As extreme weather rocks the Southern Ocean, a tumultuous mix of carbon dioxide, winds and warming waters could reach an environmental tipping point.
The world’s oceans take up more than a quarter of the carbon humans are emitting into the atmosphere, partially mitigating the greenhouse effect of that carbon. And the Southern Ocean accounts for nearly 40 percent of this marine carbon absorption, even though it makes up only one-fifth of Earth’s ocean surface area. There could be enormous consequences for our already warming planet from even a small reduction in the Southern Ocean’s ability to suck up carbon from the atmosphere.
Scientists say this reduction may already be happening, and they suspect that the westerlies in the Southern Hemisphere, which are stronger than their northern counterparts because they blow mostly over open water, are to blame. Records of actual wind speeds, as well as estimates of wind speeds from measurements of atmospheric pressure, provide clear signs that the westerlies in both hemispheres are shifting toward the poles and intensifying. Climate models show that these changes are partly due to global warming, and research is afoot to determine if they are hindering the Southern Ocean’s capacity to pull carbon out of the atmosphere.
Saunders and her colleagues are finding clues that over the past 12,000 years of Earth’s history, higher wind speeds and greater concentrations of carbon dioxide in the atmosphere have gone together. If this link between wind and CO₂ holds true today, it will lead to higher levels of the greenhouse gas in the atmosphere, which will speed up climate change, which could further intensify the westerlies.
There’s growing evidence that, at least in the winter months when the westerlies are at their most violent, some parts of the Southern Ocean are giving off more of the gas than was previously estimated. It has all the makings of a vicious cycle.
There’s one more factor that could feed into the vicious cycle that scientists fear may already be underway in the Southern Ocean. Changes in the extensive Atlantic Ocean circulation system that includes the Gulf Stream may also be exacerbating the warming of the Southern Ocean, which in turn could be adding to the poleward shift and intensification of the westerlies.
This circulation system, called the Atlantic meridional overturning circulation, or AMOC, brings warm, salty surface water to the north, where it cools and sinks and then heads back to the Southern Hemisphere through deeper ocean layers. Scientists have found evidence that the AMOC has been slowing down.
Some of the AMOC’s slowdown can be attributed to one of the consequences of global warming: an increasing supply of glacial meltwater into the North Atlantic. This fresh water reduces the salinity, and hence the density, of the surface waters. The formation of dense, salty, cold water is essential for it to sink down to the deep ocean in the North, from where it makes its return to the Southern Hemisphere. This process is being disrupted by climate change.
Because the AMOC transports heat from the south to the north, a slowdown effectively cools the Northern Hemisphere and heats up the Southern Hemisphere. This warming also intensifies the westerlies and shifts them toward the South Pole, leading to a greater vertical churning of the Southern Ocean, which may contribute to global warming by weakening the ocean’s ability to absorb CO₂. Since the likely result is even more glacial meltwater flowing into the North Atlantic, this could further weaken the AMOC. It has all the makings of a system that’s slipping out of control.
Much more detail:
https://www.smithsonianmag.com/science-nature/worlds-best-natural-defense-climate-change-make-things-worse-180974280/