A paper by NOAA and university researchers led by Martin Hoerling, Ph.D. of NOAA's Earth System Research Laboratory in Boulder, CO concluded that La Niña-related sea-surface temperature conditions were the most important factor leading to the drying out in Texas and the U.S. Southern Plains in 2011. The accompanying extreme heat, which mostly resulted from such lack of rainfall, was rare but within the range of natural temperature swings, the researchers found.

The researchers found that human-induced climate change, however, increased the chances of breaking state heat records by making the temperatures somewhat warmer than would otherwise have been the case.

”Our study shows that even in in the absence of human-caused climate change, heat waves over Texas can be substantially hotter than we’ve observed so far in the historical record,” said Dr. Hoerling.

The paper also concluded that the 2011 heat wave was well anticipated by a suite of NOAA forecast systems including official seasonal predictions.

Natural factors, primarily La Niña, drove weather patterns that increased the risk of drought and extreme heat in Texas, and were likely responsible for about 4 of the 5 degrees Fahrenheit that exceeded the state average last summer. Extreme drought beginning in the fall of 2010 and continuing into the summer played a crucial role in the extreme summertime heat, the researchers found.

Drought makes extreme heat more likely, as a region with dry soil can warm more than one with wet soil. Prior hot and dry periods over Texas during the last century often occurred during La Niña years. The study also found that climate change contributed about an additional 1 degree to the summertime warmth. Climate change did not appreciably affect rainfall patterns.

To dissect the effects of many weather and climate factors on last summer’s Texas heat wave, Hoerling and his colleagues drew on diverse tools: weather records, historical simulations of global weather patterns from sophisticated climate and weather forecast models set with the particular conditions leading up to summer 2011. His team also analyzed results from the Coupled Model Intercomparison Project, an international climate modeling program that integrates many different global climate models and model runs.

Importantly, the team found a high level of predictability of the particular severe conditions that materialized over Texas in 2011, a finding that may help improve future forecasting of extreme heat events.

Climate change will likely nudge Texas toward more extremely hot summers in the future, said Randall Dole, Ph.D., a co-author on the paper and also a research meteorologist at NOAA’s Earth System Research Laboratory. “By later in this century, even in the absence of severe drought such extreme heat will become increasingly common due to human-caused climate change.”

Co-authors of the paper, “Anatomy of an Extreme Event,” are Arun Kumar (NOAA), John Nielsen-Gammon (Texas A&M and state climatologist), John Eischeid (CIRES), Judith Perlwitz (CIRES), Xiao-Wei Quan (CIRES), Tao Zhang (CIRES), Philip Pegion (CIRES) and Mingyue Chen (NOAA). The paper is available on the Journal of Climate website.