Global Warming Contributing to Texas Drought

This blog is co-written by Jay Gulledge

Recently, President Obama quipped about GOP presidential candidate and Texas governor Rick Perry: “You’ve got a governor whose state is on fire denying climate change.” While this type of election jousting risks further politicizing an issue that should be totally non-partisan, it raises a legitimate question: Is climate change increasing the risk of drought and wildfires in Texas?

Drought through October 4. (Top)  Drought through October 11. (Bottom) Source: US Drought Monitor

Our Center has favored a risk-based approach to the linkage between climate change and extreme weather. As we have written previously, the question of whether climate change caused a singular weather event cannot fully be answered, and the correct question is whether climate change has increased or decreased the risk of extreme weather. In answering this question it is important to disentangle natural cycles from climate change, both of which are risk factors. The 2011 Texas drought offers an interesting case study.

Texas is enduring its worst one-year drought on record and has experienced the hottest summer (Jun-Aug) ever recorded in any state in the union. This extreme heat has made the drought worse by drying out grasses, trees, and soils more than the lack of rain alone would do. Unfortunately, in spite of some much needed rainfall over the Columbus Day weekend, the National Weather Service says, “widespread significant improvement in the ongoing drought is not expected,” although some parts of Texas and Oklahoma will likely be upgraded from the “exceptional drought” to the “extreme” or “severe” categories. In Dallas, which received almost four inches of rain over the weekend, this year is still on track to be the driest on record by two inches.

So what are the risk factors for drought in Texas? Natural variations in sea-surface temperatures in both the Atlantic and Pacific oceans influence temperature and precipitation in Texas. Since the ocean temperatures affect atmospheric circulation, particular patterns of sea surface temperatures increase or decrease the risk of various weather events. These natural patterns occur in cycles, so that their contribution to weather risk goes up and down over time, but would not contribute to long-term trends in any one direction. At the moment, warmer water conditions in the Pacific and Atlantic happen to both be conducive to drought in Texas.

The influence on risk from different sources of climate variability is additive, so global warming could present a new risk factor added on top of the natural ones that have always been with us. Although we don’t know yet how global warming affects the risks of some types of weather events, like tornadoes, there is strong evidence that it increases the risk of drought and wildfires in the western United States. Over time, natural cycles will wane, but global warming won’t and its contribution to risk will only increase over time. The question at hand is whether global warming has affected the risk of extreme drought in Texas.

Recently, Texas State Climatologist and Texas A&M atmospheric science professor, John Nielsen-Gammon, tried to disentangle various climatic factors contributing to the Texas drought. He made a few rough calculations aimed at determining how much of the excess heat this summer can be explained by natural cycles and by global warming. Nielsen-Gammon found that natural factors alone did not explain the full intensity of the excess heat but adding global warming to his statistical model resulted in good agreement with observed temperatures. He concluded that “the impacts of the drought were enhanced by global warming.” Global warming was not the major contributor, but it was a necessary piece.

Considering that global warming is just getting started, it is worrisome that it is probably already contributing to very extreme weather patterns. In Nielsen-Gammon’s own words, “warmer temperatures in Texas are extremely likely in the future, and based on temperatures in the first eleven years of the 21st century, those days are already here.” The implication is that the risks of extreme heat and drought have already begun to rise and will continue to rise as long as global warming continues.

Nielsen-Gammon’s analysis provides useful insights into the extreme heat/drought story in Texas and puts in place some of the building blocks that are needed for a risk analysis.  As he described himself, his work is more of the “back-of-the-envelope” type analysis that would benefit from further refinements. A full risk assessment would resolve the entire risk profile, including uncertainty around each of the risk factors involved. Nonetheless, Nielsen-Gammon’s results demonstrate that global warming is one of the risk factors that must be included to evaluate the present-day risk of extreme heat and drought in Texas.

As temperatures continue to increase,, global warming’s influence on the  risks of extreme events  will increase. Nielson-Gammon’s analysis indicates that climate change is already increasing the risks of extreme events that are causing record economic losses. If we want to avoid the escalation of these costs in the future, these risks must be analyzed, reduced and managed and the contribution of global warming must be factored in.

Dan Huber is a Science & Policy Fellow and Jay Gulledge is Senior Scientist and Director of the Science and Impacts Program.