I'm writing up an article on the possibility of a THC slowdown/ shutdown and how the state of the science contrasts to the media portrayal of the issue. It will (hopefully) be included in a new online journal/web portal aimed at improving the communication between climate scientists and science journalists.
I figured I'd bounce it off you all first, just to make sure I'm not too far off the mark. Common Climate Misconceptions: Why a "Gulf Stream" shutdown and a new Ice Age in Europe are unlikely "Britain could be heading for a climate like Alaska," the BBC reported back in 2003, painting a stark picture of life where "Our ports could be frozen over. Ice storms could ravage the country, and London could see snow lying for weeks on end." (http://www.bbc.co.uk/science/ horizon/2003/bigchill.shtml) New Scientist tells us (http:// www.timesonline.co.uk/tol/news/uk/article520013.ece) that it may "plunge the continent into a mini ice age". The potential shutdown of the Thermohaline Circulation (THC) (http://en.wikipedia.org/wiki/ Thermohaline_circulation), commonly misidentified (http:// ocean.mit.edu/~cwunsch/papersonline/naturegulfstreamltr.pdf) as the Gulf Stream, often makes the list of the most dangerous potential impacts of climate change. However, the current state of the science suggests that, rather than abruptly shutting down, the THC is likely to slow over the course of centuries, and that any cooling experienced by Northern Europe would be more than offset by the larger human- driven global warming trend. The risk of Northern Europe plunging into a new Ice Age due to global warming have been overplayed in the media to an extent not supported by the science. The THC is a global ocean circulation driven by changes in density and salinity of ocean waters. In the North Atlantic, low temperatures combined with high evaporation rates driven by strong winds moving over water increases the density of the surface water. This causes surface waters to sink, drawing in warmer surface waters from the south. These processes drive a global ocean current often referred to as the "ocean conveyor belt" (and often confused with the Gulf Stream, which primarily driven by wind). The THC is one of the main reasons that the United Kingdom and other Northern European countries enjoy such a mild climate, despite sharing the same latitude as Siberia (http://www.climate.unibe.ch/~stocker/papers/stocker02sci.pdf). We know from the paleoclimate record that the THC has shut down in the past, causing temperatures in Northern Europe to fall 3 to 6 degrees C. These shutdowns are thought to be caused by massive freshwater releases into the North Atlantic from enormous glacial lakes that develop as the world moves out of ice ages. Similarly, many scientists are concerned that increasing freshwater icemelt in Greenland and Northern Europe due to anthropogenic climate change could potentially slow or even shut down the THC. The question, like many involving climate change, is really one of timescales. Climate models show the THC slowing down over the next century by an average of 25 percent, but almost none show it actually stopping. In even the most pessimistic cases, the rate of ice melt occurring would produce an order of magnitude less freshwater than what caused past THC shutdowns (http://www.nature.com/nature/journal/ v439/n7074/full/439256a.html). While a slowdown of the THC would still have a cooling effect on Northern Europe, it would likely be more than offset by the larger global warming trend. In 2005, Harry Bryden, an oceanographer, and his research team published an article in Nature that shocked many in the climate field and generated a considerable amount of media attention (http:// www.nature.com/nature/journal/v438/n7068/abs/nature04385.html). Bryden found that, compared to data from 1957, 1981, 1992 and 1998, the volume of the THC appeared to have decreased by about 30%. His study has since been criticised by many others in the field, who argue that measurements are as of yet unable to effectively distinguish a trend from natural variability in the current. If the current had actually decreased by 30%, they argue, it would have caused measurably cooler temperatures in Europe. MIT oceanographer Carl Wunsch compares Bryden's method to "measuring temperatures in Hamburg on five random days and then concluding that the climate is getting warmer or colder." (http://www.nature.com/nature/journal/v439/n7074/full/ 439256a.html) As the folks over at RealClimate explain (http:// www.realclimate.org/index.php/archives/2007/08/who-ya-gonna-call/), "now that data has been properly published (http://www.sciencemag.org/ cgi/content/abstract/sci;317/5840/935) it confirms what we thought all along. The sampling variability in the kind of snapshot surveys that Bryden et al had used was too large for the apparent trends that they saw to be significant." Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany is strongly skeptical of the idea that a thermohaline shutdown in today's climate would lead to a situation where large parts of Europe were frozen (http://www.nature.com/nature/journal/v439/ n7074/full/439256a.html). Gavin Schmidt from NASA Goddard argues that, "while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of the Europe is a ways off yet." (http://www.realclimate.org/index.php?p=159) Wallace Broecker of Columbia University, who first posited that THC shutdowns could explain climate shifts in the distant past, puts it even more strongly: "the notion that [a modern THC shutdown] may trigger a mini ice age is a myth." (http://www.nature.com/nature/journal/v439/n7074/ full/439256a.html) Scientific debate will continue over this issues, and the recent installation of a new system for measuring THC flows should improve data on any changes that are occurring. In the mean time, journalists should be careful in painting a sensationalist picture of dramatic cooling in Northern Europe that is not supported by the current state of the science. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/globalchange -~----------~----~----~----~------~----~------~--~---
