By Michael D. Lemonick
When the National Oceanic and Atmospheric Administration issued a report on April 11 that seemed to exonerate global warming as a cause of last summer’s historic drought, a reasonable person might conclude that global warming had been exonerated. After all, NOAA is a highly respected organization, and the report’s lead author, meteorologist Martin Hoerling, is a widely respected scientist.
Judging by the reactions of other respected scientists, though, the idea that global warming is off the hook is probably too hasty. While the report did fail to find a climate-change connection, it also failed to identify any other “proximate” — i.e., direct — cause, either, leaving more questions unanswered than answered.
Some of those other scientists did point to plenty of indirect factors that might well have contributed to the drought, all of them plausibly linked to climate change. Among them: weather systems disrupted by vanishing ice in the Arctic ocean; reduced snowpack in Western mountains, leading to reduced flow in rivers that water the Great Plains; and higher temperatures that have raised the odds of heat waves that dry out the soil.
Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research, put it more straightforwardly than most in a commentary emailed to reporters. “This report,” he wrote, “has some useful material in it . . . But it is quite incomplete in many respects, and it asks the wrong questions. Then it does not provide very useful answers to the questions that are asked.”
Several climate scientists find the idea of fingering one main cause for the 2012 drought problematic in any case “It’s very unlikely in my view” said Gabriel Vecchi, a climate modeler at NOAA’s Geophysical Fluid Dynamics Laboratory, in an interview, “that an event as extreme as the 2012 drought was caused by any single factor.”
Natural climate variations were undoubtedly one of those factors but that doesn’t mean they were the only one, said Vecchi, who did not contribute to the drought report. Beyond that, which natural variations those might have been, and how they might have been made worse by climate change can be enormously tricky to tease out.
“This is always a fraught area,” said Princeton geoscientist and Climate Central board member Michael Oppenheimer, in an interview. “Attributing individual extreme events to climate forcing has only been done in a very few cases. I’ve never seen a paper that tries to attribute drought to climate change. All you can say is that it has primed the atmosphere to make drought more common.”
Conversely, he said, it’s difficult to say that a particular event was not attributable, at least in part, to climate change. The failure to see direct evidence of climate change’s influence, said Oppenheimer, “doesn’t mean it didn’t. It just means you haven’t demonstrated that it did.”
One specific criticism leveled at the NOAA report is that while it acknowledged a weather phenomenon known as a “blocking high,” which helped keep rainstorms away from the central plains last summer, it discounted that this was tied to global warming. Others scientists aren’t so sure. While it’s not definitive at this point, research published last year argues that the loss of Arctic sea ice in the summer, caused by global warming, has lessened the temperature difference between the Arctic and temperate zones, potentially altering the air flow that powers the jet stream.
That, in turn, could allow high-pressure systems to get stuck over one area for months at a time.
Another criticism addresses the fact that the NOAA report focuses mostly on last summer’s precipitation shortfall. “Precipitation is highly variable from year to year,” said Aiguo Dai, an atmospheric scientist at the State University of New York at Albany, in an interview. Dai said that makes it much trickier to find a specific cause — and, conversely, to rule one out. But since other factors could well have contributed to the drought, he argued, that narrow focus isn’t reasonable in the first place.
Hoerling and his colleagues didn’t just rule out global warming as a major cause for the 2012 drought: they also ruled out colder-than-normal ocean temperatures in the tropical Pacific. In the past, those have been associated with drought.
“That wasn’t the case last year,” Dai said, and indeed, the report argues that Pacific Ocean temperature wasn’t a factor, either. But, Dai said, “That only excludes one possibility. Greenhouse gases can affect precipitation in many other ways.”
The bottom line, he said, was that he wouldn’t have ruled out either ocean temperatures or greenhouse gases as playing a factor in the drought. (Hoerling did not respond to an interview request).
So what did the NOAA report cite in the end as the cause of the 2012 drought? “The interpretation,” write the authors, “is of an event resulting largely from internal atmospheric variability having limited long lead predictability.”
But for many scientists, that conclusion doesn’t rule out a role for climate change. For them, it’s just as likely that attributing a short-term regional climate event like the Great Plains drought to a highly complex, long-term planetary phenomenon like the global warming is simply beyond the current limits of the science.
Climate Central, based Princeton, N.J., is an independent organization of scientists and journalists researching and reporting on the science and impacts of climate change.
Image credit: Corey Raimond/flickr