In 1916, Antarctic explorer Ernest Shackleton was forced to make a treacherous voyage across the open waters of the Southern Ocean after his ship Endurance was crushed by pack ice and sank in the Weddell Sea. Along with five members of his party, Shackleton set out for the island of South Georgia in a 23-foot-long wooden lifeboat named the James Caird. Eleven days into the trip, on May 5, Shackleton thought he spied a clearing in the murky sky above. What he was actually seeing wasn’t a break in the clouds, but the white crest of a monstrous wave bearing down on them.
“During twenty-six years’ experience of the ocean in all its moods I had not encountered a wave so gigantic,” Shackleton would later write in his book South: The Story of Shackleton’s Last Expedition 1914-1917. “It was a mighty upheaval of the ocean, a thing quite apart from the big white-capped seas that had been our tireless enemies for many days.”
Shackleton’s account remains one of the most evocative descriptions of the westerly winds that circle the globe between 30 and 60 degrees latitude in both hemispheres and the colossal waves they whip up in the Southern Ocean. Today, the southern westerlies are back in focus for entirely different reasons: their potential impact on climate change.
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.