A new study on global warming’s effects on the Atlantic current has the news media scurrying to dust off their references to that post-apocalyptic cinema bomb “The Day After Tomorrow.”
The study, published in the journal Nature Climate Change, argues that further melting of Greenland’s ice sheet could be causing the Atlantic meridional overturning circulation, or the the Gulf Stream System, to slow down. The slowdown is evidenced by the cooling trend in the North Atlantic while the rest of the world heats up, according to scientists.
Okay, so what’s the problem? Stefan Rahmstorf of the Potsdam Institute, and the study’s lead author, literally wrote that the “consequences of a large reduction in ocean overturning would look nothing like the Hollywood film The Day After Tomorrow,” but noted “they would not be harmless either – e.g. for sea level… particularly along the US east coast … marine ecosystems, fisheries and possibly even storminess in Europe.”
Basically, ice melt from Greenland will put too much fresh cold water into the Gulf Stream, making the water colder and less salty — less salty water is less dense, meaning it won’t sink as much as it’s done in the past. This could slowdown the Gulf Stream and cause sea level rises and a rapid cooling of the North Atlantic.
“So if you weaken the ‘Gulf Stream’ and weaken that temperature contrast … sea level off the U.S. east coast will actually rise!” Penn State climate scientist Michael Mann (of “hockey stick graph” fame) wrote in an email to the Washington Post.
The Post notes that this is exactly the phenomenon that triggers a new ice age in the movie “The Day After Tomorrow.” Rahmstorf says not to expect anything like that to happen, but says the Gulf Stream has already slowed about 15 to 20 percent over the 20th Century.
“There is more than a 99 per cent probability that this slowdown is unique over the period we looked at since 900 AD. We conclude that the slowdown many have described is in fact already underway and it is outside of any natural variation,” Rahmstorf told the UK Independent.
So is the Gulf Stream actually weakening like scientists predicted? It’s not very clear, and Rahmstorf’s study contradicts recent ones that say the Gulf Stream has not slowed.
University of Rhode Island oceanographer Thomas Rossby published a similar study about a year ago and found no evidence the Gulf Stream was slowing down. Rossby and his colleagues spent about 20 years measuring the Gulf Stream using an acoustic Doppler current profiler (ADCP) attached to a ship that made weekly trips across the Stream and collected 1,000 measurements since 1992.
“The ADCP measures currents at very high accuracy, and so through the repeat measurements we take year after year, we have a very powerful tool by which to monitor the strength of the current,” Rossby said in a statement.
“There are variations of the current over time that are natural — and yes, we need to understand these better — but we find absolutely no evidence that suggests that the Gulf Stream is slowing down,” he added.
In contrast, Rahmstorf’s study did not use actual observations to reach the conclusion that the Gulf Stream was slowing. Instead, Rahmstorf used “multi-proxy temperature reconstruction” to show that cooling, possibly from increase ice melt, in the Gulf Stream may be slowing it down.
Rahmstorf’’s study makes no mention of Rossby’s in its footnotes, though it does refer to a 2010 study by NASA oceanographer Josh Willis who compiled measurements of the Gulf Stream from “ocean-observing satellites and profiling floats.”
But even Willis’s study found no significant slowing of the Gulf Stream over the past 15 years. In fact, Willis noted that the Stream may have even sped up a little bit during this time.
“The changes we’re seeing in overturning strength are probably part of a natural cycle,” Willis said in a statement. “The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling.”
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