longread s hezkou infografikou
In the Atlantic Ocean, Subtle Shifts Hint at Dramatic Dangers
https://www.nytimes.com/interactive/2021/03/02/climate/atlantic-ocean-climate-change.html
a spate of studies, including one published last week, suggests this northern portion of the Gulf Stream and the deep ocean currents it’s connected to may be slowing. Pushing the bounds of oceanography, scientists have slung necklace-like sensor arrays across the Atlantic to better understand the complex network of currents that the Gulf Stream belongs to, not only at the surface, but hundreds of feet deep.
“We’re all wishing it’s not true,” Peter de Menocal, a paleoceanographer and president and director of the Woods Hole Oceanographic Institution, said of the changing ocean currents. “Because if that happens, it’s just a monstrous change.”
The consequences could include faster sea level rise along parts of the Eastern United States and parts of Europe, stronger hurricanes barreling into the Southeastern United States, and perhaps most ominously, reduced rainfall across the Sahel, a semi-arid swath of land running the width of Africa that is already a geopolitical tinderbox.
...
The scientists’ concern stems from their understanding of thousands of years of the prehistoric climate record. In the past, a great weakening or even shutdown of this arm of the Gulf Stream seems to have triggered rapid changes in temperatures and precipitation patterns around the North Atlantic and beyond.
The northern arm of the Gulf Stream is but one tentacle of a larger, ocean-spanning tangle of currents called the Atlantic Meridional Overturning Circulation, or AMOC. Scientists have strong evidence from ice and sediment cores that the AMOC has weakened and shut down before in the past 13,000 years. As a result, mean temperatures in parts of Europe may have rapidly dropped to about 15 degrees Celsius below today’s averages, ushering in arctic like conditions. Parts of northern Africa and northern South America became much drier. Rainfall may even have declined as far away as what is now China. And some of these changes may have occurred in a matter of decades, maybe less.
...
The science remains relatively new, and not everyone agrees the AMOC is actually slowing. But in both scientific modeling of climate change and in the prehistoric record, a North Atlantic cooling presages a shutdown of the current. “One of the hallmarks of a shutdown is this cold blob,” says Dr. de Menocal. “The cold blob is a big deal.”
...
The clearest example began about 12,800 years ago. Glaciers that had once covered much of North America and Europe had retreated considerably, and the world was almost out of the deep freeze. But then, in just a few decades, Greenland and Western Europe plunged back into cold. Temperatures fell by around 10 degrees Celsius, or 18 degrees Fahrenheit, in parts of Greenland. Arctic-like conditions returned to parts of Europe.
The cold snap lasted perhaps 1,300 years — before reversing even more abruptly than it began. Scientists have observed the sudden changes in the pollen deposited at the bottom of European lakes and in changes in ocean sediments near Bermuda.
This forced a paradigm shift in how scientists thought about climate change. Earlier, they had tended to imagine creeping shifts occurring over many millennia. But by the late 1990s, they accepted that abrupt transitions, tipping points, could occur.
This didn’t bode well for humanity’s warming of the atmosphere. Dr. Broecker, who died in 2019, famously warned: “The climate system is an angry beast and we are poking it with sticks.”
...
WHY DID THE AMOC shut down? A leading theory is that meltwater from retreating glaciers emptied into the North Atlantic or Arctic oceans. Freshwater is lighter than saltwater, and the sudden influx of more buoyant water could have impeded the sinking of denser, saltier water — that critical “overturning” phase of the AMOC.
Today we don’t have massive glacial lakes threatening to disgorge into the North Atlantic. But we do have the Greenland ice sheet, which is melting at the upper end of projections, or about six times faster than in the 1990s. And according to one study, the subpolar North Atlantic recently became less salty than at any time in the past 120 years.
There’s little agreement on cause. Changes in wind patterns or currents may be contributing, as could greater rainfall. But Stefan Rahmstorf, a physical oceanographer with the University of Potsdam in Germany, suspects that, similar to what happened some 12,800 years ago, meltwater from Greenland is beginning to slow the AMOC.