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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. 51
    zebra says:

    Al Bundy (and Mal),

    “…why would anyone with more money than they can reasonably use…”

    Why would someone with access to a well-functioning forum to discuss the psychological, political, economic, and technological questions around mitigating climate change… insist on acquiring even more column inches from people who are trying to discuss a specific scientific question?

    But carry on Al, let’s hear some more of that fine outrage about the greedy capitalists and how their behavior is incomprehensible to you.

  2. 52
    Mal Adapted says:

    Before somebody else gets their licks in: the concept of a rational economic agent is under development, but is now understood to include cognitive motivators like risk-aversion and fairness, along with greed and the lust for power. If Economics is the scientific investigation of human behavior, that’s progress.

    Whether or not we understand their behavior, we shouldn’t suppose the shrewd businesspeople of the Koch club don’t know what they’re doing. Hey, if I’m so smart, why ain’t I rich ;^)?

  3. 53
    ASLR says:

    More food for thought:

    Dome Fuji Ice Core Project Members (08 Feb 2017), “State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling”, Science Advances, Vol. 3, no. 2, e1600446, DOI: 10.1126/sciadv.1600446

    http://advances.sciencemag.org/content/3/2/e1600446.full

    Extract: “Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation.”

    &

    Turney, et al. (2017), “Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial”, Nature Communications 8, Article No. 520, https://doi.org/10.1038/s41467-017-00577-6

    https://www.nature.com/articles/s41467-017-00577-6

    Extract: “An ensemble of transient meltwater simulations show that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train.”

  4. 54
    ASLR says:

    The linked reference discusses the finding of improved modeling near the Eocene–Oligocene transition (EOT) that are of particular interest for calibrating climate models to including Hansen’s ice-climate feedback mechanism (that is highly related to the response of the meridional overturning circulation). These new finding indicate higher climate sensitivity than projected by earlier less-sophisticated models. These findings increase the probability that Hansen’s warnings about the risks of abrupt climate change this century are correct:

    Hutchinson, D. K., de Boer, A. M., Coxall, H. K., Caballero, R., Nilsson, J., and Baatsen, M.: Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1, Clim. Past, 14, 789-810, https://doi.org/10.5194/cp-14-789-2018, 2018.

    https://www.clim-past.net/14/789/2018/

  5. 55
    Frank Sterle Jr says:

    “It’s harder to rally people around a threat to humanity than one that endangers their own backyard.” I spotted this profound yet disturbing truism, albeit perhaps logically Darwinian, in an essay (titled “Crossing Lines”) in the July/August issue of The Walrus.
    It highlights the apparently prevailing penny-wise-pound-foolish widespread human mentality when it comes to the serious manmade pollution, though immediately free from our societal view, that’s toxifying our life-sustaining natural environment and worsening an already dire global warming reality.
    Perhaps it helps explain the increase in per capita automobile ownership (including SUVs) in Canada last year, compared to 2016, especially in B.C.; it’s something that UBC’s Sauder School of Business economist Werner Antweiler describes as “a disconcerting picture”, considering serious global greenhouse gas concerns. “The number of vehicles has grown faster than the number of people in the country.”
    I often wonder whether that unfortunate aspect of our general nature that permits us our tunnel vision regarding environmental degradation, will be our eventual undoing?
    Maybe due to Earth’s large size, there seems to be a general oblivious mentality as though even the largest contamination event can somehow be safely absorbed into the environment—air, sea, and land.
    For example, it’s largely believed that when released into gritty B.C. coastal waters, diluted bitumen (dilbit) will likely sink to the bottom, as with the 2010 Michigan spill in which dilbit is still being scraped off of the Kalamazoo River floor.
    I wonder, could that sinking characteristic perhaps appeal to some people who are usually apathetic towards the natural environment deep below the water surface: i.e. it will no longer be an eyesore after it sinks—i.e. out of sight, out of mind?
    It may be the same mentality that allows the immense amount of plastic waste, such as disposable straws, to eventually find its way into our life-filled oceans, where there are few, if any, caring souls to see it.
    Could it be the same mentality that, when asked by a Global news reporter (a few months back) what he thought of government restrictions on disposable plastic straws, compelled a young male Vancouverite wearing sunglasses to retort, “It’s like we’re living in a nanny state, always telling me what I can’t do.”
    Astonished by his utter shortsightedness, I recall wondering whether he was the same sort of individual who had a sufficiently grand sense of material entitlement—a.k.a. the “Don’t tell me what I can’t waste or do, dude!” attitude—to permit himself to now deliberately dump a whole box of unused straws into the Georgia Strait, just to stick it to the authorities who’d dare tell him that enough is enough with our gratuitous massive dumps of plastics into our oceans (which are of course unable to defend themselves against such guys seemingly asserting self-granted sovereignty over the natural environment), so he could figuratively middle-finger any new government rules with a closing, ‘There! How d’ya like that, pal?!”
    And, of course, the condition is allowed to fester via a mainstream news-media, being socially liberal and/or economically libertarian, that seems to not have a problem with such childish oh-well perspectives; the same narrow-mindedness that often makes me question whether we really have plausible hope in turning around our recklessness in time?
    After all, why worry about such things immediately unseen, regardless of their most immense importance, especially when there are various undesirable politicians and significant social issues over which to dispute—distractions our mainstream media seem only too willing to provide us?
    Besides, what back and brain busting, home-mortgaged labourer sustains the energy to worry about such things immediately unseen, regardless of their most immense importance?
    I see it somewhat analogous to a cafeteria lineup consisting of diversely societally represented people, all adamantly arguing over which identifiable traditionally marginalized person should be at the front and, conversely, at the back of the line; and, furthermore, to whom amongst them should go the last piece of quality pie—all the while the interstellar spaceship on which they’re all permanently confined is burning and toxifying at locations rarely investigated.
    As a species, we really can be so heavily preoccupied with our own individual admittedly overwhelming little worlds, that we’ll miss the biggest of pictures.

  6. 56

    Mal, #54–

    Hey, if I’m so smart, why ain’t I rich ;^)?

    ‘Cause money can’t buy you love? ;-)

    Or, less cheekily, because you’re smart enough to realize that wealth is an idol of the mind?

  7. 57
    Hank Roberts says:

    > out of sight, out of mind?

    until you eat seafood with small plastic particles in it, or use sun-dried sea salt with more of the same.

    Frst law of ecology: nothing goes away

  8. 58
    Racetrack Playa says:

    Possibly relevant to this discussion, but from a more physical oceanography viewpoint, and very useful on background:

    Topographic Coupling of the Atlantic Overturning and Gyre Circulations
    Stephen Yeager
    Journal of Physical Oceanography 45(5):1258-1284 · May 2015

    https://www.researchgate.net/publication/276461242_Topographic_Coupling_of_the_Atlantic_Overturning_and_Gyre_Circulations

    In particular:

    “There is evidence both from ocean general circulation models(OGCMs; e.g., Gerdes and Köberle 1995;Häkkinen2001;Böning et al. 2006;Yeager et al. 2012;Yeager andDanabasoglu 2014) as well as from recent observations (Häkkinen and Rhines 2004;McCarthy et al. 2012)that changes in the large-scale Atlantic overturning and gyre circulations are fundamentally linked, but the dynamical connections remain obscure. Understanding the relationship between the overturning and gyre circulations, and their coordinated response to surface wind and buoyancy forcing perturbations, is of fundamental physical oceano-graphic interest, and it has broad implications for efforts to understand and predict the climate variations associated with changes in poleward oceanic heat transport.”

    And this too:

    “Furthermore, a variety of observational and modeling studies have made it clear that wind variability is the source of much of the high-frequency variance in AMOC, through adjustment to large-scale Ekman transport anomalies (Jayne and Marotzke 2001;McCarthy et al. 2012) or wind stress curlforcing (e.g., Kanzow et al. 2010; Duchez et al. 2014b).”

    This focus on the physics of the Atlantic gyre and overturning circulation is rather refreshing.

  9. 59
    ASLR says:

    The linked research provides more evidence that a slowing of the AMOC will lead to increased warming at high latitudes due to abrupt warming during the summer months; which increases Arctic Amplification:

    G. Bromley et al. (06 April 2018), “Interstadial Rise and Younger Dryas Demise of Scotland’s Last Ice Fields”, Paleoceanography and Paleoclimatology, Vol. 33, Issue 4, https://doi.org/10.1002/2018PA003341

    https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2018PA003341

    Abstract
    Establishing the atmospheric expression of abrupt climate change during the last glacial termination is key to understanding driving mechanisms. In this paper, we present a new 14C chronology of glacier behavior during late‐glacial time from the Scottish Highlands, located close to the overturning region of the North Atlantic Ocean. Our results indicate that the last pulse of glaciation culminated between ~12.8 and ~12.6 ka, during the earliest part of the Younger Dryas stadial and as much as a millennium earlier than several recent estimates. Comparison of our results with existing minimum‐limiting 14C data also suggests that the subsequent deglaciation of Scotland was rapid and occurred during full stadial conditions in the North Atlantic. We attribute this pattern of ice recession to enhanced summertime melting, despite severely cool winters, and propose that relatively warm summers are a fundamental characteristic of North Atlantic stadials.

    Plain Language Summary
    Geologic data reveal that Earth is capable of abrupt, high‐magnitude changes in both temperature and precipitation that can occur well within a human lifespan. Exactly what causes these potentially catastrophic climate‐change events, however, and their likelihood in the near future, remains frustratingly unclear due to uncertainty about how they are manifested on land and in the oceans. Our study sheds new light on the terrestrial impact of so‐called “stadial” events in the North Atlantic region, a key area in abrupt climate change. We reconstructed the behavior of Scotland’s last glaciers, which served as natural thermometers, to explore past changes in summertime temperature. Stadials have long been associated with extreme cooling of the North Atlantic and adjacent Europe and the most recent, the Younger Dryas stadial, is commonly invoked as an example of what might happen due to anthropogenic global warming. In contrast, our new glacial chronology suggests that the Younger Dryas was instead characterized by glacier retreat, which is indicative of climate warming. This finding is important because, rather than being defined by severe year‐round cooling, it indicates that abrupt climate change is instead characterized by extreme seasonality in the North Atlantic region, with cold winters yet anomalously warm summers.

  10. 60
    zebra says:

    #61 ASLR,

    Reading the “plain language” summary, I have trouble following what their conclusion is, and so your interpretation.

    “Earth is capable of abrupt high magnitude changes in both temperature and precipitation”

    Are they talking about a global measurement or North Atlantic region?

    Likewise for:

    “This finding is important because, rather than being defined by severe year‐round cooling, it indicates that abrupt climate change is instead characterized by extreme seasonality in the North Atlantic region, with cold winters yet anomalously warm summers.”

    Maybe you could clarify?

  11. 61

    #61 ASLR
    My Plain Language Summary
    The Scottish Highlands isn’t Earth (and I rather like the Scots).
    Check out on an Atlas or globe Pacific Ocean, Africa, Asia, Antarctica, Australia

  12. 62
    Hank Roberts says:

    Changes in Extreme Rainfall Over India and China Attributed to Regional Aerosol‐Cloud Interaction During the Late 20th Century Rapid Industrialization

    First published: 24 July 2018
    https://doi.org/10.1029/2018GL078308
    https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/

    =================

    … The enhanced aerosol pollution during this rapid industrialization era is the leading cause of the spatially heterogeneous extreme rainfall change by dimming surface solar radiation, cooling adjacent ocean water, and weakening moisture transport into the continental region, while GHG warming or natural variability alone cannot explain the observed changes. Our results indicate that the projected intensification of regional extreme rainfall during the early‐to‐mid 21st‐century, in response to the anticipated aerosol reduction, may be underestimated in global climate models without detailed treatment of complex aerosol‐cloud interaction.