You’ve heard it before: “We cannot make a direct connection between climate change and weather events.” Okay, that may have been technically or literally true, but only in the total science nerd sense. Because every scientist who said it also knew, deep down, that there was a connection – just no way to prove it for any given event.
James Hansen, a climatologist and head of NASA’s Goddard Institute, has published a paper in Proceedings of the National Academy of Sciences that concludes that the weather extremes we’ve been seeing over the last six decades can scientifically be attributed to climate change. How? He bypasses the admittedly near-impossible task of linking each specific weather event by itself to climate change and instead looks at the entire group of weather events as a whole. Could these abnormal weather events have occurred by chance? Were they just normal “weird weather?” What are the odds?
The way to talk about odds in groups of events is to use statistics. Hansen and his group applied statistics to the weather data for each of the six decades from 1951 through 2011 (so this data doesn’t even include all our extreme weather this year). This analysis produces a series of probability curves (see the figure) which say how probably any given temperature anomaly is in that decade. Even without getting into all the technical elements of the data, you can tell two (true) things just by looking at the curves:
- Every decade, the curve shifts to the right. This means that the average temperature has been steadily rising – no surprise there, right?
- Every decade, the curve has been getting shorter and wider. THIS is the real news.
A normal statistical distribution (the famous Bell Curve, which is what you get when events are truly random) is relatively tall and narrow – meaning that mostly, things happen near the mean, and the outliers are pretty rare. As the curve gets flatter and wider, fewer things are grouped close to the mean, and more occur out near the fringes. In temperature terms, the extreme temperatures (mainly heat waves, since the whole curve is shifting rightwards toward “hotter”) become more common.
That’s the general trend. Then you look at the actual extreme events them selves. Specifically, temperature data from the Moscow heat wave of 2010 and the Texas-Oklahoma heat wave of 2011 put them in the part of the curve where they are statistically so unlikely to have occurred “naturally” that there must have been climate forcing. And by unlikely, he means a 0.1-.02% or so chance. It’s like getting two 1000-year events back to back.
Looking at the global picture, the flattening of the curve means that the odds of these extreme heat waves occurring have gone from 2 in 1000 to about 1 in 10. In other words, what used to be extreme oddities are now 50 times more likely. 100-year floods are now likely to occur every other year.
Scary part? These curves aren’t a forecast; this “new normal” is already on the books. And because we’re still pumping CO2 into the atmosphere at prodigious rates, it’s only going to get worse.
Michael Kelberer has been reporting on climate change since 2007.