The 14 U.S. disastersin 2011 that caused at least $1 billion in damage (NOAA)

In taking stock of the extreme weather of 2011, many experts (including yours truly) have pointed to the record number of “billion-dollar disasters” as evidence that the events we saw this past year were unprecedented. As the National Oceanic and Atmospheric Administration (NOAA) announced last week, there were a record 14 such events in the U.S. that caused at least $1 billion in damage, including Hurricane Irene, multiple tornado outbreaks, and Tropical Storm Lee and its associated flooding. This beat the previous record of nine billion-dollar disasters set in 2008.

But while the number of billion-dollar disasters offers insight into the increasing economic consequences of extreme weather in the United States, it does not allow us to make any firm conclusions about global warming, nor does it provide much clarity on the question of whether global warming is causing more losses from natural disasters.

As CWG’s Jason Samenow reported, the billion-dollar disaster metric is flawed, because socioeconomic changes during the past several decades outweigh any other trends. To put it simply, we’re putting more stuff in harm’s way than there used to be, causing mounting losses for the global insurance industry, and making it more likely that a severe storm will cause at least $1 billion in damage today than, say, 20 years ago.

These socioeconomic trends are taking place at the same time that global warming is beginning to manifest itself in extreme weather events, naturally leading to some confusion.

Recent studies and online commentary have pointed out that although it’s tempting to do so, there is not enough scientific evidence to say there is a clear link between global warming and mounting losses from extreme weather events. Articles on this topic from Bill Chameides, dean of the Nicholas School of the Environment at Duke University, Nature’s Quirin Schiermeier, and New York Times blogger Andrew Revkin are worth reading.

In his blog post, Chameides explores the difficulties involved with attributing trends in large, “noisy” datasets: “It seems like the recent past has delivered some strange weather. Is it part of a long-term trend? A trend due to global warming? If you require rigorous scientific evidence, the answer, at least today, is that we just don’t know.”

Readers of Chameides’ post and the other stories might come away with a sense that global warming’s influence on extreme weather events is simply too difficult to discern at this point. But that’s not quite right, either.

There is growing scientific evidence that climate change already tilts the odds in favor of some types of extreme weather, such as heavy precipitation events and heat waves. (That this is not yet evident in global disaster losses isn’t surprising, since most studies indicate the manmade influence on extreme events is relatively small, but becoming more noticeable as time goes on.)

Although extreme event attribution is at its infancy, we already know enough to view certain extreme events, such as the barrage that struck during 2011, with justifiable suspicion.

A compelling scientific case can be made that without manmade global warming, we wouldn’t have had temperatures that were quite as hot in Texas and Oklahoma, and fewer records might have been smashed. Both states had their hottest summers on record, and Oklahoma set a record for the hottest summer of any state in the U.S. since records began in 1895.

Without manmade global warming, we might have had less rainfall from Irene and Lee, and a reduced contrast between the very wet areas of the country and the bone dry regions. During 2011, a record 58 percent of the U.S. experienced either very wet or very dry conditions. Texas had its driest year on record while seven Ohio Valley and northeastern states had their wettest years.

We already know that the balance of warm-temperature records to cool-temperature records is out of whack, and becoming increasingly lopsided in favor of warm records. For 2011 as a whole, the ratio of warm to cool records was about three to one. But during the summer months, that ratio was more like eight to one.

Record warm minimum temperatures for June-August have outpaced record cool minimum temperatures in recent decades. (NOAA)

And we already know from recent attribution studies that manmade warming has tilted the odds in favor of specific heat and flooding events during the past decade or so.

Thus, while formal climate change attribution studies have not been completed for the extreme events of 2011, it’s naive to think that global warming didn’t play a role - perhaps even a sizable one - in contributing to these and other extreme events, regardless of whether the trend is clearly visible in damage estimates.