Rising sea levels doubled the probability of the flooding we saw after Hurricane Sandy. The triple-digit heatwave that hit the U.S. last July was made four times as likely by climate change, which, globally, was also responsible for 35 percent of 2012's record-high temperatures. All in all, a new analysis from the National Oceans and Atmospheric Administration found "compelling evidence" that "the effects of natural weather and climate fluctuations played a key role in the intensity and evolution" of many of last year's extreme weather events.
For the report, published in the Bulletin of the American Meteorological Society, the NOAA compiled 19 analyses of 12 extreme weather events from the past year. They found reason to believe that about half of those events were caused by natural variations in weather and climate. Rainfall patterns, in particular, don't seem to have been our fault: Not the heavy rains in northern China and what in Britain was the rainiest summer in a century, nor the droughts that devastated the American Midwest, Kenya and Somalia. Human activity is believed to have played a secondary role, though, in some of those cases.
Attributing any cause to single weather events remains difficult to do. According to the study's authors, that they were able to do so in some of the cases here reflects researchers' growing confidence in their ability to detect the influence of human activity, and of our contribution to the increased likelihood of extreme weather events occurring.
The authors summed up the delicacy of working such things out in an elegant metaphor:
“Adding just a little bit of speed to your highway commute each month can substantially raise the odds that you’ll get hurt some day. But if an accident does occur, the primary cause may not be your speed itself: it could be a wet road or a texting driver.” Similarly, while climate models may indicate a human effect is causing increases in the chances of having extremely high precipitation in a region (much like speeding increases the chances of having an accident), natural variability can still be the primary factor in any individual extreme event. The difficulty in determining the precise sensitivity of, according to our analogy, driving speed on risks of accidents in particular conditions (wet roads, texting drivers) can explain why somewhat different analyses of the same meteorological event can reach somewhat different conclusions about the extent to which human influence has altered the likelihood and magnitude of the event.
When disaster happens, it's possible to blame it on the slippery road. But we can't deny the fact that we're driving very, very fast.