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Now, over the summer, the ice covering the Arctic Ocean was at a record low in keeping with a sharp warming trend in the far north. At the same time, the amount of the ocean around Antarctica covered by sea ice hit a record high. Of course, it is winter in Antarctica when it's summer in the Arctic. But still, why in a warming world is that wintertime ice in the south growing?
NPR's Richard Harris explains.
RICHARD HARRIS, BYLINE: The National Snow and Ice Data Center in Boulder Colorado keeps tabs on sea ice. And one week after the organization announced the record-breaking low in the summertime Arctic, it announced the new record high for wintertime sea ice around Antarctica.
TED SCAMBOS: Having these happen in the same year is a bit like having your high school football team win their game, but your NFL team loses the Super Bowl.
HARRIS: Ted Scambos at the Ice Center says it's a really big deal to have lost so much summertime ice in the Arctic. That ice plays a role in the climate, by reflecting solar energy back into space and cooling the planet. On the other hand, having a small gain in the amount of ice around Antarctica, during the dark of winter, doesn't have much of an effect on the climate, Scambos says.
SCAMBOS: There's no doubt that the more important effects are going on in the Arctic. And the ones that will have a bigger impact on where people live are going on in the Arctic.
HARRIS: Still, it is curious that on a planet that's getting steadily warmer, ice spreads out across more of the Southern Ocean than it did 30 years ago. Part of the explanation has to do with geography. The Antarctic continent is a huge mass of ice and its lot colder than the Arctic.
SCAMBOS: Antarctica itself is warming, but it's starting from a very cold place - a lot colder than the Arctic. So a little bit of warming doesn't take it to this critical threshold of reaching the melting point, especially in winter which is the season in Antarctica now.
HARRIS: But still, you'd expect ice there to be shrinking, not growing, in a warming world. And that is true for ice that's actually sitting on rock in Antarctica. But it's not true for the ice that's floating on the southern Ocean. On the whole, that wintertime extent has been growing by the very modest rate of about 1 percent per decade.
SHARON STAMMERJOHN: That's a little misleading because there's a lot of regional variability.
HARRIS: Sharon Stammerjohn, at the University of Colorado, says winter sea ice is actually shrinking rapidly in some parts around Antarctica and growing vigorously in other parts. So it's not simply a case of gradual increases in sea ice. She says both the growing and shrinking trends are due to a gradual increase in westerly winds around the southern continent.
STAMMERJOHN: If you have winds that are blowing in a west-east direction, that's going to tend to push the sea ice equator-ward. So it tends to expand out your sea ice edge.
HARRIS: So these increased winds are spreading out the ice more than it used to be. But why have the westerly winds increased? Two reasons, Stammerjohn says. First, global warming is increasing the temperature difference between the pole and the equator, and that makes the winds stronger. And the human-induced hole in the ozone layer over Antarctica is also changing winds at the surface.
STAMMERJOHN: Those two conspire to give us the changes in the atmospheric circulation that then lead to the regional sea ice changes we're seeing.
HARRIS: Bloggers who are skeptical of climate change like to point to the growth in wintertime Antarctic sea ice as evidence that the Earth isn't really warming up. But scientists who actually study this phenomenon say that's silly. But it is the case that the growth in wintertime sea ice is not predicted in the scientific computer models that forecast climate trends.
STAMMERJOHN: Most models would simulate an overall decrease in the last 20, 30 years, not the slight increase that we're observing.
HARRIS: Models really don't do a great job explaining the behavior of sea ice. In fact, the precipitous decline in the Northern Hemisphere doesn't show up in the computer simulations, either. Stammerjohn hopes the models will be improved with more data which, until recently, has been extremely sparse, especially from around Antarctica.
Richard Harris, NPR News.
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