The leading hypothesis is that a change in the location or strength of winds in the Southern Ocean near Antarctica that forced upwelling of deep ocean water, which then released CO2 to the atmosphere. Pictured: an iceberg in the Southern Ocean. Image: Flickr/Liam Quinn
At the end of each ice age, the ocean exhales carbon dioxide. Scientists believe this explains the difference in atmospheric CO2 concentrations between ice ages, which have lower concentrations of carbon dioxide, and warmer, more CO2-saturated periods like the one we're living in now.
What causes that carbon dioxide to exit the ocean when an ice age ends, though, is still a puzzle oceanographers are trying to decipher. The leading hypothesis now is that it wasn't primarily ocean circulation but a change in the location or strength of winds in the Southern Ocean near Antarctica that forced upwelling of deep ocean water, which then released CO2 to the atmosphere.
But research published yesterday in the journal Nature rebuts this idea, suggesting that it was changes in ocean circulation, not winds, that predominantly led the deep water to surface near Antarctica and exhale carbon dioxide to the atmosphere.
"In the midst of all this scientific energy and investigation towards the winds comes this paper, which says, it wasn't winds at all, actually, it was the other thing. And that's why it's exciting," said Elisabeth Sikes, an oceanographer at the Institute of Marine and Coastal Sciences at Rutgers University who discussed the paper in an accompanying "News and Views" piece in Nature.
Nele Meckler, a paleo-oceanographer at the Geological Institute of ETH Zurich in Switzerland and lead author on the study, explained why oceanographers care about this problem.
"One of the big questions is: Why was the climate and why were CO2 levels so different during ice ages than during warm times? And that's still a big puzzle in paleoclimate," she said.
Because the ocean is the place where most of the carbon dioxide that was not in the atmosphere during ice ages was stored, understanding how that CO2 gets exchanged between the ocean and the atmosphere is important.
A step that could improve climate models
A better understanding of how the atmosphere and the oceans communicate and exchange things like CO2 can also help improve climate models and predictions of the future.
In this recent study, the way Meckler suggests the carbon dioxide was prompted to leave the ocean "is quite a different explanation than we've seen before," Sikes said.
It takes a few steps to get from the end of an ice age to an exodus of carbon dioxide from the Southern Ocean. Here's Meckler's explanation.
At each ice age's end, massive ice melting triggered an enormous release of fresh water in the northern ocean, "basically forming a freshwater lid on the north Atlantic."
That lid of low-density water shut off the formation of deep water in the Atlantic. Deep water is cold, dense water that sinks to the ocean depths.
This basically stopped the normal north-to-south ocean circulation, Meckler said. So heat from the atmosphere, which would normally circulate through the ocean, started diffusing down toward to the bottom. Normally, dense water wants to sink below less dense water.
That heat, though, makes the water in the deep ocean, which is normally both very cold and dense, warmer and therefore less dense.
And around Antarctica, where even the surface ocean water is already quite cold and dense, some of that water in the ocean depths, which is also carbon rich, eventually warmed enough so that it became less dense than the water above it.
A release point in the Southern Ocean
"When that happens, Meckler said, "you automatically generate communication between the deep ocean and the atmosphere. So that then would release the CO2 to the atmosphere in the Southern Ocean."
Meckler actually figured this out by looking at an area near tropical Africa, far from either pole. She and her colleagues took sediment cores and tracked when there were significant deposits of silicates.