Common Misconceptions about Abrupt Climate Change
Q. What is the North Atlantic heat pump?
A.The North Atlantic heat pump refers to the fact that the Atlantic Ocean transports heat much farther north than, for example, the Pacific Ocean. This is because a part of its circulation is unique.
Ocean currents are mainly driven by two forces: winds and ocean density differences.
The portion of ocean flow that is driven by density is called the thermohaline circulation (temperature and salinity together determine the density of ocean water).
Global thermohaline circulation is sometimes described as a great ocean conveyor belt (see "Great Ocean Conveyor Belt") - with warm, less dense waters flowing in one direction at the sea surface and cold, dense waters flowing in the opposite direction in the deep ocean.
The critical points of this "conveyor belt" are where surface waters sink into the deep ocean.
This happens only in a few places - along the Antarctic shelf and at two sites in the northern North Atlantic (the Labrador Sea and the Nordic Seas).
In these two North Atlantic sites, as the ocean loses its heat to the atmosphere, the surface waters become so cold - and so dense - they sink downward into the deep ocean and then flow downhill over the seafloor toward the equator.
(image right: from Woods Hole Oceanographic Institution, click image for detail)
This sinking and southward flow help drive the ocean conveyor. The dense waters that are exiting to the south in the deep ocean have to be replaced.
This draws warm, surface currents farther north than they would normally flow and pumps additional heat to high northern latitudes.
Q. Is this North Atlantic heat pump constant?
A. No. If conditions change in the North Atlantic Ocean such that surface waters can no longer become dense enough to sink, then the "conveyor belt" would slow or possibly stop altogether.
The most likely agent of change is extra freshwater added to the ocean's sinking sites.
If too much freshwater is added - from melting ice and/or increased precipitation - then no matter how cold the surface waters become, they cannot become dense enough to sink. They may turn to ice, but still will not sink.
Q. Some reports talk about a "shut down" of the Gulf Stream. What does this mean?
A. Under no conditions will the Gulf Stream shut off entirely!
This strong ocean current is driven by winds as well as ocean density differences.
It is the latter portion of the flow -the thermohaline circulation-that brings ocean heat to the high northern latitudes and could be affected by salinity changes that are now taking place in the North Atlantic.
The winds will continue to blow over the ocean and the Gulf Stream will continue to flow even if the thermohaline circulation slows or shuts down. Its flow may be reduced, or its route slightly redirected, but it will continue to flow.
(note: in contrast, the following article from SciencePoles.org 2005- Ed.)
Goodbye Gulf Stream by 2200?
In a paper published in Science in June 2005, Ruth Curry of the Woods Hole Oceanographic Institution (WHOI) and Cecilie Mauritzen of the Norwegian Meteorological Institute report that patterns of fresh water accumulation in the Nordic Seas (in the Arctic) in recent decades suggest that the Gulf Stream would cease functioning in around two centuries if accumulation continued at the rate observed in their study.Significant slowing of the Gulf Stream could be under way even by the end of this century - and possibly sooner, depending on the rate of global warming and consequent impacts in the Arctic.
In an average year, around 5,000 cubic kilometres of fresh water (roughly equivalent to the annual output of the Amazon, the world's largest river) dilutes the salinity of seawater flows into the Atlantic Ocean from the Arctic.
(image above: chemical makeup of Ocean salinity, from 'Water Water Everywhere')
There were, however, a number of significant additional freshwater injections from the Arctic into the Atlantic - around 19,000 cubic kilometres in the thirty year period studied.
Some 2,500 cubic kilometres of the additional freshwater, however, accumulated in the critical upper layer of the Nordic Seas (where the Gulf Stream begins to "overturn").
This rate of accumulation in the Nordic Seas, according to Curry and Mauritzen, has not yet brought about sustained changes to the overturning rate but, if it continues, would be likely to do so over the next century.
By 2200 it would bring the Gulf Stream to a halt. Further global warming would halt the Gulf Stream sooner.
If, as a result of global warming, significant parts of the massive Greenland ice-sheet were to melt or break off, and if sea-ice formation were to shrink as fast or even faster than the rapid rate witnessed over the period 2002-05 then the timetable mooted in this study would likely have to be brought forward considerably, as noted by the authors.
(corresponding image from global-greenhouse-warming.com)
NPR- 8/24/2006: Arctic Freshwater Pouring Into Atlantic, Scientists Say
NewScientist.com- 9/26/06- Arctic Ocean continues to freshen up