THE YALE FORUM.- CLIMATE CHANGE & THE MEDIA

Common Climate Misconceptions

 

Why a ‘Gulf Stream’ Shut Down and a New European Ice Age are Unlikely

By Zeke Hausfather | October 11, 2007

 

Some in the news media may be overplaying the extent of the risk that Northern Europe might soon plunge into a new Ice Age. They risk going beyond where the best science can now take them.

“Britain could be heading for a climate like Alaska,” the BBC reported back in 2003. It painted a stark picture of a life in which “our ports could be frozen over. Ice storms could ravage the country, and London could see snow lying for weeks on end.”

New Scientist in 2005 cautioned us that stuttering ocean warm currents may “plunge the continent into a mini ice age.”

National Geographic that same year reported that “Chilling new evidence from the Atlantic Ocean is raising fears that western Europe could soon be gripped by a mini ice age.”

The potential shutdown of the Thermohaline Circulation (THC), commonly misidentified (.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 1) that the THC, rather than abruptly shutting down, is likely to slow over the course of centuries; and 2) any cooling in Northern Europe would be more than offset by the larger human-driven global warming trend.

The THC is a global ocean circulation driven primarily 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, increase the density of the surface water. As a result, surface waters sink, drawing in warmer surface waters from the south. These processes drive a global ocean current often referred to as the “ocean conveyor belt.” The THC is one of the reasons (.pdf) that the United Kingdom and other Northern European countries enjoy such a mild climate, despite sharing the same latitude as Siberia.

Paleoclimate records show that the THC has shut down in the past, causing temperatures in Northern Europe to fall substantially. These shutdowns are hypothesized to be caused by massive freshwater releases into the North Atlantic from enormous glacial lakes that develop as the world moved out of ice ages.

Similarly, many scientists are concerned that increasing freshwater icemelt in Greenland and Northern Europe as a result of 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 anywhere from zero to 50 percent, but almost none show it actually stopping. In even the most pessimistic cases, the rate of icemelt occurring would produce an order of magnitude less freshwater than what caused past THC shutdowns.

In any of these modeled cases a slowdown of the THC would still have a cooling effect on Northern Europe, but 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. Bryden found that, compared to data from 1957, 1981, 1992 and 1998, the volume of the THC appeared to have decreased by about 30 percent.

Many others in the field have since criticized Bryden’s study. They argue that measurements are not yet able to effectively distinguish a trend from natural variability in the current. An actual decrease of 30 percent in the current, these scientists maintain, would have caused measurably cooler temperatures in Europe.

M.I.T. 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.” As scientists writing in RealClimate explain, “now that data has been properly published, 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 would be frozen. And Gavin Schmidt, from NASA Goddard, maintains that, “while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of Europe is a ways off yet.”

Wallace Broecker of Columbia University, who first posited that THC shutdowns could explain climate shifts in the distant past, puts it even more strongly, pulling no punches: “The notion that [a modern THC shutdown] may trigger a mini ice age is a myth.”

Scientific debate unquestionably will continue over this issue. 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 need to be careful in painting an unjustified picture of dramatic cooling in Northern Europe, one not supported by the current state of the science.

Author
Zeke Hausfather is a regular contributor to the Yale Forum (E-mail: zeke@yaleclimatemediaforum.org).