Explorations: The Meridional Overturning Circulation
Page 69 of The AMS Weather Book, in Chapter 3, briefly describes the meridional overturning circulation (MOC) of the global system of ocean currents. The MOC is often referred to as the thermohaline circulation (from the Greek words for heat and salt) to describe the effects of cold, salty water that sinks in the northern Atlantic Ocean and in the Southern Ocean at some places around Antarctica. The colder and saltier the water, the faster it sinks. The cold, salty water flows toward the equator deep under the ocean. Warm, surface water flows from the tropics to replace the sinking water.
This description is greatly oversimplified, because it tends to ignore the role of wind in creating and powering surface ocean currents, such as the Gulf Stream in the Atlantic. For this reason, many scientists prefer to talk about the MOC. Meridional refers to generally north–south movement. Overturning reflects the role of the thermohaline part of the system, and circulation means that the water that sinks eventually returns to the surface elsewhere.
A good, nontechnical introduction to the meridional overturning circulation (MOC) is NASA’s Ocean Motion and Surface Currents Web site. As mentioned on page 69 of The AMS Weather Book, the names thermohaline circulation and oceanic conveyor belt are often used for the MOC, as NASA does on the previously mentioned Web site.
By the early 1990s, scientists were beginning to build computer models of climate that linked the oceans and the atmosphere. One of the results was a hypothesis that the effects of a warming global climate would be to slow down the thermohaline circulation, which in turn could cool Europe and eastern North America by slowing the transport of warm water into the northern Atlantic. (This was often described as “slowing down the Gulf Stream,” which isn’t quite how it would work.) By the mid 1990s, various studies seemed to confirm that such a slowdown might be already starting.
Around this time, climate scientists were also becoming more concerned about the possibility of abrupt climate change. Scientists use the term to describe changes such as the climate of New England becoming more like the climate of Washington, D.C., over a decade or two. (NOAA’s A Paleo Perspective on Abrupt Climate Change Web page explains this.) Climate researchers had evidence that changes in the MOC had been involved in past “abrupt” climate changes.
The 2002 National Academy of Sciences report Abrupt Climate Change: Inevitable Surprises takes a detailed look at the question.
Some news reporters didn’t let the subtleties of science get in the way of a good story and headlined the idea that global warming could paradoxically bring a new ice age very quickly. While no serious scientist went this far, it captured the public imagination and the attention of writers and moviemakers.
Scientists use the word abrupt when talking about climate in a particular way that is clear to them, but to most nonscientists something that happens over two decades is not abrupt. This was, however, an opening for writers and moviemakers to use a “scientific” term to cloak science fiction in the guise of science. Probably the culmination of this use was in 2004 movie The Day After Tomorrow. The movie was a target too good to ignore for those who like to engage in a reviewer’s equivalent of shooting fish in a barrel. This is what the author did in an online review that USATODAY.com published. Others had similar fun, and teachers who like to use illogical, silly, or bad science as a tool to teach real science loved the movie.
The real science isn’t hard to find. A good place to begin is with Spencer R. Weart’s book The Discovery of Global Warming, Second Edition (Cambridge, MA: Harvard University Press, 2008). It is available online in a hypertext version with free access. The chapter “Ocean Currents and Climate” lays out the story of how scientists worked out the relation between the oceans and climate, including the discovery of the MOC. This chapter puts the flap over a possible slowing of the MOC in context. It is also a good source of links to more information on the topic.