Was the 2011 Texas heat wave a “Black Swan”, a climate surprise that sprung upon the region and its inhabitants living under the assumption that such a heat wave could not happen because it had never before been observed? Such a question, similar to that posed by essayist Nassim Taleb in a more general discussion of the role of chance in human experience, asks if the hottest summer observed in the past was the hottest possible. Climate proxy records spanning several hundred years, for example, provide strong evidence of larger variability than observed in a mere century of instrumental data.

The plots below show the departure of temperature and precipitation from normal for June, July and August (JJA). The top plot shows the departure for 2011. The bottom left plot shows the summertime departure from normal for each year 1890-2011 over Texas. The bottom right plot compares the distribution of average summertime temperatures for the early 20th century and the late 20th century with an indication of where the 2011 summer value fall in those distributions. Click on a figure to get a larger view.

Figure 1. The observed 2011 June-August (JJA) averaged surface temperature departures (°C, top), the time series of JJA Texas surface temperature departures (°C, bottom, left), and the probability distribution functions of the JJA Texas surface temperatures for two sub-periods of the historical record: 1895-1954 (blue curve), and 1955-2010 (red curve). The observed 2011 JJA Texas surface temperature shown in gray tick mark. Data source is the NCDC U.S. Climate Divisions, and departures are relative to 1981-2010 means. The PDFs are non-parametric curves constructed using the R software program which utilizes a kernal density estimation and a Guassian smoother.

Was the 2011 Texas heat wave a “climate change event” whose probability was negligible without a globally averaged temperature increase? A recent synthesis report by the Intergovernmental Panel on Climate Change (IPCC) expresses medium confidence that heat waves have lengthened and become more frequent over many regions as a result of anthropogenic climate change. Addressing such a question requires quantifying how the probability of hot summers has increased in the region affected by the heat wave. It also requires reconciling the extreme magnitude of the event with the magnitude of probable human contributions to that event, and in particular determining whether the near factor of two exceedance of the previous record by the 2011 Texas heat wave could have occurred only through human influences.