Update [2/17/11 5:05 PM]: A few bloggers and scientists have taken issue with the two Nature studies, arguing that they underplay the uncertainties still at work in the climate system. Judith Curry, an atmospheric scientist at Georgia Tech who tends to be a bit more skeptical of mainstream climate research, wrote that she doubted the papers could really attribute rising precipitation to greenhouse gas emissions. Roger Pielke Jr.—a professor of environmental studies at the University of Colorado—has a post throwing cold water on the media attention the Nature papers have earned—including from yours truly. He writes:
Accepting that precipitation has increased and can be attributed in some part to GHG emissions, there have not been shown corresponding increases in streamflow (floods) or damage. How can this be? Think of it like this — Precipitation is to flood damage as wind is to windstorm damage. It is not enough to say that it has become windier to make a connection to increased windstorm damage — you need to show a specific increase in those specific wind events that actually cause damage. There are a lot of days that could be windier with no increase in damage; the same goes for precipitation.
Put simply, there’s still a lot of uncertainty around these conclusions—uncertainty that can be found in the body of the papers themselves, but not in their abstracts, nor in the headlines of the news pieces written about the research. Andrew Revkin of Dot Earth makes the same point in greater detail, criticizing the papers and the coverage for what he calls “front page thought“:
To me, as a reporter, the authors and journal are trying to have things both ways — including definitive statements in abstracts and summaries that draw the attention of the press and public, but then saying, no, this is not definitive… please note the uncertainties in the final line (even though that line isn’t actually necessarily linked, rhetorically, to the definitive statements).
I agree that the papers, on closer examination, are still a long way from really fingerprinting climate change on extreme weather events. As Pielke points out, the causes of a flood go well beyond just rain, and these papers weren’t able to take those additional factors into account. That said, as Revkin notes, these papers are part of a growing chorus showing how manmade greenhouse gas emissions are impacting the weather, as we enter what’s been called the Anthopocene Era. If we were in a court of law we still couldn’t convict climate change for extreme events like the Pakistani floods or the Russian heat wave—and maybe we’ll never be able to do it. But we’ll still have to live with a changing climate.
Original post: Perhaps the biggest challenge facing climate scientists today is fingerprinting—proving that extreme weather events like droughts, heat waves and floods are actually being caused by manmade climate change. Climate scientists have a great deal of confidence about the impacts of global warming over a long period of time, decades to centuries—higher temperatures, changes in precipitation, rising sea levels. But if you actually ask a climatologist whether an unusually strong hurricane or a crippling heat wave is actually connected to global warming, you’ll always get the same answer: we can’t prove it.
But that’s beginning to change. In the first major paper of its kind, a new study in the February 17 Nature has found that heavy precipitation is at least partly due to the growing concentration of manmade greenhouse gas emissions in the atmosphere. A team of scientists from Canada and Scotland used powerful computers to analyze the causes behind the rise in storms and heavy snowfall over the past half century. They found that the likelihood of extreme precipitation on any given day rose by 7% between 1951 and 1999—the years covered by the study. That’s outside the bounds of normal variability, and the increase only make sense if rising greenhouse gas emissions and climate change are taken into effect. “Human influence on the climate system has the effect of intensifying precipitation extremes,” said Francis Zwiers, a climate researcher at Environment Canada in Toronto and the study’s lead researcher.
That conclusion shouldn’t be that surprising—climatologists have predicted an uptick in extreme weather events as greenhouse gas concentrations in the atmosphere increase. Since warmer air can carry more water vapor, a warmer planet should see heavier rain and other precepitation—and that’s what we’ve begun to see with actual weather. As an accompanying article to the Nature studies points out, for every 1 C in warming near the Earth’s surface, scientists expect a 2-3% increase in total global precipitation.
But it’s not global precipitation patterns that should really worry us—it’s local rainfall, either too much or too little. We need scientists to be able to tell us whether a certain flood or a certain drought actually can be tied to global warming. And in another study published in Nature this week, that’s exactly what a team of climatologists set out to do. With help from computer time donated by the public, the researchers analyzed the severe rains that flooded England and Wales in 2000, leading to some of the worst flooding in British history. They looked at the climate as it actually existed at the time, and then compared it to hypothetical climates as they would have been if human beings had never begun adding greenhouse gases into the atmosphere. The researchers concluded that the chances of such a major flood happening at that time were roughly doubled by the rise in greenhouse gases. “Greenhouse gas emissions due to human activity have affected the odds of floods in England and Wales,” said physicist Pardeep Pall of the Swiss Federal Institute of Technology (ETH Zurich), who led the Nature research.
Given the billions of dollars for climate adaptation at stake, it’s vital for scientists to deliver more rapid and accurate analysis of extreme weather events—like climate change CSI. But we’re not there yet—the Nature study of the 2000 floods took a decade of work and incredibly complex computer simulations. But we’ll need better analysis not just to predict coming extreme weather events and prepare for them—something that could have averted much of the damage from the terrible floods in Pakistan last summer—but in assigning cause and blame. That will matter for the business world—insurance companies will want to know just what caused devastating floods or storms as they pay out their claims. But it will also impact international climate negotiations. If developing countries—already on the front lines of global warming—can prove that carbon emissions from rich countries are causing what we once referred to as natural catastrophes, they may well be justified in demanding the equivalent of carbon reparations.