Thursday, March 24, 2016

The Atlantic Meridional Overturning Circulation Is Going To Collapse And Destroy Civilization ... Or Not

The Atlantic has published an article entitled "The Struggle of Clear Climate Communication." A somewhat innocuous title that, ironically, serves to obfuscate the author's real message that we are all doomed unless something drastic is done now! In particular, the author focuses on a paper published by James Hansen, a climate scientist and the former director of NASA’s Goddard Institute for Space Studies, who claims that even a 2 degree Celsius increase in global temperatures will cause the shut down of the Atlantic Meridional Overturning Circulation (AMOC) current. The article states: "Once this current is shut down, then the Atlantic near the Arctic will stay cold and heat will build up in the southern latitudes, creating the potential for extreme weather of mythical proportions." 
[The paper] warns that, by 2100, the planet’s natural system could change so dramatically that enormous “superstorms,” sometimes powerful enough to hurl ocean boulders hundreds of feet into the air, will form in the Atlantic Ocean. Seas could also rise so quickly that they will inundate coastal cities—including New York, Washington, and San Francisco—rendering them unlivable before the end of the century.
However, Hansen adds:
“Have we passed a point of no return? I doubt it, but it’s conceivable,” he adds. “But if we wait until the real world reveals itself clearly, it may be too late to avoid sea-level rise of several meters and loss of all coastal cities.”
Hansen's predictions are based on computer models. But, as they say, garbage-in, garbage-out (GIGO). Anthony Watts reports on a study showing "There is no real evidence for a diminishing trend of the Atlantic Meridional Overturning Circulation." That study notes that accurate measurements of AMOC are only recent, but even using other proxies, there is no evidence that variations of the strength of the currents are anything but normal variations.
Every approach proposed so far for the AMOC has indices that may or may not represent the long term trend in the AMOC depurated of the variability. Every approach interprets changes in the indices as supporting either no AMOC slowdown or a clear evidence for a slowdown. Either may be right or wrong, but they cannot both be right. Here we argue that there is no unquestionable evidence of any change in the AMOC signal if not variability. 
The long-term sea level variations along the east coast of North America appear to be different north and south of Cape Hatteras. And the differences in north-south sea level change can be argued to reflect changes in the AMOC which then adjust the sea surface temperature (SST) patterns that make up the Atlantic Multi Decadal Oscillation (AMO). A stronger AMOC should lead to warmer temperatures in the Atlantic marking a positive AMO so the AMOC and AMO should be linked. Long-term AMO oscillations then argue for an oscillating AMOC over the past 50 years without a long-term trend. 
There is no reliable measure of the AMOC direct or based on proxies that covers a sufficient time window to show a clear trend beyond inter-annual and multi-decadal variability. Claims of strengthening or reducing of the AMOC are therefore pure speculation.
 (Underline added).

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