Antarctic sea ice has hit its second all-time record maximum this week. The new record is 2.112 million square kilometers above normal. Until the weekend just past, the previous record had been 1.840 million square kilometers above normal, a mark hit on December 20, 2007, as I reported here, and also covered in my book.

Mark Serreze, director of the National Snow and Ice Data Center, responded to e-mail questions and also spoke by telephone about the new record sea ice growth in the Southern Hemisphere, indicating that, somewhat counter-intuitively, the sea ice growth was specifically due to global warming.

“The primary reason for this is the nature of the circulation of the Southern Ocean – water heated in high southern latitudes is carried equatorward, to be replaced by colder waters upwelling from below, which inhibits ice loss,” Serreze wrote in an e-mail. “Upon this natural oceanic thermostat, one will see the effects of natural climate variations, [the rise] appears to be best explained by shifts in atmospheric circulation although a number of other factors are also likely involved.”

There was one part of his response that was hard for me to understand. What would heat the water at high latitudes, those closest to the South Pole? (I also didn’t understand why he was talking about ice loss being inhibited when what was happening was the record growth of ice.)

Over the phone, I asked Serreze if he could clarify what was heating the water. His full response is below:

What we’re talking about is water that is 60 degrees south and more southerly than that, and so the basic thing is you have got surrounding the Antarctic continent a band of fairly strong and somewhat steady west-east winds, which they call the Roaring 40s, but then you’ve got this thing called the coriolis force, which wants to turn things to the left. What happens is that water at the high latitudes, what happens is that as we heat that water, you set up what’s called an Ekman drift, which at the surface transports that water from the high southern latitudes toward the equator.

What happens is you have to set up a continuity that has to occur so that what happens is that there’s an upwelling of cold waters from below, there’s a whole circulation loop where water sinks in the lower southern latitudes, then there’s a return flow that brings the same amount of mass to the higher latitudes.

Basically, what happens is that in the Arctic you can warm that surface water up and it doesn’t get transported away. It stays there, and it helps melt more ice, but in the Antarctic, the water gets carried away.

I thanked Serreze for his response but told him that I still didn’t know what heated the water at high latitudes. Was it, simply, global warming?

“Exactly!” he said.

“How many degrees is the water heated, before it is transported toward the equator?” I asked.

“I don’t have data on that,” Serreze said. He indicated that Marika Holland, a sea ice specialist and climate modeler at the National Center for Atmospheric Research, would possibly have some data as well as, perhaps, a fuller description of the mechanism warming the water nearest Antarctica and the associated growth of sea ice.

Holland did not respond to multiple requests for comment.

Gavin Schmidt, director of Goddard Institute for Space Studies, also did not respond to multiple requests for comment.