Bill Chameides for The Huffington Post
Whenever an extreme weather event occurs these days, the question almost inevitably asked is: Was it caused by global warming? For years, the less-than-satisfying scientific response went something like this: We don't know; even though global warming will increase the likelihood of extreme events, directly linking a specific event to global warming is not possible.
What Had Been Impossible Approaches the Possible
But that was then and much has changed of late. While for some the words "not possible" mean don't even bother trying, for others it is merely a challenge to go about proving the perceived impossible possible.
And so the World Climate Research Programme -- a joint project of the World Meteorological Organization and the International Council for Science whose "main objectives ... are to determine the predictability of climate and to determine the effect of human activities on climate" -- has identified predicting extreme weather events and attributing them to climate change (or not) as one of six "grand challenges" in climate research.
In answer to that challenge, "Explaining Extreme Events of 2012 from a Climate Perspective," the second [pdf] in a series of papers edited by Thomas Peterson of NOAA's National Climatic Data Center and colleagues, was released by the Bulletin of the American Meteorological Society on September 5, 2013. (A similar analysis of extreme events from 2011 was released last year.)
Connecting the Dots to Climate Change or Ruling Out a Connection in a Given Event
So how do scientists do the "impossible"? How do they establish a causal link between an extreme weather event and climate change? The methodology is probabalistic rather than deterministic, similar to the way weather predictions provide probabilities of rainfall (i.e., a 30 percent chance of rain) rather than "rain" or "no rain" absolutes.
In this case, the analyses determine how much more likely a given event is as a result of global warming. This is done using a model (most often a climate model) to simulate the extreme event for two cases: one with preindustrial conditions including preindustrial greenhouse gas concentrations and the other for current enhanced concentrations of greenhouse gases. Each case is run repeatedly, and the number of times the model produces an event like the one being studied is recorded. If the model with current conditions produces the event more often than that of preindustrial conditions, it is concluded that global warming made the event more likely and probably contributed to its occurrence.
Probably the first instance of this methodology was the study of the 2003 heat wave in Europe that led to the premature deaths of more than 20,000 people. Writing in the journal Nature, Peter Stott of the Hadley Centre for Climate Prediction and Research and co-authors concluded that "it is very likely (confidence level >90%) that human influence has at least doubled the risk of a heatwave exceeding" a mean summer temperature for 2003.
There is a caveat to all this: these analyses are based on computer model simulations and as a result are only as good as the models' ability to simulate the processes that led to the extreme event in the first place. This probably does not represent a huge caution for events involving extremes in temperature, but I am a lot less sanguine when it comes to events involving more complex phenomena like heavy rainfall.
NASA’s image of the day for Friday, February 8, 2013 is a shot of the massive winter storm taking aim at the northeastern U.S.
Article continues at The Huffington Post.
Bill Chameides is the Dean of Duke University's Nicholas School of the Environment.
Photo: A satellite image of a massive winter storm taking aim at the northeastern U.S., February 8, 2013. (NASA)