Running the Numbers on Antarctic Sea Ice National Snow and Ice Data CenterAlthough the difference may seem slight, Antarctic ice cover set a 33-year record on Sept. The agency that tracks polar ice reported Tuesday that winter coverage of sea ice in Antarctica has set a 33-year high. The ice hit its maximum extent on Sept. 26, at the peak of Antarctic winter, when it covered 7.5 million square miles of the Southern Ocean. That's a half-percent increase over the previous record, set in 2006. The National Snow and Ice Data Center uses a five-day moving average to track such matters, and always waits a few days before announcing a minimum or maximum in sea ice at either pole. That is to make sure the low or high point for the year has really been reached, given that sea ice can change abruptly in response to winds and other factors. The five-day averaging also helps smooth out small errors in the satellite tracking data. This longstanding practice has been explained publicly many times, but that has not stopped climate-change contrarians from asserting that the snow and ice center had been trying to hide this year's record in Antarctica by supposedly failing to make any announcement. Skeptic blogs have been humming for days about the likelihood of a record, and claiming that the growth in Antarctica offsets the disappearance of sea ice in the Arctic. This is a claim the climate contrarians tend to make almost every time Arctic sea ice sets a record or near-record low. In reality, the trends in Antarctic sea ice are pretty small compared to what's happening in the Arctic. For readers without much background, it might help to appreciate the big differences between the Arctic and the Antarctic. The Arctic is a confined, relatively shallow ocean basin surrounded by land, whereas the Antarctic is a massive, ice-covered continent surrounded by thousands of square miles of deep, open ocean. The Arctic Ocean is connected by relatively narrow gateways to the rest of the world's seas. That means warm water from lower latitudes has a hard time flowing in. Historically, a great deal of sea ice tended to survive the winter there, exerting a big influence on the reflectivity, or albedo, of the planet. The white polar ice cap would reflect sunlight during the summer, when the sun shines 24 hours a day at high latitudes, helping to keep the Arctic cold. That, in turn, influenced the climate of the Northern Hemisphere mid-latitudes, where the bulk of the world's people live. Remember all those weather maps you've seen on television of cold Arctic air swooping down into the United States? The Arctic has effectively been our air conditioner, which may make for chilly winters but can bring relief in the summer. In Antarctica, when winter sets in and the sun drops low in the sky, sea ice can grow unimpeded over the huge ocean surface. But then, in contrast to the historical situation in the Arctic, about 80 percent of the Antarctic ice melts in the summer. So the Antarctic ice has always come and gone in an annual rhythm. Most of it does not hang around to reflect sunlight back to space at the time of year when Southern Hemisphere sunlight is strongest. The tendency of Arctic sea ice to persist through the summer, and of Antarctic sea ice to disappear, are both factors in establishing the climate we all grew up with. The big question is, how much are things changing as the world warms? Climate skeptics, as they have been doing this year, often compare summer ice in the Arctic with winter ice in the Antarctic. And they are always thrilled to report that Antarctic ice is growing by leaps and bounds at exactly the moment the Arctic ice is shrinking. But this is an apples-to-oranges comparison. Scientists explain that, to the extent comparison between poles makes sense, the correct way to do it is to look at summer minimums and winter maximums in each hemisphere. Given how the seasons work, the relevant figures for each pole always occur about six months apart. The satellite record of precise measurements begins in 1979. One can calculate trends in all sorts of ways, but for each pole and each season, I'm going to calculate a simple arithmetic average of sea ice in the first five years of the satellite record, then do the same for the most recent five years. That will give us the before-and-after picture. And, to give a sense of scale, I'm going to compare those numbers to the overall size of the ocean basin in which that ice grew. First, let's examine summer sea ice in the Arctic, which has been a big news story in recent years because it is falling so precipitously. For the first five years of the satellite record, before global warming had sent the ice into sharp decline, more than half the Arctic Ocean tended to be covered by ice at the peak of the summer melt season. To be precise, the summer minimum of sea ice in the Arctic from 1979 to 1983 averaged out to 2.76 million square miles, or 51.1 percent of the surface of the Arctic Ocean. In the most recent five years, by contrast, the summer coverage fell to an average of 31.6 percent of the ocean surface. This year, when we hit a new record low for the satellite era, the ice fell to 24 percent of the ocean surface. Unless there's a big recovery in coming years, in other words, we will have lost fully half the sea ice in the Arctic. The change represents a potentially massive reduction in the albedo of the planet, which explains why climate scientists are so troubled by it. Compared to the change in the Arctic, what does the Antarctic summer minimum look like? Well, the trend there is definitely opposite to the Arctic trend - summer sea ice is growing through time. But not by that much. And this past summer melt season did not set any record; it was only the sixth-highest minimum in the satellite era. Again, let's average the first five years of the satellite record, from 1979 to 1983. In that period, the sea ice left at the end of the summer melt season covered about 13.8 percent of the surface of the Southern Ocean. In the most recent five years, the average rose to 14.6 percent of the ocean. So, expressed as a percentage of ocean cover, the decline going on in the Arctic is almost 25 times the increase going on in the Antarctic. Walt Meier, a top scientist at the snow and ice center, told me, "It should be pretty clear that the change in sea ice in the Arctic is much more substantial than what is happening in the Antarctic." We'll do similar math for the winter maximum in Antarctica. In the first five years of the satellite era, the average sea ice peaked at 91.9 percent of the surface of the Southern Ocean. In the most recent five years, counting this record-high year, it increased to cover 92.9 percent of the ocean. How does that compare to the trend in the Arctic for the same season? I'll spare readers another deluge of numbers, but in percentage of ocean cover, the decline in Arctic winter ice is eight times as fast as the increase in Antarctic winter ice. A search of skeptic blogs for that particular statistic comes up empty, somehow. Now, don't get me wrong: What is happening to sea ice in the Antarctic is very much an interesting scientific question. Why is it growing slightly even as the planet, over all, is warming up? Far from hiding anything, mainstream climate scientists are all over that issue, and have been for years. For a summary of their research, check out this fascinating article from the magazine Oceanography. Finally, to compare the trends at the poles with your own eyes, go to this NASA Web site and click the radio buttons to move through time. The video embedded at the top of the post gives a sense of the change in Antarctic winter maximums through time. And the image below shows how much the summer sea ice in the Arctic has declined.