Climate and Weather Extremes: Asking the Right Questions

The still-developing field of attribution science examines specific weather events and short-term atmospheric patterns in a broader, longer-term climate context. In such research, communication is key; it’s vital to understand exactly what questions are being asked. A case in point is an article in the July issue of BAMS. “Explaining Extreme Events of 2011 from a Climate Perspective” gives long-term context to some of the significant weather events of 2011 featured in the new State of the Climate, which is also part of the July BAMS.
The authors write:

One important aspect we hope to help promote …is a focus on the questions being asked in attribution studies. Often there is a perception that some scientists have concluded that a particular weather or climate event was due to climate change whereas other scientists disagree. This can, at times, be due to confusion over exactly what is being attributed. For example, whereas Dole et al. (2011) reported that the 2010 Russian heatwave was largely natural in origin, Rahmstorf and Coumou (2011) concluded it was largely anthropogenic. In fact, the different conclusions largely reflect the different questions being asked, the focus on the magnitude of the heatwave by Dole et al. (2011) and on its probability by Rahmstorf and Coumou (2011), as has been demonstrated by Otto et al. (2012). This can be particularly confusing when communicated to the public.

So the new attribution paper in BAMS strives to answer a very specific questions–a series of them, as it turns out, since the paper is actually a collection of a number of studies by different teams, representing several of the cutting-edge approaches to researching attribution in rapid response to the extreme weather. Most of the authors, but not all, seek to answer questions about how global climate change changes odds that extreme events might occur.
Last week, NOAA held a briefing to discuss both the State of the Climate and the new BAMS article. Two coauthors of the article, Tom Peterson of NOAA’s National Climatic Data Center and Peter Stott of the Met Office Hadley Centre, discussed the answers they found. They noted that in some cases, such as the rainfall that caused flooding in Thailand, there was no connection between human activities and the extreme weather. But other events exhibited a clear human influence that increased the possibility of that event occurring. One example is the prolonged heat wave in Mexico and the southwestern United States, which was the region’s hottest and driest growing season on record by a significant margin. The steamy temperatures were connected to the La Niña that was prominent last year, and the study found that such a heat wave is 20 times more likely in La Niña years today than it was in 1960. As the coauthors noted in the briefing, the answer might be completely different in years without a La Niña , pointing out the importance of context–and understanding the questions being asked–in this study.
The State of the Climate itself documents the weather extremes of the recent past and give them context in the historical record. The 282-page peer-reviewed report, compiled by 378 scientists from 48 countries around the world, also provides a detailed update on global climate indicators and other data collected by environmental monitoring stations and instruments on land and ice, at sea, and in the sky. It used 43 climate indicators to track and identify changes and overall trends to the global climate system. These indicators include greenhouse gas concentrations, temperature of the lower and upper atmosphere, cloud cover, sea surface temperature, sea level rise, ocean salinity, sea ice extent, and snow cover. Each indicator includes thousands of measurements from multiple independent datasets.
Among the highlights of this year’s SOC:

  • Warm temperature trends continue: Four independent datasets show 2011 among the 15 warmest since records began in the late nineteenth century, with annually-averaged temperatures above the 1981–2010 average, but coolest on record since 2008. The Arctic continued to warm at about twice the rate compared with lower latitudes. On the opposite end of the planet, the South Pole recorded its all-time highest temperature of 9.9°F on December 25, breaking the previous record by more than 2 degrees.
  • Greenhouse gases climb: Major greenhouse gas concentrations, including carbon dioxide, methane, and nitrous oxide, continued to rise. Carbon dioxide steadily increased in 2011 and the yearly global average exceeded 390 parts per million (ppm) for the first time since instrumental records began. This represents an increase of 2.10 ppm compared with the previous year. There is no evidence that natural emissions of methane in the Arctic have increased significantly during the last decade.
  • Arctic sea ice extent decreases: Arctic sea ice extent was below average for all of 2011 and has been since June 2001, a span of 127 consecutive months through December 2011. Both the maximum ice extent (5.65 million square miles on March 7, 2011) and minimum extent (1.67 million square miles, September 9, 2011) were the second smallest of the satellite era.
  • Ozone levels in Arctic drop: In the upper atmosphere, temperatures in the tropical stratosphere were higher than average while temperatures in the polar stratosphere were lower than average during the early 2011 winter months. This led to the lowest ozone concentrations in the lower Arctic stratosphere since records began in 1979 with more than 80 percent of the ozone between 11 and 12 miles altitude destroyed by late March, increasing UV radiation levels at the surface.
  • Sea surface temperature and ocean heat content rise: Even with La Niña conditions occurring during most of the year, the 2011 global sea surface temperature was among the 12 highest years on record. Ocean heat content, measured from the surface to 2,300 feet deep, continued to rise since records began in 1993 and was record high.
  • Ocean salinity trends continue: Continuing a trend that began in 2004, and similar to 2010, oceans were saltier than average in areas of high evaporation, including the western and central tropical Pacific, and fresher than average in areas of high precipitation, including the eastern tropical South Pacific, suggesting that precipitation is increasing in already rainy areas and evaporation is intensifying in drier locations.

The report also provides details on a number of extreme events experienced all over the globe, including the worst flooding in Thailand in almost 70 years, drought and deadly tornado outbreaks in the United States, devastating flooding in Brazil and the worst summer heat wave in central and southern Europe since 2003.