Take a look at the map below. Direct from the National Oceanic and Atmospheric Administration (NOAA), it shows the blended land and sea surface temperatures (in °C) for the months of December 2014 through February 2015 (the warmest winter on record).
Notice anything odd in the colors up there? Most areas are trending upwards, heat-wise, except for a spot in the upper Atlantic that is at its record coldest. If you consult various heat trend maps from NASA or NOAA, you will see this spot appear again and again.
This cooling region in the subpolar Atlantic between Newfoundland and Ireland has been predicted since the 1980s, and it may signal a major shift in how the ocean works and how our climate behaves. It could be evidence that the Gulf Stream System is weakening, a condition that both the fourth and fifth IPCC report have agreed is “very likely” to occur in this century. Here’s why that’s bad:
The Atlantic Meridional Overturning Circulation (AMOC)
The AMOC is a major current in the Atlantic Ocean that has two main components: A northward-flowing upper layer of warm water and a southward-flowing lower layer of colder water. This current has a significant influence on the Earth’s climate system, as it transfers heat energy from the tropics in the Southern Hemisphere to the atmosphere of the Northern Hemisphere. You can see a model of the AMOC in the NASA video below.
Scientists hypothesize that fluctuations in the AMOC have contributed to major climate shifts throughout history. Unfortunately, direct evidence for this is lacking due to various technical and natural factors, but those fluctuations have been linked to significant and troubling sea level rise.
How Does the AMOC Affect Sea Level Rise?
Between 2009 and 2010, the east coast of the United States experienced an inter-annual sea level rise that scientists declared “unprecedented in tidal gauge records.” The coastal sea level above New York City rose over five inches and the average magnitude of sea level rise (SLR) across northeast New England was 3.9 inches.
The event caused “persistent and widespread coastal flooding even without apparent weather processes,” resulting in beach erosion “almost as significant as some hurricane events.” This phenomenon has been linked to a fluctuation in atmospheric pressure combined with a slowdown in the AMOC (which also spurred severe hurricane and winter conditions in northwestern Europe).
Stephen Griffies, a NOAA climate and ocean modeler who contributed to the study, recently told ClimateCentral that a further slowdown in the AMOC is “inevitable.”
“If the overturning circulation slows down further,” he said, “these extreme sea-level events on the East Coast will become more frequent.”
Why Is the AMOC Cooling Down?
Greenland is melting.
Last year, satellite data from the European Space Agency’s CryoSat mission revealed that both the Greenland and Antarctic ice sheets are melting at an “unprecedented rate.” A new study published in the journal Nature Climate Change suggests this influx of cold water is affecting the slowdown in the AMOC.
Like most climate systems, the AMOC is subject to variability over time. Between 1970 and 1990, the current experienced a sharp slowdown followed by a partial recovery. However, it has failed to recover to its pre-Industrial Revolution levels, suggesting that man-made greenhouse gas emissions and the resulting warming of the globe have made an impact. And while the Intergovernmental Panel on Climate Change (IPCC) has predicted the AMOC’s slowdown, the heavy influx of cold Greenland meltwater is believed to be exacerbating the condition ahead of schedule.
Using data gathered from ice cores, tree rings and coral samples, the research team (led by the Potsdam Institute for Climate Impact Research in Germany) posits that weakness in the AMOC after 1975 “is an unprecedented event in the past millennium.”
The exact consequences will be difficult to predict, but it will definitely have an impact on marine life, which benefits from the nutrients the AMOC delivers up from the ocean depths. “The most productive region, in terms of availability of nutrients, is the high latitudes of the North Atlantic,” said Mann. “If we lose that, that’s a fundamental threat to our ability to continue to fish.”
Hurricanes and nor’easters like the recent Winter Storm Juno that brought Snowmageddon to the East Coast could also become more common, he warned.
“If you shut down this mode of ocean circulation, you’re denying the climate system one of its modes of heat transport,” said Mann. “if you deny it one mode of transport, it’s often the case that you will see other modes of transport increase.”
Why Should We Be Worried?
The Greenland ice sheet contains enough water to raise sea levels 23 feet if it completely melted. And while a combination of global warming and pollution is accelerating Greenland’s melt rate, most climate change scenarios say it will take thousands of years for the ice sheet to disappear.
However, disrupting the AMOC with cold water will have a serious if unpredictable impact on our climate. As Al Gore pointed out in his film, An Inconvenient Truth, the last time the ocean’s “heat conveyor belt” was doused with a massive volume of frigid water, it prompted a 900 to 1,000 year ice age in Europe in the span of 10 years.
But, given the planet’s current warming trend, would disrupting the AMOC induce the same shift? We simply don’t have enough data to say. Yet despite our limited data, we do have plenty of evidence that the current’s speed is declining to a level not seen in over a thousand years. And that merits more than a little investigation.