More on the Difficulties of Attributing Extreme Events

And here are more important points on extreme events, from a recent article in EOS by John M. Wallace:

Claims that the severity of heat waves, droughts, floods, and storms is increasing dramatically in response to global warming should, in fact, be viewed with caution for the following reasons:

• The subtle early warning signs of global warming are beginning to be detectable in the statistics of extreme events such as droughts and episodes of heavy rainfall averaged over intervals of a few decades, but the changes reported thus far do not qualify as “dramatic” [Easterling et al., 2000; Intergovernmental Panel on Climate Change (IPCC), 2007].
• The attribution of individual events—such as the current droughts in the Horn of Africa and Texas, the floods in Pakistan and Thailand, or the 2009 summer heat wave in Russia—to global warming is problematic because extreme events would occur from time to time even in an unchanging climate.
• Even in the presence of climate change,extreme events do not occur often enough to enable scientists to track decade-to-decade changes in their statistics in real time, as they successfully do with more aggregated quantities such as global mean temperature and sea level [Palmer and Räisänen, 2002].

Via Dot Earth