The summer of 2011 in Texas was pretty brutal. Some cities had over 90 days in a row with temperatures exceeding 100ºF. June, for example, had over 1,200 high temperature records set by various weather stations throughout the state. August had almost 2,000 high temperature records.
It didn’t rain the entire summer… and by summer, I mean March to October. By December, some weather stations had recorded less than an inch of rain since January 1st. It was the worst drought year for over 117 years.
I lived through it. Some nights, it would almost get down to 95ºF. The HVAC companies were doing a huge business as residential and commercial AC units were dying left and right. I was sent home from work twice due to the building’s AC units going out.
In every way, it was one of the worst summers I’ve ever lived through.
The million dollar question (actually 5.3 billion dollars, as that the estimated impact in Texas from the heat wave) is, “Did global warming cause that heat wave?”
This paper, “Anthropogenic influence on the changing likelihood of an exceptionally warm summer in Texas, 2011*” by David Rupp et. al. sought the answer.
The drought conditions were generally caused by La Niña conditions in the Pacific. The La Niña effect had begun in the fall of 2010 and extended into the spring of 2011. By the time summer hit, the La Niña conditions had faded, but the lack of water was already a problem.
Dry summers cause hotter summers for two reasons. The first is direct solar heating due to lack of cloud cover. The second is the lack of ground water absorbing heat energy. Instead of evaporating water, the solar heat is directly and indirectly heating the atmosphere.
Global warming can exacerbate the problem both directly (the increased retention of infrared energy) and indirectly. The first is lower precipitation, exacerbating the problems mentioned above. The second is positive feedback by an increase in the evapotranspiration rate. The soil dries out faster and, again, links back to the conditions above.
Different studies have reached different conclusions on the status of man-caused global warming effects on these heat waves.
To answer the question once and for Rupp et. al. compared simulations of the actual events of 2011 with alternate worlds with differing characteristics.
The base model (using the UK Meteorological Office’s program) was used with all observed greenhouse gases, observed sea surface temperatures, observed atmospheric conditions, and sea ice. This is the “all forcings” model. Five other scenarios were developed. None of the five “natural forcing” scenarios used man-caused greenhouse gases. They did have some variations that would allow better discrimination among the models.
The results were quite significant.
First of all, the soil moisture deficits were actually higher in the natural forcings models. That means that the temperatures should have been higher in those models due to lower soil moisture.
But the actual temperature difference between the all forcings model and the natural models is about 0.5-1.0ºC higher for the all forcings model.
What this says is that, conditions that should have resulted in slightly cooler temperatures actually resulted in much higher temperatures. The only difference was the effect of greenhouse gases.
That seems like an awful lot of work to say “greenhouse gases emitted by humans are the direct case of warmer than normal summer temperatures”.
But the value of this isn’t so much in the conclusion, but in the work itself. This has shown us that greenhouse gases have a direct impact, that actually reduces mitigating factors (like soil moisture), on the temperature.
This study, mentions seven similar studies (for the Russian 2012 heatwave, the 2012 US heatwave, the 2013 Australian heatwave, and the Asian heatwave of 2013) that all are done with similar processes and have very similar results, despite using different models and different data sets. This indicates that this particular study is pretty robust and the conclusion put forward is on very firm scientific ground.
Increased greenhouse gas emissions result in increased temperatures.
Rupp, D. et al. Anthropogenic influence on the changing likelihood of an exceptionally warm summer in Texas, 2011. Geophys. Res. Lett. (2015). doi:10.1002/2014GL062683