feelings on that matter at face value and genuine (until proven otherwise.) He/she is certainly best placed above all to make that call. When I combine my own personal observations with the feedback from Tung (which came later) I get a match. You do not. But that does not fit the context nor the framing I see imo.

    I do not know for “certain” but it appears to me sitting in the “peanut gallery” there is much more going on here than merely a “factual critique” of someone’s Paper and the errors you and Mann feel need to be pointed out/exposed. Something doesn’t add up here. It’s not a paper by C Monckton and T Ball. So I get a sense something is going on behind the scenes that readers are not privy to. But I’ll leave you with it, it’s your business, and I’ll butt out.

  34. 34
    Carrie says:

    Of course I can appreciate accusations of myself being the pot calling the kettle black. Life’s full of contradictions. :)

  35. 35
    Ka-Kit Tung says:

    Response to Stefan’s inline Response to #30: We discussed in the Methods section of our paper the same issues that you mentioned. We said that the subpolar part of AMO is AMOC driven, while the subtropical part is partly influenced by the atmosphere, especially at higher frequencies. It is also influenced by the northward displacement of the Gulf Stream which is itself a consequence of the slowing of AMOC. This part was also discussed in your paper, Caesar et al (2018), to explain why the “fingerprint” of a weakening AMOC is a dipole in SST: cooling in the subpolar and warming in the subtropical region. Your statement above (I maybe confused by the sentence structure) seems to imply that the subtropical part of your “fingerprint” is uncorrelated with your subpolar part, and unrelated to the AMOC weakening, contrary to the claims made in your paper that the dipole is a fingerprint of a weakening AMOC.

    I am familiar with the debate between Rong Zhang on one hand, and Amy Clement and Mark Cane on the other hand on the other hand, who claimed that AMV can be explained by atmospheric red noise without ocean dynamics (discussed in the AGU meeting). The latest publication on this debate is the 2017 GRL paper by Zhang. The Abstract is:

    “Abstract This study identifies key features associated with the Atlantic multidecadal variability (AMV) in both observations and a fully coupled climate model, e.g., decadal persistence of monthly mean subpolar North Atlantic (NA) sea surface temperature (SST) and salinity (SSS) anomalies, and high coherence at low frequency among subpolar NA SST/SSS, upper ocean heat/salt content, and the Atlantic Meridional Overturning Circulation (AMOC) fingerprint. These key AMV features, which can be used to distinguish the AMV mechanism, cannot be explained by the slab ocean model results or the red noise process but are consistent with the ocean dynamics mechanism. This study also shows that at low frequency, the correlation and regression between net surface heat flux and SST anomalies are key indicators of the relative roles of oceanic versus atmospheric forcing in SST anomalies. The oceanic forcing plays a dominant role in the subpolar NA SST anomalies associated with the AMV. ”

    These issues could be discussed more in a post with figures, including various different definitions of AMO/AMV. We have been using EOF to define our AMO, which has a stronger subpolar emphasis. Enfield’s definition of SST mean of Atlantic north of the equator linearly detrended includes a little more of the subtropical atmospheric emphasis.

  36. 36
    Al Bundy says:

    Mal Adapted: , by interests whose vast personal and family wealth stems from socializing a global cost of their business.

    AB: This puzzles me no end. Why on Earth would someone with more money than they could reasonably spend want more money? Even more, why would they be willing to degrade their own future in order to acquire more money that they will never spend? It sounds totally insane to me. Ideas?

  37. 37
    Hank Roberts says:

    The in-phase correlation between AMOC and AMO, and between AMOC and Northern Hemisphere mean surface temperature CHANGED when there is a persistent top-of-atmosphere forcing due to CO2. There is some interesting, and perhaps counterintuitive, science there.

    Perhaps Tamino could suggest how to look into that?

  38. 38

    Xianyao Chen & Ka-Kit Tung, Nature Vol. 559 July 19, 2018 states “Evidence from paleoclimatology suggests that abrupt Northern Hemisphere cold events are linked to weakening of the Atlantic Meridional Overturning Circulation (AMOC)…” and “…a weakened AMOC, as in 1975-1998, should have led to Northern Hemisphere cooling”. Are these correct and is it of sufficient geographic definition ? Is it the consensus that a weakened AMOC should lead to cooling of China, India & Russia (all in the Northern Hemisphere) ? Or should the statements have been “Evidence from paleoclimatology suggests that abrupt Northern Hemisphere cold events in the north Atlantic region are linked to weakening of the Atlantic Meridional Overturning Circulation (AMOC)…” and “…a weakened AMOC, as in 1975-1998, should have led to cooling in the north Atlantic region” ?

  39. 39

    Xianyao Chen & Ka-Kit Tung, Nature Vol. 559 July 19, 2018 states “…storing heat in the deeper Atlantic, buffering surface warming for the planet as whole.”. This makes no sense because the concept on decadal and longer time scales violates the understanding by climate scientists of “Earth’s energy budget” and any climate scientist publishing with that huge change proposed needs to clarify their new science modifying “Earth’s energy budget” concept, which is:
    – Time-averaged global solar input (quantity and spectral distribution)
    – Earth’s average albedo
    – Quantity and type of atmospheric greenhouse gases
    The 3 items above are the lot for Earth’s balanced GMST, nothing about heat going into oceans. It’s a very big deal to state that you’ve proved that incorrect. ENSO is a cycle that takes heat temporarily down and cools the surface not by taking heat down into the ocean (I vaguely recall Trenberth 300m) but by revealing colder water ultimately from the Antarctic at the tropical Pacific Ocean surface. It definitely does not cool the surface by sinking the heat in the Pacific Ocean “instead of” warming the surface. It cools the surface by revealing colder water ultimately from the Antarctic at the tropical Pacific Ocean surface (you see this colder-than-average surface water over a vast area of the tropical Pacific Ocean in temperature pictorials of La Nina years), not the same thing at all. This concept of lowering GMST by “taking heat down” instead of heating the surface is different than ENSO and seems nonsensical to me.

    The reason is that heat “buried in the oceans” goes there instead of passing through Earth’s atmosphere and being radiated to space. It does not get “buried in the oceans” instead of sitting for decades or even years in Earth’s atmosphere.

    Summary:
    – Earth’s GMST cannot be lowered by “taking heat down into the oceans instead”
    – Earth’s GMST can be lowered internally by an energy flow within Earth’s ecosphere by bringing to the surface some of the “store of cold” in the oceans which got there by water radiating heat to space near Antarctica & Greenland over the last 3,300 years
    – The above 1st line contradicts the universally-accepted principle of “Earth’s energy budget” and the 2nd line above does not.

    Now, anomaly in the other end(s) of the THC can make sense (Indian, Pacific Oceans ?). For each 1 km**3 surface water stored in the deeper there must be 1 km**3 of deeper ocean water moved up and 1 km**3 of new surface water from below else all Earth’s oceans would end up squashed in the Marianas Trench and the sea bed would be dry (sorry). so more surface water descending anywhere on Earth must mean more below-surface water surfacing some other place(s) on Earth. Ocean average Potential Temperature 3.5 degrees and GMST is (I forget) ~15.5 degrees, which is much warmer. What about that ?

    Incidental: geothermal heat into the oceans is 1.4 Zettajoules / year, a very steady small warming attempt. If the oceans went stagnant geothermal would warm by that additional

  40. 40
    Carrie says:

    Possibly related?

    Consensus climate models need to improve their representation of the stratosphere-troposphere coupling in order to better project coming extreme weather events, including those in Siberia:

    Pengfei Zhang et al. (25 Jul 2018), “A stratospheric pathway linking a colder Siberia to Barents-Kara Sea sea ice loss”, Science Advances, Vol. 4, no. 7, eaat6025, DOI: 10.1126/sciadv.aat6025
    http://advances.sciencemag.org/content/4/7/eaat6025

    Abstract: “Previous studies have extensively investigated the impact of Arctic sea ice anomalies on the midlatitude circulation and associated surface climate in winter. However, there is an ongoing scientific debate regarding whether and how sea ice retreat results in the observed cold anomaly over the adjacent continents. We present a robust “cold Siberia” pattern in the winter following sea ice loss over the Barents-Kara seas in late autumn in an advanced atmospheric general circulation model, with a well-resolved stratosphere. Additional targeted experiments reveal that the stratospheric response to sea ice forcing is crucial in the development of cold conditions over Siberia, indicating the dominant role of the stratospheric pathway compared with the direct response within the troposphere. In particular, the downward influence of the stratospheric circulation anomaly significantly intensifies the ridge near the Ural Mountains and the trough over East Asia. The persistently intensified ridge and trough favor more frequent cold air outbreaks and colder winters over Siberia. This finding has important implications for improving seasonal climate prediction of midlatitude cold events. The results also suggest that the model performance in representing the stratosphere-troposphere coupling could be an important source of the discrepancy between recent studies.”

    See also:
    Cartier, K. M. S. (2018), Why are Siberian temperatures plummeting while the Arctic warms?, Eos, 99, https://doi.org/10.1029/2018EO103403. Published on 26 July 2018.
    https://eos.org/articles/why-are-siberian-temperatures-plummeting-while-the-arctic-warms?utm_source=eos&utm_medium=email&utm_campaign=EosBuzz072718

    Extract: “The team is currently exploring whether the stratosphere plays a similar role in linking regional sea ice loss to extreme weather events in the northern reaches of North America.”

  41. 41
    patrick says:

    @21 [Response] > I am glad we agree on the presence of an anthropogenic trend and it would be good if you emphasise this point to any media contacts. I can tell you that all the media that contacted me had misunderstood that point – they assumed that your intention was to contradict our published papers by suggesting there is no anthropogenic weakening. A number of articles have appeared with claims and headlines like “The slowing of Atlantic overturning is not part of global warming but a natural cycle”. You cannot expect that the public or many journalists read the full paper or understand the difference between a long-term climatic trend and short-term decadal variability, and that both are present at the same time and your paper was only looking at the latter. [etc.] –Stefan

    Thank you for your clarity on this, and elsewhere.

  42. 42

    No rapid global warming. Just the opposite. I would like to see commenting on my findings on solar activity and climate. Well my 2005 climate model has been the only model to project dropping temperatures. I saw some comments about my hypothesis that planets tidally perturbate the solar surface. They state that this would be a centimeter only change in solar surface. But this is not correct because solar surface is not any liquid but a very low density gas and any perturbation can lead to thousands of degrees change.
    I give some history of the ideas:
    already as of 2006 I shared my findings with the community via Climate Audit blog but the blog people refused to reproduce these fine findings.
    This is my original 2005 thesis at the ITIA-NTUA library (National Technical University of Athens) in Greek
    https://www.itia.ntua.gr/en/docinfo/680/
    and this is a more recent paper of 2014 in the same ITIA-NTUA library in English explaining the sun-climate connection phenomena.
    https://www.itia.ntua.gr/en/docinfo/1486/
    In my blog I provide links to my more recent papers.
    http://dimispoulos.wixsite.com/dimis
    climate modeling can’t be accurate if not attributing to the correct phenomena.
    if you don’t spread the truth and scientifically documented discoveries, you can’t solve anything but preserve a vicious state

  43. 43

    AB, #36–

    AB: This puzzles me no end. Why on Earth would someone with more money than they could reasonably spend want more money? Even more, why would they be willing to degrade their own future in order to acquire more money that they will never spend? It sounds totally insane to me. Ideas?

    It seems insane, and arguably it is, but for some people, wealth is not really about things to be used and enjoyed, or about changing the world in some positive way, or even about power and control directly; it’s about winning, which really means it’s about personal worth. As the attitude was put the son of an acquaintance, “He who dies with the most toys, wins.”

    For me, that’s wrong on so many levels that I don’t care to try to unpack it all. But apparently, some do feel that. And one consequence is that it abolishes ‘enough’, since it’s not about actual utility anymore. It’s about competition and ‘ego’–about not being a ‘loser.’ Life becomes a competition in which ‘no lead is safe.’

  44. 44
    Hank Roberts says:

    Al B: “Why on Earth would someone with more money than they could reasonably spend want more money?”

    look up: Supernormal stimulus

  45. 45
    Hank Roberts says:

    the only model to project dropping temperatures

    Not the only one to have predicted global cooling, but those that did have thus far been wrong about that:
    https://skepticalscience.com/future-global-cooling.htm

    https://www.skepticalscience.com/solar-activity-sunspots-global-warming.htm

  46. 46
    Victor says:

    #21 Stefan’s comment:

    “I am glad we agree on the presence of an anthropogenic trend and it would be good if you emphasise this point to any media contacts. I can tell you that all the media that contacted me had misunderstood that point – they assumed that your intention was to contradict our published papers by suggesting there is no anthropogenic weakening. A number of articles have appeared with claims and headlines like “The slowing of Atlantic overturning is not part of global warming but a natural cycle”. You cannot expect that the public or many journalists read the full paper or understand the difference between a long-term climatic trend and short-term decadal variability, and that both are present at the same time and your paper was only looking at the latter.”

    From the paper in question:
    “Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability6, rather than an anthropogenically forced trend.”

    Looks like the media got it right after all. Changes “since the 1940’s” seem pretty long-term to me, but what do I know?

  47. 47
    Carrie says:

    I request a critical but factual review by RC authors of this paper please.

    Hansen et al. 2016

    Hansen, J., M. Sato, P. Hearty, R. Ruedy, M. Kelley, V. Masson-Delmotte, G. Russell, G. Tselioudis, J. Cao, E. Rignot, I. Velicogna, B. Tormey, B. Donovan, E. Kandiano, K. von Schuckmann, P. Kharecha, A.N. LeGrande, M. Bauer, and K.-W. Lo, 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. Atmos. Chem. Phys., 16, 3761-3812, doi:10.5194/acp-16-3761-2016.

    https://pubs.giss.nasa.gov/abs/ha04710s.html
    and
    https://www.atmos-chem-phys.net/16/3761/2016/

    eg in the abstract – “These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.

    But I am much more interested in an deep analysis of the all Data presented which feeds into the “highly controversial” conclusions of Hansen et al., compared with the much more conservative “consensus view” of the IPCC and RC scientists.

    eg Is Hansen et al right, wrong, unclear, definitive, uncertain, valid or reasonably accurate; and, is it based on solid scientific evidence and high quality scientific analysis or not?

    I recall seeing M Mann in an interview being equivocal about this paper and yet maintaining a respectful tone of Hansen nevertheless. I would like to know what he and others really think about this important Paper which gained a lot of media attention and yet very little sustained support from the body of climate scientists in the field.

    Potentially related cites to be discussed in light of Hansen et al & other papers about the AMOC weakening:

    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

    Let’s focus critically on the actual science being communicated – who is more right (scientifically) that is? Thanks

  48. 48
    Carrie says:

    30 Ka-Kit Tung; I’d like to see their rebuttal in full. It’s fair and reasonable to allow that to happen here.

  49. 49
    Mal Adapted says:

    Al Bundy:

    AB: This puzzles me no end. Why on Earth would someone with more money than they could reasonably spend want more money? Even more, why would they be willing to degrade their own future in order to acquire more money that they will never spend? It sounds totally insane to me. Ideas?

    Well, yeah. Economically speaking, AGW is a Tragedy of the Commons: it’s the aggregate result of rational individual choices by producers and consumers of fossil fuels. The present, private benefits of socializing the climate-change costs of each fuel transaction on the ‘free’ market are greater than the transactors’ anticipated share of the total socialized costs. We consumers buy as much energy as our budgets allow at the lowest per-joule prices we can find, to heat our homes, cook our meals and run our crosstown errands, then freely emit all that fossil carbon out our private tailpipes. Fossil fuel producers, meanwhile, collect annual profits in the $100s of billions by removing the stuff from geologic sequestration and selling it for all the traffic will bear, leaving it to consumers to actually transfer it to the atmosphere.

    Fossil fuel producers and their investors, IOW, control the kind of wealth that effectively buffers them from the impacts of climate change, and gives them the political power to block collective action which would curtail their revenues and strand their assets. They don’t really expect their futures to be degraded, and the net present benefit of BAU is highly positive for them, as individuals and families. The future’s uncertain and the end is always near anyway. Under the circumstances, they’d be fools not to do what they’re doing. We’re the fools, for letting them keep doing it.

  50. 50

    #47 Hank, it’s been the only climate model that projects dropping temperatures.
    Solar irradiance has been decreasing since 1950 yet the planet warmed considerably since. My model accounts for it and only projects a cooling in the coming decades. Why? Because I have accounted for the solar wind effect on climate too. Solar wind is becoming stronger and stronger since 1930 and is the reason for nowadays high temperatures. Though temperatures shall fall down again.
    There is a very strong mathematical documentation of the described phenomena. Within this frame I have even explained the complete solar wind mechanisms and theoretically calculated all it’s properties (temperature, velocity etc).
    This is the correct approach to climate variability for I have attributed for the correct phenomena.