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Tuesday, May 17, 2011

Unusually wet or dry may become usual

David Brooks

This has been, as you have probably noticed, a really wet spring, with borderline flooding.

But last spring, as you may recall, was really dry, with borderline drought – quite a contrast.

I pondered this difference as I slogged through the rain on the way to my car the other day, and I realized that shifts between one extreme and another is just the sort of pattern predicted by some models of climate change. A warmer globe puts more moisture in the atmosphere from increased evaporation, which is linked to heavier falls of rain or snow.

But I also realized that switching from dry to wet is just the sort of thing that has long seemed routine for New England – you know, “wait five minutes and the weather will change.”

So while I tried to dry out my shoes, I wondered whether there is any way to judge objectively which one it might be: New England normality or climate-change oddity?

On a scientific level, the answer is no, just because New Hampshire is so small that nobody knows how climate change will affect our weather, at least over a decade or two.

“For the resolution of climate models, when you get down to smaller and smaller resolutions, you need bigger and bigger computers to run them,” said state climatologist Mary Stampone. “The best you feasibly do in New England is basically a gridbox. You can’t pinpoint Concord in these things.”

But I’m a journalist, not a scientist, so I decided to give it a shot.

I gathered 110 years of statewide precipitation totals collected by the National Weather Service at its station in Concord. You can find it via the state climatologist office Web site.

Then I did three calculations for the period of 1901-2010: I calculated the annual average (the mean, to be precise) for the period; then calculated how much each year varied from that average; then calculated the average of all those annual variations.

This gave me a measure of a “normal” year: One in which the total precipitation was 43.1 inches, plus or minus 4.52 inches.

Years with totals outside that range were labeled as “extreme,” although “abnormal” is probably a better term. Note that it doesn’t matter whether a year was abnormally wet or dry; all that mattered was whether the difference was large.

Finally, I tried to measure whether there has been a change in the frequency of those years over the 110-year period – in other words, whether abnormal years are becoming more common or less common.

My finding? I saw no particular change over the decades up through 2000, but encountered an eyebrow-raiser over the past decade.

Between 2001 and 2010, eight of the 10 years had precipitation levels outside the normal range. That includes the wettest, second-wettest and third-wettest years on record, as well as the third-driest year on record.

No other decade had more than six years outside the average, and most had fewer than five.

Has our climate pattern altered to favor extreme precipitation? I can’t really say, alas, because it was based on just one weather station and because my methods, shall we say, don’t quite follow research standards.

Climate-statistics folks calculate change out to the second standard deviation rather than just the mean (more number-crunching than I could handle), and I estimated extreme-weather frequency simply by counting the number of abnormal years per decade and comparing decades, which isn’t exactly sophisticated.

Still, it hints that we shouldn’t be surprised down the road if “unusual” amounts of rain fall – or don’t fall.

Time to get a new umbrella, I think.

Granite Geek appears Mondays in the Telegraph, and online at David Brooks can be reached at 594-5831 or