Extreme events of 2011: climate change a major factor in some, but not all

The science of quantifying how climate change changes the odds of extreme weather events like droughts and floods took a major step forward Tuesday with the publication of NOAA's annual summary of the past year's weather. The 2011 State of the Climate report contains a separate peer-reviewed article published in the July issue of the Bulletin of the American Meteorological Society titled, Explaining Extreme Events of 2011 From a Climate Perspective. In the paper, a group of scientists led by Peter Stott of the Met Office Centre in the United Kingdom looked at how climate change may have changed the odds of occurrence of some of 2011's notable weather extremes. These kinds of attribution studies require huge amounts of computer time and take many months to do, but the scientists plan to start making this a regular part of the annual NOAA State of the Climate report. Some of their findings for 2011:

- Determining the causes of extreme events remains difficult. While scientists cannot trace specific events to climate change with absolute certainty, new and continued research help scientists understand how the probability of extreme events change in response to global warming.

- La Niña-related heat waves, like that experienced in Texas in 2011, are now 20 times more likely to occur during La Niña years today than La Niña years fifty years ago.

- The UK experienced a very warm November 2011 and a very cold December 2010. In analyzing these two very different events, UK scientists uncovered interesting changes in the odds. Cold Decembers are now half as likely to occur now versus fifty years ago, whereas warm Novembers are now 62 times more likely.

- The devastating 2011 floods in Thailand caused an estimated $45 billion in damage, making it the world's most expensive river flooding disaster in history. The study found, however, that the amount of rain that fell in the catchment area was not very unusual, and that other factors such as human-caused changes to the flood plain and the movement of more people into flood-prone areas were more important in causing the disaster. "Climate change cannot be shown to have played any role in this event," the study concluded, but warned that climate models predict an increase in the probability of extreme precipitation events in the future in the region.

- The deadly drought in East Africa, which killed tens of thousands of people in 2011, was made more likely by warming waters in the Indian Ocean and Western Pacific. While the scientists did not specifically tie the warming of these waters to human-caused global warming, they noted that climate models predict continued warming of these waters in the coming decades, and this will likely "contribute to more frequent East African droughts during the spring and summer."


Figure 1. An SH-60F Sea Hawk helicopter assigned to Helicopter Anti-Submarine Squadron (HS) 14, flies around the Bangkok area with members of the humanitarian assessment survey team and the Royal Thai Armed Forces to assess the damage caused by the 2011 floods. Image credit: Petty Officer 1st Class Jennifer Villalovos.

Weather on steroids
One interesting aspect of the paper was the scientists' use of the baseball player-steroids analogy to help explain how climate change can increase the odds of extreme weather: "One analogy of the effects of climate change on extreme weather is with a baseball player (or to choose another sport, a cricketer) who starts taking steroids and afterwards hits on average 20% more home runs (or sixes) in a season than he did before (Meehl 2012). For any one of his home runs (sixes) during the years the player was taking steroids, you would not know for sure whether it was caused by steroids or not. But you might be able to attribute his increased number to the steroids. And given that steroids have resulted in a 20% increased chance that any particular swing of the player’s bat results in a home run (or a six), you would be able to make an attribution statement that, all other things being equal, steroid use had increased the probability of that particular occurrence by 20%. The job of the attribution assessment is to distinguish the effects of anthropogenic climate change or some other external factor (steroids in the sporting analogy) from natural variability (e.g., in the baseball analogy, the player’s natural ability to hit home runs or the configuration of a particular stadium)."



Video 1. National Center for Atmospheric Research scientist Dr. Jerry Meehl explains how climate change's impact on extreme weather is like how steroids affect a baseball player.

Jeff Masters