The Gulf Stream is part of a much larger system, however. As that warm water flows northeast, it gradually cools, and in cooling, compresses and sinks. Eventually, in the Labrador and Greenland Seas, it becomes dense enough that it plunges down thousands of meters into the deep ocean. There it becomes a new current, running back south. It can remain in this deep-ocean current for many years until it eventually upwells at the equator or in the Southern Ocean.
This global conveyor belt of water is AMOC, and it is critical to the world’s climate. (Most scientists pronounce it as AY-mock.)
When AMOC is strong, it sends millions of cubic meters of ocean water north every day. A strong AMOC seems to shape the entire planet’s climate systems. It moderates the intensity of Atlantic hurricanes, lessens the risk of drought in North America, and assures the health of monsoons in India. AMOC also ferries warm weather from the equator to Western Europe, where it helps bring the region unusually mild winters. (Consider that temperate Berlin is about as far from the equator as the snowy Chilean city of Punta Arenas.)
Crucially, the entire AMOC system depends on cool, dense water “overturning” in the northwest Atlantic Ocean. Without cooled water plunging into the deep ocean near Greenland, and turning back south, the entire conveyor belt will stop.
About 30 years ago, climate researchers became concerned that AMOC could suddenly shut down as a result of anthropogenic climate change. The “paleoclimatic record”—that is, what the planet’s geology and fossil record reveal of previous global climates—showed that the AMOC has rapidly collapsed in the past. “Rapidly” here means “within the span of a human lifetime.”
The crumpling of AMOC could potentially cause big problems for the global economy. AMOC’s disappearance would quickly worsen sea-level rise on the U.S. East Coast and subject the Southeast to unusually intense tropical storms. It could upheave agriculture in India, Europe, and the African Sahel.
But as climate models improved, those fears dissipated. “No current comprehensive climate model projects that the AMOC will abruptly weaken or collapse in the 21st century,” wrote a team of NOAA researchers in 2008. “We therefore conclude that such an event is very unlikely.”
Thomas Delworth was the lead author of that report. Delworth is a researcher at the NOAA Geophysical Fluid Dynamics Laboratory and a professor of atmospheric and oceanic science at Princeton University. He says that scientists are now re-examining those old conclusions.
“Some recent work now is challenging that consensus. It suggests that the real climate system may be less stable than [the models] think,” Delworth told me.
The most attention-getting of this work: a paper last year by James Hansen and 18 other scientists that argued the AMOC’s collapse could threaten global civilization this century. The paper built on older work showing that huge injections of freshwater have historically destabilized AMOC, essentially by flooding the Atlantic with cold water and screwing up its finely tuned density cycle. Hansen and his colleagues argued that as the Greenland ice sheet melts, it would be able to provide exactly such a pulse—and that, crucially, climate models failed to account for this physical process.