New Study: Atlantic Circulation Less Stable than Thought (“Gulf Stream” slowdown)

January 5, 2017

New study on behavior of Global warming’s effect on critical circulation in the Atlantic Ocean – what many laypeople think of as “the Gulf Stream”, but scientists more correctly call the “Atlantic Meridional Overturning Circulation”, or AMOC.

I produced the video (above) not long ago on a similar study by Stefan Rahmstorf, Mike Mann, and Jason Box – which looked at the possible effect of melting Greenland ice on the same circulation.  The current study looks at greenhouse gases and global temps generally, but does not include Greenland melt.

Main point is, while most folks think of global warming as a long, slow process, climate change can have unexpected, paradoxical, and sudden effects, as it can cause relatively rapid changes in ocean and/or atmospheric circulation.

Science News:

Spewing too much carbon dioxide into the atmosphere could shut down the major ocean current that ferries warm water to the North Atlantic, new climate simulations suggest. While not as extreme as the doomsday scenario portrayed in the movie The Day After Tomorrow, such a shutdown could cause wintertime temperatures to plummet by an estimated 7 degrees Celsius or more in northwestern Europe and shift rainfall patterns across the globe.

Many previous climate simulations predicted that the Atlantic circulation would remain largely stable under future climate change. But those simulations failed to accurately portray how relatively freshwater flows between the Atlantic and Southern oceans, an important mechanism as the climate warms. After fixing that inaccuracy, Yale University climate scientist Wei Liu and colleagues set up an extreme climate scenario to test the current’s robustness. Doubling CO2concentrations in the atmosphere shuttered the Atlantic current in 300 years, the researchers’ simulation showed.

While such a rapid CO2 rise is unrealistic, the new simulation demonstrates that the current isn’t stable after all, the researchers conclude January 4 in Science Advances. “The next step is to use a more realistic warming scenario to predict what the future will look like,” Liu says.

Even with a more realistic scenario, the applicability to the real world will be hampered by a lack of direct long-term observations of the Atlantic circulation, says Gerald Meehl, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. Observations help improve simulations, but such data for the Atlantic current don’t go back “for more than a decade or two,” he says.

Known as the Atlantic Meridional Overturning Circulation, the Atlantic current is a colossal conveyor belt. It carries warm water from the South Atlantic northward along the ocean surface into the North Atlantic. Near Greenland where the current makes a U-turn, cold water sinks and flows southward into the South Atlantic. These two halves of the AMOC form a loop that keeps northwestern Europe warm and drives rainfall across the tropical Atlantic.

Warming due to climate change in the North Atlantic makes the waters there less dense and less likely to sink. This change slows the AMOC. In many previous climate simulations, the current’s speed bounces back. That’s because these simulations incorrectly show that rain-freshened water flows from the Southern Ocean into the Atlantic Ocean. In these simulations, as the AMOC weakens, this influx of freshwater slackens and the Atlantic becomes saltier. Like cold water, salt-laden water is denser and more likely to sink, helping the AMOC recover.

But ocean observations show that freshwater flows from the Atlantic into the Southern Ocean, not the other way around. Liu and colleagues updated an existing simulation by manually correcting the flow direction. After doubling CO2 concentrations in the simulation compared with 1990 levels, the researchers found that the North Atlantic warmed and the AMOC slowed. With less warm water moving northward, countries such as England and Iceland cooled even when taking into account the greenhouse warming from the added CO2.

The researchers also found that as the AMOC slowed, less freshwater from the Atlantic flowed into the Southern Ocean. That decreased the Atlantic’s saltiness, further weakening and ultimately collapsing the AMOC. The same simulation without the flow-direction change did not show a disrupted current, the researchers found. Meltwater from shrinking Greenland ice may also freshen the Atlantic, suppressing the AMOC, though the researchers didn’t look at this effect.




31 Responses to “New Study: Atlantic Circulation Less Stable than Thought (“Gulf Stream” slowdown)”

  1. Okay, good, but I don’t understand one part …

    Doubling CO2concentrations in the atmosphere shuttered the Atlantic current in 300 years, the researchers’ simulation showed.

    While such a rapid CO2 rise is unrealistic …

    Preindustrial CO2 was 288 ppm. Call it 300 ppm. Current is 405 ppm or so. With non-CO2 GHGs accounted for, including those like CH4 that become CO2, presently stands at 490 ppm (Ref Nobel laureate and former Sec DOE Stephen Chu). At present rate of emission (no increase in it) of GHGs, will get to 650 ppm by 2050. 650 ppm is more thsn 2X 300 ppm. Doubling.

  2. Hypergeometric, their first scenario which they test their model against NCAR is based on an instantaneous doubling from current levels.

    Please see:

    We conduct two parallel doubling CO2 experiments (the CTLCO2 and ADJCO2) based on two versions of the CCSM3 (see Materials and Methods for details). The atmospheric CO2 concentration is instantaneously doubled (at year 201) from the present-day level and then remains constant thereafter. This warming scenario is between the Representative Concentration Pathways (RCP) 4.5 and RCP6.0 scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report (1). The two models show similar climate responses during the first three decades. The AMOC strength reduces (Fig. 1C)

    Open Access: W. Liu et al. Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate. Science Advances. Published online January 4, 2017. doi: 10.1126/sciadv.1601666.

    • So the issue then isn’t the doubling, it’s the impulse response from rapid doubling, and whether or not the AMOC as a system can tell the difference between an all-in-one-year impulse and the same forcing over 40 years.

      It might not, given as big as it is, but that’s a different calculation.

      And the point is the article in SCIENCE NEWS left that out.

  3. hypergeometric wrote:

    So the issue then isn’t the doubling, it’s the impulse response from rapid doubling…

    Yep. The first is essentially a toy scenario, similar to the toy models of all ocean or only two layers of atmosphere. The purpose of the unrealistic scenario is to simply to explore the principles involved where the contrast should be greatest. Instantaneous doublings are a pretty common way of exploring the behavior of models.

    Oftentimes the people who do the pop summaries, who may bounce from science topic to science topic, won’t really be that familiar with the science and will leave things out or just get them wrong. Another possibility is that the individual was familiar enough they forgot to explain something they automatically take for granted. Or perhaps they were just being human. Since this is a pop summary I might go for a combo of 1 and 3.

  4. Bob Doublin Says:

    GREAT post,Peter. And reminder of how complex it all is. Thanks.

  5. indy222 Says:

    Good, physical, intuitive-level explanation of a phenomenon people need to understand. Very well done.

  6. redskylite Says:

    Thanks for the great post Peter. A lot of people have a rosy picture of Climate Change/Global Warming, and (apart from pesky flooding) AGW seems to promise the ability of growing of luscious grape vines and exotic tropical fruits in previously temperate areas. Today’s topic is very much reminding us that climatic influences can be caused by the transfer of heat through ocean currents, and it is not just a simple equation of sun intensity/ghgs/aerosols/cloud cover etc. The climate of some geographic areas are very vulnerable.

    As someone who lived and enjoyed the mild climate of Dorset, U.K this following recent article seems very related to the AMOC slowdown/shift/change.

    Climate change is here: what does this mean for Dorset?

    “In the longer term, what will happen if we get global warming, is that the Labrador current on the East Coast of America, will go further South and because it is denser than warm water it will cut off the gulf stream which comes from the West Indies to here.”

    • redskylite Says:

      Another part of Britain that enjoys a mild climate, thanks to the influence of the Gulf Stream” is Cornwall. If you take this influence away the county would be devastated.

      “The subtropical weather in Cornwall means new exotic crops such as quinoa and Japanese persimmon are now more likely to succeed, according to a new technique developed by University of Exeter experts to monitor the climate.

      Parts of Cornwall have become subtropical since 2000 and this could create opportunities to grow new, unusual plants. Sunflowers, maize, grapevines and tea are already grown in the Duchy.”

  7. vierotchka Says:

    Up to about thirty-five or so years ago, winters reached temperatures considerably lower than since then – at least 7°C to 10°C lower than in recent years. In Geneva, where I live, we used to get abundant snow until about 35 years ago. Since then, winters have been gradually warmer and shorter, and we’ve even had spring flowers in December instead of March-April. In the past ten years, we’ve either had about 3 centimeters of snow that vanished almost overnight, or none at all.

  8. grindupbaker Says:

    “ocean observations show that freshwater flows from the Atlantic” – common sense dictates that fresher water must be flowing south, not north, in the Atlantic because the Arctic Ocean is totally enclosed by land (Bering paddling pool 0.8 Sv inbound) and precipitation exceeds evaporation and rivers run into it. That fresh rain and river water needs to get back to the tropics (even the Indian and Pacific) where it belongs and the only way is south through the Atlantic. Unless that 0.8 Sv inbound through the Bering Strait is highly saline. A huge feature of the Atlantic is that it has the 15,000,000 km**2 of Arctic above it that is not a warm evaporative surface so water cannot much leave it via atmosphere. It has to get out through the Atlantic.

  9. Tom Bates Says:

    I would point out this is all a model and ignores the real world climate of the last 12000 years. The number of assumptions built into this model are rather large and unproven. The observation of actual flows above is not common sense it is the actual flows. Fresh water is not flowing north unless a black hole exists somewhere in the arctic which is unlikely.

    • grindupbaker Says:

      Unless evaporation from the relatively warm surface water at 58N-62N or some such range, driven north by wind in the Atlantic has exceeded the net of precipitation – evaporation for the Arctic land and ocean. I’m wondering what the quantities are and whether there’s been some significant change in fresh water flux here (other than from Greenland changes) over the last few decades with the ecosphere warming.

  10. Tom Bates Says:

    Average monthly weather in Geneva, Switzerland. Geneva’s high altitude results in pleasantly warm summers, mild winters (with temperatures not going much below freezing), and year round rainfall. … The weather is pleasant with mild temperatures, however September is the wettest month.

    If this description of the weather is correct the poster making different claims lives on some other planet or perhaps we are talking about different Geneva’s.

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