This year’s Midwest heat wave and some other recent extreme weather events are no fluke of nature, but a consequence of a warming planet, according to an analysis of climate data by NASA scientists. The odds of an unusually hot summer have doubled since mid-century, according to the research by NASA’s James Hansen and two colleagues, published today in the journal Proceedings of the National Academy of Sciences.
Furthermore, the researchers found that the extremes of heat are getting even hotter, creating in essence a new “normal” for climate. On average, 10 percent of land area across the planet now experiences these more extreme temperatures, a more than tenfold increase from 1951 to 1980, when less than 1 percent of global land area reached this extreme.
“This summer people are seeing extreme heat and agricultural impacts,” Hansen said. “We’re asserting that this is causally connected to global warming, and we present the scientific evidence for that.”
Hansen and colleagues analyzed mean seasonal temperature in June, July and August during the last 30 years and showed that the odds have increased for an anomalously hot summer – a period of average temperatures that reach more than 0.43 standard deviations from the norm. Specifically, 75 percent of global temperature anomalies now fall into the hot category, compared to just 33 percent from 1951 to 1980.
Hansen has used the “climate dice” analogy to explain this: In the 1980s, he looked at the summers between 1951 and 1980; he put the 10 hottest into one category, the 10 coldest into another, and the 10 falling in between into a third. These, he said, could be characterized by the six sides on dice: Two sides red for hot summers, two sides blue for cool, and two sides white for “normal.” For the period there was an equal chance of any of the three occurring.
Compared to that 30-year period, the dice now are loaded for hot summers: Four sides red, one blue and one white.
The researchers based their analysis on real temperature data, not on climate models. For more on the paper, and the science behind it, follow these links:
A “popular science” write-up of the paper, by Hansen and colleagues.
The Proceedings of the National Academy of Sciences paper can be found at the PNAS site online; this is a subscription service, so you may only be able to read the abstract.
The NASA-Goddard Institute for Space Studies has on its web site two animations to illustrate the science underlying the new paper.
On the ABC News website, this story on global warming and extreme weather offers four straightforward analogies to help understand the science behind it, including an explanation of the “climate dice.”
A related paper from NOAA, published by American Meteorological Society in July 2012, “Explaining Extreme Events of 2011 from a Climate Perspective.”