Study: Decades-Long Atlantic Meridional Overturning Circulation Collapses Possible With Increasing Temperatures

Before getting into the crux of this article, it’s probably necessary to explain that the climatic changes that have occurred over the last tens of millennia (observable in the paleoclimate record) haven’t always been gradual, widespread, or consistent.

To put that another way, there have often been rapid, fluctuating, extreme changes in temperature and precipitation in different regions over the last few hundred thousand years that have occurred over timespans of just decades — owing to changes in oceanic circulation, the breaking of ice dams, extensive ash falls, etc.

Via The Verge: “AMOC in the North and South Atlantic oceans. Red lines: surface flows. Yellow and green: intermediate flows. Blue and purple: deep flows. NOAA, adapted from Speich 2009, Lumpkin 2007”

One of the subjects that’s sometimes broached by modern climate scientists — mostly very tentatively — is the possible effect that rising temperatures and melting ice in the Arctic could have on the Atlantic Meridional Overturning Circulation (AMOC). The AMOC, for those unfamiliar, is the conveyer-belt-like current in the Atlantic Ocean that’s responsible for the very mild temperatures of Northwestern Europe (rather than temperatures similar to those of other regions near the same latitude). The AMOC brings relatively warm waters from further south up north, and thus warms the region. This warm water then cools in the northern latitudes, sinks, and then circulates back to the tropics. (The temperature differential is the cause of the circulation.)

New research from Yale University is showing, though, that if temperatures warm enough, big changes could be in store — the AMOC is much less stable than previously assumed.

The Verge provides more:

“(Wei) Liu’s work suggests that most models don’t accurately reflect how much freshwater (which, in the ocean, just means slightly less salty water) travels in and out of the Atlantic. Freshwater is hard on the circulation because it’s not as dense as saltier water, so it doesn’t sink very well. Too much of it, and you lose the churn that keeps the AMOC going. …

“Basically, according to Liu’s work, the AMOC is a camel, and it’s already carrying a boatload of straws that current climate models don’t consider. Global warming could be the straw that breaks the metaphorical AMOC-camel’s back. That’s because if the air is too warm in the North Atlantic, the water in the AMOC won’t be able to cool down by transferring its heat to the atmosphere, so it won’t sink and circulate back to the tropics like it does now. In fact, Liu calculated that if atmospheric carbon dioxide increases to 710 parts per million it could be enough of a blow to collapse the AMOC within 300 years of that spike. For comparison, last week’s levels came in at 405 parts per million. That’s up from 355 ppm in 1990.”

If this was to happen (the collapse of the AMOC), then winters in the North Atlantic could “drop as much as 7° Celsius in the wintertime.”

This far out, though, it’s pretty much impossible to say what this would mean exactly — the context of it all, that is. What will the Arctic be like 300 years from now? Will people be around? Will the region be an ecologically depleted shell of what it now is? Littered with chemical, mining, and nuclear wastes? A trade and/or pirating hub scattered with sail boats?

The new findings are detailed in a paper published in the journal Science Advances.