Science Linking Drought to Global Warming Remains Matter of Dispute
In delivering aid to drought-stricken California last week, President Obama and his aides cited the state as an example of what could be in store for much of the rest of the country as human-caused climate change intensifies.
But in doing so, they were pushing at the boundaries of scientific knowledge about the relationship between climate change and drought. While a trend of increasing drought that may be linked to global warming has been documented in some regions, including parts of the Mediterranean and in the Southwestern United States, there is no scientific consensus yet that it is a worldwide phenomenon. Nor is there definitive evidence that it is causing California’s problems.
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Dennis Falaschi, left, of the Panoche Water District in California, and Aaron Mandell, of WaterFX.
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In fact, the most recent computer projections suggest that as the world warms, California should get wetter, not drier, in the winter, when the state gets the bulk of its precipitation. That has prompted some of the leading experts to suggest that climate change most likely had little role in causing the drought.
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Cattle in Rancho Murieta, Calif., one of the areas suffering from drought. Max Whittaker/Reuters
“I’m pretty sure the severity of this thing is due to natural variability,” said Richard Seager, a climate scientist who studies water issues at the Lamont-Doherty Earth Observatory of Columbia University.
To be sure, 2013 was the driest year in 119 years of record keeping in California. But extreme droughts have happened in the state before, and the experts say this one bears a notable resemblance to some of those, including a crippling drought in 1976 and 1977.
Over all, drought seems to be decreasing in the central United States and certain other parts of the world, though that is entirely consistent with the longstanding prediction that wet areas of the world will get wetter in a warming climate, even as the dry ones get drier.
What may be different about this drought is that, whatever the cause, the effects appear to have been made worse by climatic warming. And in making that case last week, scientists said, the administration was on solid ground.
California has been warming along with most regions of the United States, and temperatures in recent months have been markedly higher than during the 1976-77 drought. In fact, for some of the state’s most important agricultural regions, summer lasted practically into January, with high temperatures of 10 or 15 degrees above normal on some days.
The consequence, scientists say, has been that any moisture the state does get evaporates more rapidly, intensifying the effects of the drought on agriculture in particular. “We are going through a pattern we’ve seen before, but we’re doing it in a warmer environment,” said Michael Anderson, the California state climatologist.
The White House science adviser, John P. Holdren, said in a briefing last week: “Scientifically, no single episode of extreme weather, no storm, no flood, no drought can be said to have been caused by global climate change. But the global climate has now been so extensively impacted by the human-caused buildup of greenhouse gases that weather practically everywhere is being influenced by climate change.”
The drought eased a bit with heavy rains in Northern California this month, but many major reservoirs have only half the water expected for this time of year. “I think the situation is still pretty severe,” said Prof. Alex Hall, who studies climate at the University of California, Los Angeles.
California gets much of its water from snow in the winter along the western slopes of the Sierra Nevada. That means 38 million people and a $45 billion agricultural economy are critically dependent on about five heavy storms a year.
Dry Western States
Rain and snow in California have been so minimal over the last three years that 95 percent of the state is in drought. Neighboring states like Nevada have also been hard-hit.
