“It’s been often stated that Greenland melt could be very devastating for the AMOC, but no one has really looked at it in a systematic way,” says Pepijn Bakker, a climate scientist at the University of Bremen in Germany and author of the new study.
So Bakker teamed up with ice sheet modelers and others to calculate how the AMOC would respond to Greenland melting under a number of different emissions scenarios.
In one scenario, Bakker calculated what would happen if anthropogenic carbon dioxide emissions peak around the year 2040 and then decline. In that situation, the AMOC weakens by about 18 percent, but does not collapse.
In another scenario, carbon emissions continue to rise at their current pace throughout the next century. If this happens, the AMOC has a 44 percent chance of collapsing entirely by 2300, he says. The research hints at some of the hugely negative consequences that could lie in wait if humanity chooses to do nothing to mitigate climate change.
Like a sudden drop in blood pressure, a weakening or shutdown of the AMOC could cut off vast swaths of the ocean from a vital supply of heat and nutrients. Much of the planet could experience wild changes in temperature and productivity, altering entire ecosystems.
Conversely, Bakker says the study suggests that if humans take action to limit greenhouse gas emissions, the AMOC should survive.
The effect of Greenland ice melt has not been widely included in previous studies of the AMOC because ice melt predictions are generally not included in climate models.
Even today’s most sophisticated climate simulations can’t account for all of the potential complexities of ice sheets. Some have their own river systems or particular microclimates, for instance, and the ways in which the ice interacts with warm and cold ocean waters is not well understood.
The only way to include Greenland ice melt (as well as Antarctic ice melt) in climate models is to make assumptions about the rates at which the ice sheets are melting, says Michael Mann, a climate scientist at Pennsylvania State University who studies the AMOC and was not involved in the new work. Mann says Bakker’s predictions of this fresh water influx are more realistic than previous estimates of an AMOC shutdown, most notably those made in a paper published earlier this year by former NASA climate scientist James Hansen.
Also complicating matters: scientists don’t fully understand the AMOC itself and how it might respond to not only an influx of fresh water, but also to shifting winds, rising sea levels, and changes in atmospheric and ocean temperatures.
Notably, Bakker’s calculations of how much AMOC-suppressing meltwater could flow into the North Atlantic were more conservative than some previous estimates, including Hansen’s widely reported paper. Hansen’s group projected massive freshwater runoff from Greenland and a catastrophic collapse of the AMOC. That paper also predicted violent superstorms, mega-waves throwing massive boulders onto shorelines, and a stratification of the ocean with cold water on top and warm water on the bottom.
Bakker says he thinks Hansen’s paper was unrealistic, but adds that a collapse in the AMOC would have devastating consequences. Sediment cores drilled from Greenland and Antarctica show that during the last ice age, the AMOC weakened suddenly and dramatically, nearly to the point of collapse, just before periods of wild climate change in the northern and southern hemispheres.
“It definitely cools Europe, which perhaps combined with global warming is not such a bad thing, some would argue,” Bakker says. “It would reduce warming in the North Atlantic region and over Europe. But it would also change many other things: the rain belts over Africa and Central America would shift. The ocean circulation itself impacts all the biology that lives in there. I’m not sure if we really know all the consequences of such things.”
Published January 3, 2017
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