Extreme Weather Adds Up to Troubling Future

The drought that afflicts the U.S. Southwest has been going on for more than a decade, but if the distant past is any guide, the region could be in for much worse.

Extreme weather and climate-related events already have cost the United States billions of dollars. Speakers at a symposium Friday focused on the hard facts of what we know and don’t know about the causes, and how changing climate affects agriculture, water supplies, wildlife and our economy. The panel was part of the 2013 annual meeting of the American Association for the Advancement of Science, being held in Boston.

The drought offers an example of how difficult is can be to tease out the impacts of human-induced climate change from those of natural climate variations.

Climate scientist Richard Seager, Lamont Research Professor at the Lamont-Doherty Earth Observatory, said the current drought stems primarily from natural variability in the climate. The two main natural drivers of drought in the Southwest are periodically cooler sea surface temperatures in the tropical Pacific Ocean, known as La Niña, and warmer sea surface temperatures in the tropical Atlantic, he said.

But combined over time with the effects of human-induced climate change, Seager said, the odds are growing for a hotter and drier climate throughout that region.

Impacts from human-induced climate change, primarily from heat-trapping gases produced by burning fossil fuels, is now “equivalent to what has happened in past megadroughts,” Seager said. “But the climate models are predicting something that will happen forever more”—as long as the global-warming carbon dioxide stays in the atmosphere. This greenhouse gas-driven warming is introducing a new type of North American drought, he said, driven by global changes in the hydrological cycle and atmospheric circulation that do not depend on patterns of sea surface temperature change.

There has been a significant increase in “billion dollar” disasters such as floods, hurricanes, tornadoes and wildfires, “and it’s costing the country a great deal of money,” said Donald J. Wuebbles, a scientist from the University of Illinois at Urbana-Champaign. And the trend is likely to continue.

The recent drought in Texas began in the early fall of 2010, said John Nielsen-Gammon of Texas A&M University, the state climatologist. By June 2011, “we had run out of colors” to represent temperature extremes on the maps, he said. Low rainfall was “way off the scale” and high temperatures set records. The dry spell led to $7.6 billion in agricultural losses, nearly $800 million in timber losses and $535 million in insured property losses, Nielsen-Gammon said.

The UN’s Intergovernmental Panel on Climate Change has predicted that as a result of climate change, wet regions will grow wetter, while drier areas will become drier, and subtropical dry regions will expand toward the poles. A fresh IPCC assessment, due within the next year, reinforces that, Seager said.

While the 20th century was relatively wet, the climate record going back for a millennium, derived from tree rings and other evidence, shows the region has experienced much longer periods of drought, lasting decades at a time. “There’s no reason to think that can’t happen again,” he said.

The variations do not affect everywhere at once. The Great Plains region experienced what he calls a megadrought from the mid-12th to mid-13th century. But the plains have been dry at times when the Southwest was relatively wet. In the southeastern U.S., a region we now think of as a wet climate, tree ring records show that the period from 1050 up to about 1870 was overwhelmingly drier than it is now, he said. That region has had problems with drought in recent years, as well.

What causes these variations in climate is still unclear. He and other scientists are working to unravel the history of past climate and the natural forces behind the swings from wet to dry. Climate changes in the distant past were the result of natural forces. Now the effects of human-induced warming have to be factored in.

In some cases, Seager suggested, the human influence and natural trends might moderate each other. But they could also amplify the impact, making things much worse. That’s what happened during the Dust Bowl of the 1930s, when soil erosion from agricultural practices fed huge dust storms that in turn intensified the drought and actually pushed it northward through the central part of the United States.

Seager was principal author on a study published recently in Nature Climate Change that predicted a 10 percent drop in the Colorado River’s flow in the next few decades. With the river already taxed by demands from farms and cities across the Southwest, that could prove disastrous.

He said the effect of human-induced climate change combined with natural forces could cause the collapse of the Colorado River system.

“The cost of these droughts is enormous, but it’s invisible,” Seager said—at least compared to the impact of a storm such as Hurricane Sandy. The cost of that storm is counted visibly in homes and businesses lost, and in the price of reconstructing buildings and shorelines. But the drought bears its own costs: billions of dollars in crop insurance payments, reduced water supplies and impeded barge traffic on the Mississippi River, for instance.

Extreme events also have had severe impacts on ecosystems and wildlife. Many species have migrated to cooler latitudes, or higher up into mountains to escape the heat, said Camille Parmesan of the University of Texas in Austin. “We’re looking at data for 4,000 species around world, and we’re seeing a high percentage of change, consistent with changes in climate, “ she said. That includes extinctions. “The drivers are not the mean annual temperatures; it is extreme events driving those changes.”