Given the categorical difference between “climate” and “weather,” it rarely helps to point to a heat wave and call it global warming. After all, as Kurt Vonnegut said, “Welcome to Earth. It’s hot in the summer and cold in the winter,” and this still seems to be the case. So what is giving NASA scientists the confidence to point to specific events like last year’s heat wave in Texas and 2010’s heat wave in Russia as evidence of manmade global warming?
An overall up-tick in what used to be statistical anomalies.
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NASA scientists have analyzed worldwide trends dating back to the 1950s, and have concluded that without manmade climate change, we wouldn’t be experiencing this bout of “hot,” “very hot” and “extremely hot” summers.
It’s a little confusing because, hey, sometimes it’s just hot, and the odds are next summer won’t be as hot as this one. But as Dr. James Hansen, the lead author of the paper, pointed out in today’s press conference, “The natural variability of weather and seasonal mean temperature is very large. What I suggested is that people have to pay attention to the frequency at which we see extreme events and unusually warm or usually cool seasons.”
Looking at worldwide data collected since the 1950s, the researchers discovered that the percentage of the Earth’s surface that has experienced “extremely hot summers” has risen. “Extremely hot” summers were defined as a mean summer temperature experienced by less than one percent of Earth’s land area between 1951 and 1980, which forms the baseline for the study.
Since 2006, about 10 percent of the land area across the Northern Hemisphere has experienced these temperatures each summer. Over the past decade 75 percent of the Earth’s land area is experiencing “hot” summers, as opposed to 33 percent from 1951 to 1980.
It grows increasingly likely that the next summer will be an extremely hot one, but it won’t always be. Hansen’s paper employs the bell curve which shows the number of extreme variations grow, as the whole chart shifts to the right, to a pronouncedly warmer world than the baseline formed from ’51-’80.
Some climate change skeptics point to this baseline – rather than the deviations – as the anomaly. John Christy at University of Alabama-Huntsville told told NBCnews.com that the period was a quiet time for extremes. Clifford Mass at the University of Washington told told USA Today that the study’s omission of the Dust Bowl-creating heat waves in the ’30s hurts its credibility.
At the press conference, Hansen pointed to the global scope of the data sets, and explained that while the central United States did experience a few years of extremely hot summers in the 1930s, “the United States covers 1.5 percent of the world’s area. And if you look at the total data at the time, it was not that anomalous…It just so happened that the place with the largest anomalies was the United States in the ’30s.”
One of Hansen’s favorite metaphors is that of the climate dice. The baseline dice are two sides cool summers, two sides average summers and two sides hot summers. Hansen’s model for the 21st Century had four sides devoted to hot summers (leave “cool” and “average” with one each), but he’s since amended them to make one of the sides the extremely hot summer, for the extreme anomalies characterized by drought and wildfire.
“Every season is going to be a crapshoot,” he said. “You roll the dice, but they’re loaded. They’re substantially loaded.”
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