As hundreds of local temperature records have been smashed from Atlanta to Colorado Springs, there's been lots of discussion about whether the recent molten weather can be "blamed" on global warming. Isn't it at least possible this heat wave is just a random outburst? Or are we really seeing the effects of all that carbon dioxide humans have put into the atmosphere?
Perhaps a chart can help clarify matters. The National Climatic Data Center has just released its "State of the Climate" report for June 2012. The last 12-month period on the mainland United States, it notes, were the warmest on record. What's notable, however, is that every single one of the last 13 months were in the top third for their historical distribution--i.e., April 2012 was in the top third for warmest Aprils, etc.
"The odds of this occurring randomly," notes NCDC, "is 1 in 1,594,323." (Note: This might be a bit high; see the update below.)
Meteorologist Jeff Masters puts it this way: "These are ridiculously long odds, and it is highly unlikely that the extremity of the heat during the past 13 months could have occurred without a warming climate."
Taking this a step further and connecting with everything else we know about climate science: This means it's unlikely that this 13-month warm period could have happened had humans not loaded the atmosphere with carbon dioxide and heated the surface temperature of the planet by 0.8 °C (1.4 °F) above pre-industrial levels. If this is just a freak outburst by Mother Nature, then it's a fairly improbable one.
Of course, that's looking a longer 13-month period. What about last week's heat surge? Or this record? Or that record? Climate scientists often use cautious language whenever connecting individual events with global warming. "Humans have made some extreme weather events more likely, and they are happening," writes Richard Alley, a geosciences professor at Penn State. "Just as a back-street gambler might beat someone in an honest game but has a better chance with loaded dice, Nature might have caused this summer’s weather but we gave it a boost." Scientific efforts to attribute specific, short-term events to long-term climate change have only recently gotten underway.
On the other hand, as Grist's David Roberts argues, ordinary people rarely talk this way when discussing the causes of most things. No one hedges and says that there are many factors that cause lung cancer and that no one incidence of cancer can be definitively blamed on cigarette smoke (even though this is more scientifically precise). We just say, "smoking causes lung cancer" and leave it at that. Why, Roberts asks, should climate change be any different?
Lately, it seems many scientists are trying to re-calibrate their language along those lines. The AP's Seth Borenstein recently interviewed a dozen climatologists on how we should think about the droughts, wildfires, and unprecedented heat waves hitting the United States right now. “This is what global warming is like," said Jerry Meehl of the National Center for Atmospheric Research, in a formulation echoed by many of his peers. "And we’ll see more of this as we go into the future.”
*Update: My colleague at Capital Weather, Jason Samenow, has a smart post passing on some criticisms of NCDC's 1 in 1.6 million figure, with one analyst suggesting that the true probability is likely closer to "less than 1 in 100,000," since the months aren't uncorrelated with each other. Fair point. The record heat was unlikely, but not 1 in 1.6 million unlikely. Samenow also has a longer breakdown of how improbable the recent heat wave in D.C. was from a statistical point of view.
Elsewhere, NOAA climatologist Martin Hoerling, who's done careful research on the attribution of extreme weather events, explains why "there's an 80 percent chance that the record high you experienced was due to climate change.”