Carl Zimmer has posted an interesting response to the points that were raised repeatedly in yesterday’s comment thread on George Will’s climate change column: A Wrinkle In Ice (or Not).

In his reply to me, Chapman explained that the two research centers, NSIDC and ACRC, both use the SSMI satellite readings, but they have different methods for building their time series. Chapman and his colleagues at ACRC use a composite of three sequential days for their ice cover readings. If a swath of data is missing on one particular day, they can go back to the previous day’s concentrations. If there are still missing regions, they go one more day back.

“Missing regions or swaths of data have always occurred from time to time in the SSMI record, which is why we set it up this way,” Chapman explained.

Despite the recent trouble with the SSMI satellite, Chapman said the three-day-composites have still been meaningful. “As one check, we have been comparing our time series with those from the independent data source AMSR-E. They are just about identical so we are comfortable that our time series remain solid. Our time series and therefore the statement are unaffected by the recent satellite problems. If the sensor degrades a lot more, our numbers will be affected, but to date, they are not.”

I then asked what he thought about the Washington Post’s support of Will’s claim about ice. (To recap again, their support was decidedly roundabout. A January 1 post on a blog called Daily Tech claimed that global ice cover in late 2008 were unchanged from 1979. In response to that blog post, the Center posted a pdf on their web site explaining that “observed global sea ice area, defined here as a sum of N. Hemisphere and S. Hemisphere sea ice areas, is near or slightly lower than those observed in late 1979.” But then the scientists also explained that climate models predict a decline in Arctic ice, but are less certain about Antarctica, with some even suggesting an increase–making measurements of global sea ice not terribly relevant to the question of climate change. The Post ignored that part.)

Here’s Chapman’s reply:

Since their statements were based on the end of the previous year, and more importantly the end of 1979, the statement ‘global sea ice levels now equal those of 1979′ just didn’t make sense any more. We have received 80-100 emails from confused people who had read George’s column and looked up the graphs on the Cryosphere Today [one of the center’s web pages] and said they came to a different conclusion, or, could we point them to the report that said that Feb 1979 and Feb 2009 sea ice area was nearly the same. We had to post the current and corresponding 1979 values to avoid the inconsistency that readers were noting. After doing some googling, it appears that Daily Tech article got repeated on a lot of blogs, so it’s not surprising George Will came across it at some point. Still it was sloppy for them to not double check with the original source and it really points out the danger of making any conclusions on climate change based on any two days in history. I really wish they would have contacted us at some point to avoid this.

Our goal is to present the data in as concise and useful format as possible for interested users. Whether the Washington Post decides to publish a correction is up to them.