Since 1900, global SST (sea surface temperature) has increased by an average of 1oC with the exception of an area of the North Atlantic off the southeast coast of Greenland that has cooled by about the same amount. This area has been dubbed the “cold blob”, abbreviated (as all climate phenomena seem to be) as “CB”; and, while you might think that any cold news is good news, in this case, it isn’t.
The Gulf Stream
The North Atlantic is warmed by the Gulf Stream that flows up the east coast of North America. This warms the air over the North Atlantic and makes western Europe, which is at the same latitude as Canada, a warmer and more climatically hospitable place than our northern neighbor.
The Gulf Stream is part of the North Atlantic Meridional Overturning Circulation (AMOC). AMOC is a continuous flow that consists of warm surface waters that move generally northward and cold deep waters that move southward. (Meridional means that the circulation is generally north-south, and overturning refers to the fact that at certain points the water dives down about one mile – called “deep water formation” or “downwelling” – from the surface to begin its underwater flow.)
By distributing heat from lower latitudes to higher latitudes, AMOC reduces the temperature differential between tropical, temperate, and arctic zones. This has several beneficial effects. As mentioned earlier, it makes Europe a more climatically hospitable place; the vertical mixing due to overturning also brings nutrients to the surface which enhances marine life; by reducing the heat content in the lower latitudes, it reduces the horizontal temperature gradient which reduces the energy available to power storms; and it is the largest carbon sink in the northern hemisphere, sequestering nearly a billion tons of carbon a year.
The AMOC is one of the strongest current systems on the planet. On its way north, the flow of the Gulf Stream increases from about one billion cubic feet per second to over 5 billion cubic feet per second. This is more than 100 times as much as all rivers of the earth combined.
The AMOC Index
The AMOC index measures the difference between the surface and deep-water temperature in the subpolar Atlantic. Here is a record of the AMOC index since 900 AD. (Fossilized foraminifera provide proxy data for temperature.)
Atlantic meridional overturning circulation index
You will note that the index fluctuates naturally, but that for the past 200 years (coincident with the beginning of the industrial revolution) the index has been on a steady decline. The natural fluctuation is due to the way in which deep water formation occurs. The warm water from lower latitudes is saltier than the colder northern waters and so it has a higher specific gravity. As it flows north and cools, it becomes denser than the surrounding, less salty water, which makes it heavier and allows it to make the dive to the deep ocean and begin its underwater journey. This downwelling is what causes the current to continue. Any interruption of the deep-water formation process will therefore slow the AMOC current.
As with virtually all climate phenomena, there is a feedback mechanism that controls the cyclicality of the AMOC index. When the index is high, the current is robust, and the heat transferred to the arctic is at its maximum. The resulting warming of the arctic causes increased melting of ice. As more ice melts, albedo is reduced, engendering more ice melt.
The greatest store of ice in the arctic is on Greenland. As the rate of ice melt increases, more freshwater flows into the ocean. This freshwater dilutes the saltiness of the water at the northern extreme of the AMOC where downwelling occurs. As the water becomes less salty, it becomes less dense, so it does not sink as rapidly. This slows downwelling and therefore the speed of the AMOC current. As the current slows, less heat is delivered to the artic, the rate of ice melt slows down, freshwater runoff declines, dilution of the saltwater decreases, and the AMOC flow naturally picks up again.
Referring once again to the chart, you can see that, although the cyclical nature of the AMOC index continues, it does so against a background of an overall reduction in the index and therefore a slowing of the AMOC current. This trend is due to the effects of anthropogenic global warming due to the production of greenhouse gases in quantities that cannot be naturally absorbed and that therefore build up in the atmosphere and prevent the Earth from emitting enough heat to remain in thermodynamic equilibrium.
The Thermohaline Circulation
AMOC is part of the global thermohaline circulation current – a continuous conveyor belt that transfers heat (thermo) and salt (haline) around the world. The current is driven by downwelling which is driven by differences in density between the current and the surrounding water which is, in turn, controlled by the temperature and salinity of the water. Consequently a slowdown in AMOC will cause a slowdown in the entire thermohaline circulation current which will affect climate not just in the north Atlantic, but globally.
Consequences of an AMOC Shutdown
The 2004 disaster move “The Day After Tomorrow” uses the stoppage of the AMOC current as its plot device. Unfortunately the producers opted for theatrical impact at the expense of a more realistic portrayal of the likely consequences, a curious choice since the real consequences would have been dire enough.
The consequences of a stoppage of the AMOC are the flip side of the benefits listed earlier: The climate in Europe will become more like that of Canada; diminished nutrients will adversely affect marine life in the north Atlantic; one of the world’s largest carbon sinks will be lost, exacerbating the effects of greenhouse gas emissions; and the horizontal temperature gradient will increase and drive superstorms of unprecedented power. There is evidence that such a shutdown of the AMOC occurred 118,000 years ago during the previous interglacial period (the Eemian). On the Bahamian island of Eleuthera, seven massive 1,000-ton boulders are positioned atop a 15-meter-high cliff. According to several studies, they were transported there by super storms.
How Soon, How Bad?
The paleontological record shows that the relative stability of the Holocene climate and sea level is the exception rather than the rule. As the climate inevitably changes as a result of our ever-increasing greenhouse gas emissions, we should not expect change to continue to be gradual. History shows that feedbacks will trigger bursts of rapid climate changes that will occur in decades, not centuries. Our experience and our intuition do not prepare us for such an eventuality.
No one can make date-specific predictions, but it is clear that our continued burning of fossil fuels in ever-greater quantities is sealing our fate and that we can, sorrowfully but confidently, say that catastrophic consequences will befall our children and grandchildren this century. The caveat “unless we change” has become an increasingly meaningless coda.