Assessing Climate Change in a Drought-Stricken State
By Kate Galbraith (The New York Times) - August 26, 2011
Texas has endured its worst one-year drought in recorded history. And the hottest July. August is on course to be hotter still, setting another record.
So, is this the result of climate change?
Scientists hedge, particularly when it comes to the drought, because they are reluctant to pin any single weather event on climate change. They point to La Niña, an intermittent Pacific Ocean phenomenon that affects storms, as the immediate cause.
“We can’t say with certainty whether this particular drought is in and of itself a product of climate change,” said David Brown, a regional official with the National Oceanic and Atmospheric Administration.
However, Dr. Brown added, these kinds of droughts will have effects that are “even more extreme” in the future, given a warming and drying regional climate.
Climate change, or global warming, has become a hot topic on the presidential campaign trail. Most scientists, including Dr. Brown, say humans are altering the climate by adding heat-trapping gases like carbon dioxide and methane to the atmosphere. Even so, Gov. Rick Perry, campaigning this month in New Hampshire, declared himself a “skeptic” that climate change is the result of human actions.
Drought and high temperatures are consistent with climate-change forecasts for Texas. According to John Nielsen-Gammon, the state climatologist who was appointed by Gov. George W. Bush in 2000, about 80 percent of the models that were run for a 2007 report by the Intergovernmental Panel on Climate Change, a group overseen by the United Nations, predict declining precipitation for Texas.
Climate change is already raising temperatures, Dr. Nielsen-Gammon said. Texas is one to two degrees warmer than in the 1970s, he said, and “by the middle of the century, it should be another two to three degrees warmer, give or take.”
But many uncertainties remain. For example, how climate change will affect La Niña is “one of the big questions in climate science right now,” Dr. Brown said.
Scientists already have trouble predicting more than a few months in advance when La Niña and El Niño, another Pacific Ocean phenomenon that affects Texas rainfall, will appear. A La Niña or El Niño event is present roughly three of every five winters, Dr. Nielsen-Gammon said.
Other unknowns include the effect of climate change on cloud cover, which affects temperatures, and on hurricanes, though there is some evidence that hurricanes will become more intense, scientists say.
Also, there is some disjuncture between climate predictions and recent patterns. For example, as difficult as it is to believe given the current dry conditions, rainfall across Texas has actually been increasing over the past 100 years, contrary to the models, Dr. Nielsen-Gammon said.
No one knows how long the current drought will last. Much depends on whether La Niña returns this fall. Forecasters say there is about a 50 percent chance that will occur, which might continue to keep things dry. But Dr. Brown put the odds of experiencing the same degree of extreme dryness again as “very small.”
La Niña was present for four years during the 1950s drought, which still ranks as the worst in Texas history due to its longevity. Unusually high surface temperatures in the tropical Atlantic Ocean also helped cause that drought, and those have also been present in recent years.
“The main factors that contributed to the 1950s drought are also in place right now,” Dr. Nielsen-Gammon said, adding that Texas is “likely to be” at the start of a multiyear drought, though it is hard to know with certainty.
Tree rings suggest that some past droughts that occurred well before records began in 1895 were even worse than the one in the 1950s.
Meanwhile, storms are always possible in the coming months. Cold fronts stalling above the state while moisture arrives from the gulf can produce rain, Dr. Nielsen-Gammon said. And hurricane season is revving up.
“Hopefully, we’ll get clobbered by a tropical storm or two,” he said. “It doesn’t have to be windy; it just has to be wet.”