ROBERT SIEGEL, host:
Some climate change skeptics explained their doubts by citing temperature trends. They point out that global temperatures have plateaued over the past 10 years. Well now, a new study has found at least a partial explanation why.
And as NPRs Richard Harris reports, the answer is above our heads.
RICHARD HARRIS: Global warming has not stopped. The last decade is the warmest since temperature record keeping began, and 2009 was one of the warmest years ever recorded. But if you look over the course of the last decade, there is no strong warming trend in that time period. Susan Solomon at the National Oceanic and Atmospheric Administration in Colorado says that leads to an obvious question.
Dr. SUSAN SOLOMON (Senior Scientist, National Oceanic and Atmospheric Administration): People very reasonably have asked me, you know, why is it that in the last decade, it just doesnt look like its gotten that much warmer, even though CO2 has continued to increase, and in fact has increased quite fast.
HARRIS: The short answer is theres lots of natural variability in the climate. Tropical ocean patterns called El Ninos and La Ninas can have strong warming or cooling effects. The sun even gets slightly brighter or dimmer. And now, Solomon pinpoints another cause in a new study. It has to do with vapor way up high in the stratosphere.
Dr. SOLOMON: There have been some surprising changes in stratospheric water vapor that have really packed a wallop as far as surface climate goes.
HARRIS: It turns out, starting in the year 2000, a narrow layer of the stratosphere dried out quite rapidly. And water in the atmosphere traps heat, like glass in a greenhouse. So less stratospheric water means less warming.
Dr. SOLOMON: Its amazing that the stratosphere, which is so far removed from the surface, can exert such a big effect.
HARRIS: In fact, Solomon calculates that the loss of water in the stratosphere has offset about a quarter of the warming that would otherwise have occurred.
Dr. SOLOMON: I hasten to say it is not the whole answer probably to the reason why there has been so little obvious warming in the last decade, but I think it's probably a part of it.
HARRIS: Solomon figures that the stratosphere is dry because there have been fewer towering thunderstorms in the tropics to push water up there.
But Andrew Dessler at Texas A&M University says this is almost certainly a temporary state of affairs.
Professor ANDREW DESSLER (Atmospheric Sciences, Texas A&M University): This cant keep cooling or offsetting carbon dioxide forever.
HARRIS: Not forever since, first, the stratosphere can get only so dry. And second, the weather patterns that caused the stratosphere to dry out are bound to change. So this is clearly part of a shorter-term variation in the climate. Dessler compares it to the gyrations of the stock market.
Prof. DESSLER: Youve got sort of day-to-day or month-to-month ups and downs, but theres this long-term trend, whether its going up and down, and thats really what you care about in the stock market and in the climate.
HARRIS: Still, its very useful to identify the factors that drive the short-term ups and downs. That way you arent fooled into thinking that a temporary change is actually part of a long-term trend.
Prof. DESSLER: You can often be confused with what looks like a trend, that may go on for a long time, but turns out not to be a trend. I mean the housing market, thats the problem in a nutshell. People saw it was going up. They thought it was going up forever, but it wasnt.
HARRIS: The long-term trend of climate change is obvious. The most recent decade was the hotter than the 1990s, the 1990s were hotter than the 1980s, and so on. And Susan Solomon says the science behind that long-term trend is well understood, and water in the stratosphere is not driving it.
Dr. SOLOMON: But its really helpful and fascinating, I think, to better understand the ups and downs that may happen from one year to another, from one decade to another. Theres a lot more that we need to understand there.
HARRIS: Her new research published online in Science magazine is a piece of that puzzle.
Richard Harris, NPR News.
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