Last week, Nature published a climate science study that came to a very surprising conclusion – one that other climate scientists struggle with. Two other scientists wrote a critical response and posted it to Real Climate the same day, detailing their problems with the study’s findings.
Arguments like these could continue to play out among scientists, but the BBC News covered the research without skepticism, so we thought it would be worth explaining what the arguments are about.
AMOC run amok
The study by Xianyao Chen of the Ocean University of China and Ka-Kit Tung of the University of Washington focused on the large-scale movement of water in the Atlantic Ocean. In this part of the ocean “conveyor belt” that wraps around the globe, surface water is carried toward the pole before mixing down around Greenland and moving south along the ocean floor.
That downward mixing can get stuck and interfere with the northward movement of warmer water that plays an important role in the regional climate. (You may recall that this mechanism was greatly exaggerated in the disaster movie) The day after tomorrow.) Scientists have been closely monitoring this current system, the “Atlantic Meridional Overturning Circulation,” or AMOC.
Overall, the AMOC is expected to slow as the world warms further and melting glacial ice from Greenland continues to flow on a conveyor belt. Measurements have shown that it may already have slowed down slightly due to global warming. But a power interruption is thought to be unlikely.
In the new study, Chen and Tung claim that: The day after tomorrow had it right back. Although the stalling of the current has caused cold conditions around the North Atlantic in the past, the researchers say it could be different all at once. pre workout climate. Because the Atlantic conveyor belt helps transport a large amount of heat energy held by our greenhouse gases to the deep ocean, slowing the conveyor belt should allow more heat energy to build up near the surface, fueling global warming. temperature rises.
Most of Chen and Tung’s evidence for this comes from comparing Atlantic circulation data with global temperature data and noting that temperatures seem to have risen faster during periods when circulation slowed. They conclude that data shows little sign of human impact on circulation, so we must be in another period of naturally low circulation that could continue for another twenty years or so — with global warming faster than we’ve seen in the last few years. decades have seen.
Feel the flow, it’s circular
Ka-Kit Tung also co-authored a 2013 study (which we covered) that attributed most of the fluctuations in global temperature to the Atlantic circulation. That caused man-made global warming to go back almost linearly to at least 70 years, despite increasing greenhouse gas emissions.
That study has been widely criticized for the way it calculated the influence of the Atlantic, somewhat circularly assuming most of the fluctuations were attributable to the Atlantic, eliminating other well-studied factors such as sunlight-reflecting aerosol pollution, eruptions, and circulation in the Pacific. were sidelined. That same calculation appears in this new study, which underlies their comparison between the strength of the Atlantic circulation and changes in the rate of global warming.
In their positions at Real Climate, Michael Mann of Penn State and Stefan Rahmstorf of Potsdam University — who have both studied the Atlantic circulation — disagree with this and several other aspects of the new paper.
They say the idea that slowing circulation would speed up North Atlantic warming rather than cooling it runs counter to so many studies that it requires “extraordinary claims requiring extraordinary evidence” to tread territory. But, they argue, there is a complete absence of significant evidence: “Chen and Tung also do not show model simulations to provide evidence that their mechanism may actually work, nor do they discuss the various published model results that have come to the opposite conclusion.”
So should you value the new study’s prognosis for the next twenty years? Probably not, argue Mann and Rahmstorf.
“It’s hard not to think about the prediction of Keenlyside et al. in Nature in 2008,” they write. “These authors made headlines around the world by predicting a phase of global cooling, ironically also based largely on a forecast of weak AMOC […] At the time, the Realclimate team had solid reasons to predict that the prediction would turn out to be wrong – which it indeed was. This time again, we have no doubt that rapid global warming will continue until we severely reduce greenhouse gas emissions, but for reasons unrelated to the AMOC.”
Sometimes a study going to publication makes a bunch of researchers in that field pretty cranky — not every newspaper inspires a go-get-the-good champagne toast. It’s confusing to the public when these studies make the news without clarifying the context, but “grumpy” is generally a productive mode for scientists. Substantive argumentation forces existing ideas to be clearly formulated, which sometimes brings up a point that can be solidified by the right test.
However, when only one side of these debates makes the press, it can be difficult for the public to know that an argument is underway.
Nature2018. DOI: 10.1038/s41586-018-0320-y (About DOIs).