To attribute or not to attribute?

Scientists are increasingly trying to determine whether they can attribute specific extreme weather events to human-caused climate change – something that researchers hesitated to attempt not long ago – and to what extent.

The punishing Texas drought and heat wave of 2011 is, unsurprisingly, a popular focus for such efforts. Three studies recently in the news have linked the drought to manmade global warming. Considered together, they illustrate the emerging nature of scientific efforts that seek to identify and measure the extreme-event fingerprint of that warming trend, as well as the inevitable expert disagreements about a young and evolving branch of science.

The only study of the three that has been formally released was issued last week by the National Oceanic and Atmospheric Administration (NOAA) and will be published in the Bulletin of the American Meteorological Society. The researchers used computer modeling to reach the conclusion that “conditions [related to the La Niña weather pattern] leading to droughts such as the one that occurred in Texas in 2011 are, at least in the case of temperatures, distinctly more probable than they were 40-50 years ago.”

The Texas study, conducted by researchers in Oregon and Britain, was issued as part of a group of findings on climate change’s ties to several extreme weather events last year. A New York Times article was typical of media coverage in saying that the researchers had determined “that global warming made the severe heat wave that afflicted Texas last year 20 times as likely as it would have been in the 1960s.”

Actually, the researchers didn’t precisely say that. Since data from modeling simulations for 2011 were not yet available, they used data from 2008 as a “proxy” for 2011, comparing it to 1964, 1967 and 1968.

“We found that extreme heat events were roughly 20 times more likely in 2008 than in other La Niña years in the 1960s and indications of an increase in frequency of low seasonal precipitation totals,” the scientists wrote, adding that this finding “suggests” a similar conclusion for 2011.

The study [PDF] ended with this cautionary note:

Modeling studies such as this allow us to quantify how much the probability of extreme hot and dry conditions in Texas has changed. Quantifying the absolute probability of such extreme conditions is much more difficult, since the models we use are subject to bias, particularly affecting tails of distributions [of data on a probability graph], and data records are too short to quantify absolute probabilities empirically. Hence, while we can provide evidence that the risk of hot and dry conditions has increased, we cannot say that the 2011 Texas drought and heat wave was “extremely unlikely” (in any absolute sense) to have occurred before this recent warming.

The leader of the research team on the Texas study, Philip W. Mote, director of Oregon State University’s Climate Change Research Institute, put it this way in a comment to the Times:

This is hot new science. It’s controversial. People are trying different methods of figuring out how much the odds may have shifted because of what we have put into the atmosphere.

Authors of a second study on climate change and the 2011 Texas drought, which was submitted in May to the Journal of Climate for review, did not hesitate to criticize the study by Mote and others.

One of this second, as-yet-unpublished study’s authors is John Nielsen-Gammon, Texas state climatologist and a professor at Texas A&M University. He told Texas Climate News that he and his co-authors “find a significant global warming contribution [to the 2011 drought], but not as large as theirs.”

The Mote paper “almost certainly attributed much too high a role to global warming because it overestimates the local change in temperatures and ignores contributions from Atlantic Ocean variability.”

The computer model used in the study “simulates a 1.4 Celsius increase from the 1960s La Niña years to 2008,” Nielsen-Gammon said by email. “This big temperature jump accounts for the greatly enhanced likelihood of breaking a temperature record in 2008. However, the observed temperatures in Texas only increased by 0.6 Celsius over the same period, so for whatever reason the model greatly exaggerates the warming in Texas and thus greatly exaggerates the contribution of warming to the 2011 heat.”

The model also does not account for the fact that the Atlantic Ocean, where most of Texas’ summertime air comes from, was in a neutral or cold phase in the 1960s and is in a warm phase now, he added.

Last month, in a far-reaching interview with Texas Climate News, Nielsen-Gammon said that the findings that he and his NOAA co-authors have submitted to the Journal of Climate agree with his own, preliminary calculations in 2011 – that last year’s temperatures in Texas “were warmer than you’d expect, even given the lack of rainfall, and the contribution (by climate change) to that is probably about in the neighborhood of a degree Celsius, or half a degree Celsius to a degree Celsius. Which means that essentially we would have broken the record with this drought in terms of high temperatures even without climate change, but we ended up breaking it by quite a comfortable margin with climate change.”

[Update, July 23, 2012: Nielsen-Gammon elaborated on his critical analysis of the Mote paper in a post on his blog.]

A third, also not-yet-published study – this one by the famous NASA climatologist and outspoken climate-protection activist James Hansen and others – became a flashpoint for scientific debate in May when Hansen alluded to its findings in an op-ed column in the Times.

In response, one of Nielsen-Gammon’s co-authors, NOAA meteorologist Martin Hoerling, criticized Hansen’s op-ed assertions in strong terms in an article by Times blogger Andrew Revkin. (Hoerling also later criticized the Mote study in the Times article that reported on it last week.)

In his Times column – in which he argued for “a gradually rising carbon fee, collected from fossil fuel companies,” with all proceeds going directly to U.S. citizens – Hansen asserted:

The global warming signal is now louder than the noise of random weather, as I predicted would happen by now in the journal Science in 1981. Extremely hot summers have increased noticeably. We can say with high confidence that the recent heat waves in Texas and Russia, and the one in Europe in 2003, which killed tens of thousands, were not natural events – they were caused by human-induced climate change.

Hansen did not explicitly cite it in the column, but on the same day it was published, a blog on Time magazine’s website reported that he was referring to a peer-reviewed paper that had been submitted to “a leading scientific journal” and made available to Time.

The Time blog post explained that the Hansen study’s attribution of the 2011 Texas and 2010 Russia heat waves to climate change “with a high degree of confidence” was derived from calculations based on “decades worth of readings from more than 1,000 weather stations around the world as well as satellite observations and measurements from Antarctic research stations. The aim: to figure out how often temperatures varied from the mean – and how far they varied – during two periods.”

Extremely hot summertime conditions meeting a specific statistical standard covered “much less than 1 percent of Earth’s surface” in historical records, but now “typically cover about 10 percent of the land area,” Hansen and his co-authors wrote in a draft of their paper, posted here [PDF], on Columbia University’s website. “Thus there is no need to equivocate about the summer heat waves in Texas in 2011 and Moscow in 2010 [which met that standard] – it is nearly certain that they would not have occurred in the absence of global warming.”

Hoerling, in Revkin’s Dot Earth blog on the Times website, labeled “patently false” Hansen’s op-ed statement that “the global warming signal is now louder than the noise of random weather.” He added that it would be “more scientifically justifiable” to say, “at least for the U.S. and extratropical land areas … that daily weather noise continues to drum out the siren call of climate change on local weather scales.”

Regarding Hansen’s op-ed assertion that the Russia and Texas heat waves “were caused” by manmade climate change, Hoerling countered:

Published scientific studies on the Russian heat wave indicate this claim to be false. Our own study on the Texas heat wave and drought, submitted this week to the Journal of Climate, likewise shows that that event was not caused by human-induced climate change. These are not de novo events, but upon scientific scrutiny, one finds both the Russian and Texas extreme events to be part of the physics of what has driven variability in those regions over the past century. This is not to say that climate change didn’t contribute to those cases, but their intensity owes to natural, not human, causes.

Revkin asked another prominent climate scientist, Kerry Emanuel of the Massachusetts Institute of Technology, to comment on Hansen’s column and Hoerling’s critique. In part, Emanuel wrote:

I see overstatements on all sides. Extreme weather begets extreme views. On the Russian heat wave, [Hoerling] is citing a single paper that claims it had nothing to do with climate change, but there are other papers that purport to demonstrate that events of that magnitude are now three times more likely than before the industrial era.

This is a collision between the fledgling application of the science of extremes and the inexperience we all have in conveying what we do know about this to the public. A complicating factor is the human psychological need to ascribe every unusual event to a cause. Our Puritan forebears ascribed them to sin, while in the 80s it was fashionable to blame unusual weather on El Niño. Global warming is the latest whipping boy.

– Bill Dawson

Image credit: Texas Parks and Wildlife Department