A Nasty Surprise in the Greenhouse. New Paper, New Video.

March 23, 2015

The disaster movie “The Day After Tomorrow” was based on long term scientific concerns about global warming’s impact on the North Atlantic Current, also called the Atlantic Meridional Overturning Circulation, – what most people think of as “The Gulf Stream” – although that is a simplification.
The movie was obviously over the top in terms of the projected impacts, but after a decade in which science has downplayed the possibility of such an event, a new paper shows that the circulation is indeed slowing down.
This could signal potential impacts on weather, the food chain, and circulation of oxygen and nutrients throughout the ocean.
I’ve been interviewing key authors of the paper, Lead author Stefan Rahmstorf, as well as paleoclimate expert Mike Mann, and Glaciologist Jason Box.
This is a paper that could have substantial impact, and might very well be distorted or sensationalized,  – so bookmark this post as a damper for overhyped speculation, as well as a warning about real impacts.

Lead Author Stefan Rahmstorf in RealClimate:

The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al. 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years.

The whole world is warming. The whole world? No! A region in the subpolar Atlantic has cooled over the past century – unique in the world for an area with reasonable data coverage (Fig. 1). So what’s so special about this region between Newfoundland and Ireland?

amoctemptrend

Fig. 1 Linear temperature trend from 1900 to 2013. The cooling in the subpolar North Atlantic is remarkable and well documented by numerous measurements – unlike the cold spot in central Africa, which on closer inspection apparently is an artifact of incomplete and inhomogeneous weather station data.

It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway.

Difficult to measure

Climate models have long predicted such a slowdown – both the current 5th and the previous 4th IPCC report call a slowdown in this century “very likely”, which means at least 90% probability. When emissions continue unabated (RCP8.5 scenario), the IPCC expects 12% to 54% decline by 2100 (see also the current probabilistic projections of Schleussner et al. 2014). But the actual past evolution of the flow is difficult to reconstruct owing to the scarcity of direct measurements. Therefore, in our study we use data on sea surface temperatures in order to infer the strength of the flow: we use the temperature difference between the region most strongly influenced by the AMOC and the rest of the northern hemisphere.

northatcurrent

Fig. 2 Schematic of the Atlantic circulation. Surface currents in red, deep currents in blue, sea-ice cover in winter in white. (Source: Rahmstorf, Nature 1997)

Now we are not the first to have inferred from temperature data that the flow must have weakened. Evidence for this was already presented by Dima and Lohmann 2010 or Drijfhout et al. 2012, among others (for further references see the introduction of our paper).

What is new is that we have used proxy reconstructions of large-scale surface temperature (Mann et al, 2009) previously published by one of us (study co-author and RealClimate co-founder Mike Mann) that extend back to 900 AD (see “What we can learn from studying the last millennium (or so)”) to estimate the circulation (AMOC) intensity over the entire last 1100 years (Fig. 3). This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99 per cent probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.

paleo500

Fig. 3 Time series of the temperature difference between the subpolar North Atlantic and the entire northern hemisphere, which can be interpreted as an indicator of the strength of the Atlantic circulation.

Also in 2014 we again find a remarkable cold bubble over the northern Atlantic – as a look at the NASA website shows. 2014 was globally the warmest year on record, 1 °C warmer than the average for 1880-1920. But the subpolar Atlantic was 1-2 °C colder than that baseline! And even more recently, NOAA last week released the stunning temperature analysis for the past winter shown in Fig. 4. That winter was globally the warmest since records began in 1880. But in the subpolar North Atlantic, it was the coldest on record! That suggests the decline of the circulation has progressed even further now than we documented in the paper.

noaa_2014

Jason Box’s Meltfactor blog:

While global surface temperatures are increasingly dominated by warm anomalies, a conspicuous area of cold has persisted south of Greenland and Iceland visible at the ocean surface in sea surface temperature observations. The abnormal cold there has been more anomalous than the US northeast winter. While the most recent northern winter was the warmest on record globally, the ocean surface area south of Greenland & Iceland had the lowest temperatures in the 136 year record. How could this be?

A new study estimates the Atlantic Meridional Overturning Circulation (AMOC) using the sea surface temperature difference at that cold spot south of Greenland/Iceland with the Northern Hemispheric temperature from NASA GISS instrumental records since the 1880s (Hansen and others 1999) and from coral-based proxies after Sherwood and others (2011) that span years since 500 AD.

Based on this AMOC reconstruction, the study finds that the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) after 1975.

1. appears unprecedented in the past millennium;

2. is expected to continue, even intensify through year 2100, as simulated with the MPI-ESM-MR global climate model of the Max Planck Institute in Hamburg (Jungclaus and others, 2013)

3. may result to a large degree from Greenland melting.

My contribution was my work of 6 years, a 172 year Greenland mass balance

reconstruction published in a 3 part series in the Journal of Climate (Box and others 2013; Box, 2013;

Box and Colgan, 2013), enabling Greenland melting to be brought more into context of its ocean thermohaline perturbation.

box_runoff

Mass Balance terms of the Greenland Ice Sheet. Data from Box and Colgan. Cumulative anomaly relative to the mean over 1840-1900, a pre-industrial period during which the Greenland Ice Sheet was approximately in balance.

Melt from Greenland produces water that is lighter and colder than the sea surface waters. The meltwater is light enough to float above the saltier sea surface waters. Because Atlantic surface waters flow northward (see map below), an increasing ice melt freshwater supply ( blue line in the figure above) may pile up near the sea surface, capping or backing up the Gulf Stream North Atlantic Drift current that a.) delivers warmth to northwestern Europe and b.) is part of a global ocean heat conveyer. While Bamber and others (2012) set the stage with “Recent large increases in fresh- water fluxes from Greenland into the North Atlantic”, the new study more directly quantifies the possible impact from Greenland on the ocean thermohaline circulation.

northatcurrent

Why should we care?

The North Atlantic ocean circulation is an important part of a global ocean circulation that exchanges heat from the equatorial surplus to the poles where the energy is lost by thermal radiation to space. A slowed global oceanic ‘conveyer belt’ may further destabilize our changing global climate. We expect no new ice-age – but major negative effects are possible. The effects could be on global climate, fisheries, or also for example storminess.

Influence on weather?

The study will stimulate discussion and research on how the large area of negative sea surface temperatures anomalies south of Greenland and Iceland may influence European and downstream weather. Given some heat exchange between a warm air mass with an anomalously cold North Atlantic sea surface, some strengthening of winds, lowering of central pressure of cyclonic systems, should result from increased “baroclinic instability” (see for example Holton et al. 1992) arising from an increased temperature difference between sea surface and atmosphere. As the subpolar North Atlantic cools and the atmosphere warms, the physics are set to strengthen cyclonic circulation in warm air masses. Converseley, cold air masses drifting off of N America would be less prone to baroclinic deepening. In any case, the perturbation may be felt not just in that part of the world, but downstream, and like the proverbial butterfly (seagull) flapping its wings, alters atmospheric and oceanic flow, with certain though hard to predict downstream consequences.

UPDATE: Washington Post now covering another aspect of this story.

However, there are many other effects, ranging from dramatic impacts on fisheries to, perhaps most troubling of all, the potential for extra sea level rise in the North Atlantic region.

That may sound surprising, but here’s how it works. We’re starting out from a situation in which sea level is “anomalously low” off the U.S. east coast due to the motion of the Gulf Stream. This is for at least two reasons. First, explains Rahmstorf’s co-author Michael Mann of Penn State University, there’s the matter of temperature contrast: Waters to the right or east of the Gulf Stream, in the direction of Europe, are warmer than those on its left or west. Warm water expands and takes up more area than denser cold water, so sea level is also higher to the right side of the current, and lower off our coast.

“So if you weaken the ‘Gulf Stream’ and weaken that temperature contrast…sea level off the U.S. east coast will actually rise!” explains Mann by e-mail.

But there’s another factor, too, involving what is called the “geostrophic balance of forces” in the ocean. This gets wonky, but the bottom line result is that “sea surface slope perpendicular to any current flow, like the Gulf Stream, has a higher sea level on its right hand side, and the lower sea level on the left hand slide,” says Rahmstorf.

We’re on the left hand side of the Gulf Stream. So weaken the flow, and you also raise the sea level. (For further explanation, see here and here.)

Indeed, researchers recently found a sudden, 4-inch sea level rise of the U.S. East Coast in 2009 and 2010, which they attributed to a slowdown of the Atlantic overturning circulation. Rahmstorf says that “for a big breakdown of the circulation, [sea level rise] could amount to one meter, in addition to the global sea level rise that we’re expecting from global warming.”

39 Responses to “A Nasty Surprise in the Greenhouse. New Paper, New Video.”


  1. […] the post:  “New video produced by climate hawk Peter Sinclair and featuring top scientists Stefan Rahmstorf, Michael Mann, and Jason Box, issues warnings about […]

  2. redskylite Says:

    Another report from U.K’s National Oceanography Centre supports the Potsdam Institute for Climate Impact Research (PIK) findings. After 10 years of observation they conclude . .. . .. . .

    “the ocean sensors have detected that the AMOC is now declining faster than anticipated (Smeed et al. 2014), which could potentially have a long-term impact on Britain’s climate.”

    Another abrupt climate change event approaching … another factor in our changing climate

    http://noc.ac.uk/news/ten-years-ocean-monitoring-uncovers-secrets-changing-uk-winters

    • redskylite Says:

      Today’s findings make the research from Exeter University much more compelling:

      “The researchers developed a model to investigate how the uncertainty surrounding tipping points should influence climate policy. Based on expert input, the likelihood that human activities will push the climate system past a tipping point increases from 2.5% in 2050 to nearly 50% in 2200 in their baseline scenario.

      The potential climate tipping points considered in the study were a collapse of the Atlantic meridional overturning circulation”

      http://www.exeter.ac.uk/news/featurednews/title_443001_en.html


  3. Looks like we are in for “AMOC Time”! (For all you Star Trek fans)


  4. […] Atlantic Meridional Overturning Circulation.  If you haven’t seen that video and post yet, go here. Historically, scientists believe that a complete shutdown of that current happened about 12000 […]


  5. […] More from my extended interview with Mike Mann. If you did not see the video this week on Mike’s headline grabbing new paper (with Jason Box and lead author Stefan Rahmstorf) covering the North Atlantic Current, make sure you see that now. […]


  6. […] in the Atlantic Meridional Overturning Circulation (AMOC).  If you have not seen the post on that, go there now. For most folks, and in many media accounts, this circulation sounds very much like what we think of […]

  7. takver Says:

    One of the impacts that hasn’t been raised is that the slowing of the Atlantic Meridional Overturning Circulation will reduce the ocean uptake of carbon dioxide. While CO2 uptake is increasing acidification (another issue), the ocean carbon sink moderates the extent of the greenhouse effect.

    A study by Pérez et al (2013) – Atlantic Ocean CO2 uptake reduced by weakening of the meridional overturning circulation – highlighted that the slowdown of the meridional overturning circulation was largely responsible for the reduction in carbon uptake. Here is the abstract in full:

    Uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean declined rapidly between 1990 and 2006. This reduction in carbon dioxide uptake was related to warming at the sea surface, which—according to model simulations—coincided with a reduction in the Atlantic meridional overturning circulation. The extent to which the slowdown of this circulation system—which transports warm surface waters to the northern high latitudes, and cool deep waters south—contributed to the reduction in carbon uptake has remained uncertain. Here, we use data on the oceanic transport of volume, heat and carbon dioxide to track carbon dioxide uptake in the subtropical and subpolar regions of the North Atlantic Ocean over the past two decades. We separate anthropogenic carbon from natural carbon by assuming that the latter corresponds to a pre-industrial atmosphere, whereas the remaining is anthropogenic. We find that the uptake of anthropogenic carbon dioxide—released by human activities—occurred almost exclusively in the subtropical gyre. In contrast, natural carbon dioxide uptake—which results from natural Earth system processes—dominated in the subpolar gyre. We attribute the weakening of contemporary carbon dioxide uptake in the subpolar North Atlantic to a reduction in the natural component. We show that the slowdown of the meridional overturning circulation was largely responsible for the reduction in carbon uptake, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic CO2 storage in subpolar waters.
    Source: http://www.nature.com/ngeo/journal/v6/n2/full/ngeo1680.html


  8. […] Sinclair has posted a short video on our paper with interview clips with Mike Mann, Jason Box and […]


  9. […] video produced by climate hawk Peter Sinclair and featuring top scientists Stefan Rahmstorf, Michael Mann, and Jason Box, issues warnings about […]


  10. Takver:

    You may be interested to know that study at the University of Wisconsin-Madison confirms what you write about CO2.

    An Aug 2010 Scientific American article (“Threatening Ocean Life from the Inside Out”), described how our burning fossil fuels is making the oceans more acidic, via the absorption of Carbon Dioxide (CO2). “This imperils the growth of species form plankton to squid”, warned Scientific American, which has been reporting about Science since 1845.”

    But whether the ocean can continue mopping up human-produced carbon at the same rate is uncertain. Previous studies on the topic have yielded conflicting results, says University of Wisconsin-Madison assistant professor Galen McKinley.
    See:
    “Climate Change Reducing Ocean’s Carbon Dioxide Uptake” July 11, 2011
    http://www.redorbit.com/news/science/2077100/climate_change_reducing_oceans_carbon_dioxide_uptake

    In an analysis published online July 10 [2011] in Nature Geoscience, McKinley and her colleagues identify a likely source of many of those inconsistencies and provide some of the first observational evidence that climate change is negatively impacting the ocean carbon sink.

    The ocean reduces atmospheric carbon dioxide and its associated global changes.

    “The ocean is taking up less carbon because of the warming caused by the carbon in the atmosphere,” says McKinley, an assistant professor of atmospheric and oceanic sciences and a member of the Center for Climatic Research in the Nelson Institute for Environmental Studies.

    The analysis differs from previous studies in its scope across both time and space. One of the biggest challenges in asking how climate is affecting the ocean is simply a lack of data, McKinley says, with available information clustered along shipping lanes and other areas where scientists can take advantage of existing boat traffic. With a dearth of other sampling sites, many studies have simply extrapolated trends from limited areas to broader swaths of the ocean.

    McKinley and colleagues at UW-Madison, the Lamont-Doherty Earth Observatory at Columbia University, and the Universite Pierre et Marie Curie in Paris expanded their analysis by combining existing data from a range of years (1981-2009), methodologies, and locations spanning most of the North Atlantic into a single time series for each of three large regions called gyres, defined by distinct physical and biological characteristics.

    They found a high degree of natural variability that often masked longer-term patterns of change and could explain why previous conclusions have disagreed. They discovered that apparent trends in ocean carbon uptake are highly dependent on exactly when and where you look on the 10- to 15-year time scale, even overlapping time intervals sometimes suggested opposite effects.

    “Because the ocean is so variable, we need at least 25 years’ worth of data to really see the effect of carbon accumulation in the atmosphere,” she says. “This is a big issue in many branches of climate science :“ what is natural variability, and what is climate change?”

    Working with nearly three decades of data, the researchers were able to cut through the variability and identify underlying trends in the surface CO2 throughout the North Atlantic.

    During the past three decades, increases in atmospheric carbon dioxide have largely been matched by corresponding increases in dissolved carbon dioxide in the seawater.

    The gases equilibrate across the air-water interface, influenced by how much carbon is in the atmosphere and the ocean and how much carbon dioxide the water is able to hold as determined by its water chemistry.

    But the researchers found that rising temperatures are slowing the carbon absorption across a large portion of the subtropical North Atlantic. Warmer water cannot hold as much carbon dioxide, so the ocean’s carbon capacity is decreasing as it warms.

    In watching for effects of increasing atmospheric carbon on the ocean’s uptake, many people have looked for indications that the carbon content of the ocean is rising faster than that of the atmosphere, McKinley says. However, their new results show that the ocean sink could be weakening even without that visible sign.

    “More likely what we’re going to see is that the ocean will keep its equilibration but it doesn’t have to take up as much carbon to do it because it’s getting warmer at the same time,” she says. “We are already seeing this in the North Atlantic subtropical gyre, and this is some of the first evidence for climate damping the ocean’s ability to take up carbon from the atmosphere.”

    She stresses the need to improve available datasets and expand this type of analysis to other oceans, which are relatively less-studied than the North Atlantic, to continue to refine carbon uptake trends in different ocean regions. This information will be critical for decision-making, since any decrease in ocean uptake may require greater human efforts to control carbon dioxide levels in the atmosphere.


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