We’re not creating weather patterns we’ve never experienced before, but they're exacerbated by a warming climate.

The drought that’s turned most of the United States into a dessicated hotbox may be a symptom of climate change, a brutal blowback from carbon pollution.

Climate scientists, who prefer to speak in terms of probabilities and trends rather than single events, are reluctant to point fingers at any one cause — but signs point to human influence making a natural dry spell unnaturally severe.

“In any single event, it’s hard to really know if you’re just seeing a natural variation or climate change,” cautioned climatologist Chris Funk of the University of California, Santa Barbara. With that caveat, Funk said when asked if human activity exacerbated the drought, “Tentatively, the answer is yes. To some extent, it is.”

Public sentiment has already linked the drought, which has turned much of the Great Plains and Midwest into disaster areas, wrecking crops and driving food prices dangerously upwards, to unnatural climate fluctuation. Belief in climate change is now at an all-time U.S. high, and while explaining the causes of any large weather pattern is always difficult, enough is known about climate to make some educated guesses.

Funk’s specialty is the dynamics of sea surface temperatures in the Indian Ocean and western Pacific Ocean. Over the last century, and in particular the last two decades, these rose by an average of 1.25 degrees Fahrenheit. Ocean temperature trends can be tricky to interpret, but there’s little scientific disagreement about Indian Ocean warming: It’s almost certainly man-made, a result of greenhouse gases trapping heat in Earth’s atmosphere.

The consequences are significant. Heated air holds extra water, supercharging monsoon systems and producing events like 2010′s Pakistan floods. Water vapor is also a greenhouse gas itself, trapping heat and creating a feedback loop of local warming.

When the western Pacific is especially warm and the central Pacific is especially cool — the latter a natural, cyclically occurring condition known as La Niña, which has prevailed since 2010 — the temperature gradient causes changes in atmospheric circulation.

Dry air is pushed westward toward the Horn of Africa, which in 2011 experienced a massive drought. There also seems to be an eastward ripple effect, interacting with other weather patterns to produce drought conditions in mid-latitude regions.

La Niña generally produces dry spells in southern North America, but adding a warm-to-cool Pacific gradient generates what some scientists call “the perfect ocean for drought,” spreading it far and wide. This occurred between 1998 and 2002, when a similar warm-to-cool Pacific gradient existed and drought struck the United States and mid-latitude regions worldwide. Another, lesser gradient occurred in 2007 and 2008, just before another U.S. drought.

The latest warm-to-cool gradient occurred in 2010 and 2011. Martin Hoerling, a research meteorologist at NOAA’s Earth System Research Laboratory who coined the “perfect ocean for drought” term, said it may well help explain the current disaster.

“The 2011 drought in Texas was part of the La Niña effect, and we’ve carried it on here,” he says. “When background conditions in the tropical Atlantic and Indian Ocean are warm, it leads to the worst of all possible worlds for droughts in the mid-latitudes. I can’t confirm that’s been driving the conditions we’ve been seeing, but it’s an useful first guess.”

Drought severity in the U.S. as of July 24, 2012

If that dynamic is at work, then fossil fuel pollution is implicated. “Some part of it is related to extra water vapor that wouldn’t be there” if not for human greenhouse gas emissions, Funk says. “If we didn’t have all that extra anthropogenic water vapor, the western Pacific would be cooler, and the gradient wouldn’t be as great.”

Bin Guan, a drought specialist at the California Institute of Technology, struck a cautionary note on early interpretations. “Drought development is a long, complicated process,” he said. “Its response to greenhouse gases is more complicated than temperature alone because it’s a combination of temperature, precipitation, evaporation, soil moisture, and other conditions.”

Whether the current drought’s severity is linked to greenhouse gas pollution is “difficult to say with certainty,” Guan said. “It could be a combination of both natural forces and human impact, but we can’t be sure, at least for now.”

Also fueling the drought was the exception warmth of the past winter and spring, which by drying U.S. soils reduced the amount of moisture entering regional weather systems through evaporation. The extent to which that reflects climate change is unknown, as is the provenance of a high-low pressure system now sitting off the U.S. west coast that funnels warm, dry air from approaching storm systems, said Hoerling.

Anticipated annual precipitation levels in 2050 as modeled for moderate greenhouse gas emission levels. (Actual emission levels are on a more severe trajectory.) Dark red, or a value of minus-4, represents extreme drought. Image: National Center for Atmospheric Research

Those factors may prove unrelated to human activity, but represent the type of patterns with which climate change interacts, ultimately producing a world that — regardless of this drought’s origin — is expected to become a much hotter, drier place. “We’re not creating weather patterns we’ve never experienced before,” Hoerling says. “But when these patterns do materialize from time to time, through the vagaries of weather, they’re materializing in a warming climate."

Photo credit: Lance Cheung, USDA/Flickr

This story first appeared on Wired and is reproduced here as part of the Climate Desk collaboration.

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