If you want to better understand whether global warming really causes droughts, hurricanes, and other extreme weather, don't think about meteorology. Think about unemployment.
Between 2008, when the housing market imploded and the stock market tumbled, and late 2009, the U.S. civilian seasonally adjusted unemployment rate shot up from under 6.0 percent to double digits. It hung around 9.6 percent for most of 2010, and has since slid slightly down to about 9.1 percent. Asked for a three-word explanation for why so many people are out of work, virtually everyone would say "the bad economy."
But wait -- is the economy really to blame? If we look at any individual lost job, is it truly certain that the housing market and credit crunch woes sufficiently explain why that worker was let go? Maybe if he had worked harder and done more to prove his worth, he would have kept his job. And in the case of big layoffs, aren't those job losses really more directly attributable to poor management, which might have done more to buffer a company against economic fluctuations?
And so on. Zooming in on individual events, one can always come up with reasons to doubt that a macro trend is responsible. In many cases, those specific factors may even be the more immediate or direct causes. But in aggregate, it's absurd to deny that the larger economic factors are a primary cause of joblessness. If nothing else, the trend creates conditions in which the missteps at the individual or corporate level are more likely to lead to disaster.
Global warming's connection to extreme weather events has always been as diffuse as the economy's tie to unemployment. The difference is that no one has tried to pretend that the economy is irrelevant.
For many years, the standard, balanced, responsible line on climate and extreme weather has been a modified version of "it's hard to say." No one event could be attributed with certainty to climate change, but patterns of weather events in keeping with those larger changes were to be expected.
Some scientists, clearly frustrated with the problem of conveying the urgency on climate they feel to the public, have become less restrained. James E. Hansen of the NASA Goddard Institute for Space Studies and two colleagues posted a paper online on Nov. 10, "Climate Variability and Climate Change: The New Climate Dice," that emphatically argues many recent events fall into "a new category of extreme climate outliers" for which warming is the only plausible explanation.
Kevin E. Trenberth of the National Center for Atmospheric Research has even published a provocative proposal in Wiley Interdisciplinary Reviews: Climate Change that recognizing new, unusual weather patterns as human in origin should be now science's default (or null) hypothesis. (Two accompanying papers by Judith Curry of the Georgia Institute of Technology and Myles Allen of the Univerity of Oxford argued to the contrary, though they have their disagreements, too. Journalist James Hrynshyn has a concise summary of their views on his "Class M" blog.)
Pinning down the cause for an event is difficult, and not just because the physical dynamics of something like a weather event are enormously complex. Nonlinear systems like the one that produces earth's weather are famously plagued with chaotic "butterfly effects," in which long chains of escalating perturbations can sometimes allow whispers of air in one hemisphere to stir hurricanes in another.
Definitively assigning blame to any one factor is therefore always open to argument. Certainty seems even more remote given that our understanding and knowledge of the weather system is always incomplete, so one can always question the importance of unobserved factors. Moreover, the philosopher David Hume argued for the impossibility of perfectly determining the cause of any effect on the basis of experience. One can always object that some unseen, unregistered factor might be the true cause.
My point is not that determining the cause of weather disasters (or anything else) is impossible or hopeless. Rather, it's that identifying a specific cause beyond doubt is impossible. Holding out for perfect proofs that global warming is causing droughts, floods, or storms -- rather than probabilistic ones based on models -- is unreasonable. It becomes a strategy for climate change deniers and climate action delayers to extend debate by simply refusing to concede.
Those skeptical of climate science frequently want proof that global warming's influence on events is dispositive -- that is, so clear that it can be distinguished from other sources of normal weather variation. They want proof that an event tied to global warming might not be simply an example of normal weather variation. They might as well be asking for proof that only the economy can be blamed for someone's job loss.
Ultimately, though, asking whether global warming caused specific past or current weather disasters is the wrong question. The better question is, will global warming cause more such droughts, floods, hurricanes, and so on in the future? The answers aren't uniform for all kinds of weather events, but in many cases, a strong case exists that particular regions will see more such problems.
Bad choices, not bad weather
Some of those who dispute the connection between global warming and weather disasters don't quarrel with warming's role in the weather -- only in the disaster. Roger A. Pielke, Jr., of the Cooperative Institute for Research in Environmental Sciences, is perhaps the most prominent advocate for the argument that the expanding human population and economic development are far more directly responsible than changes in the climate. He has shown, for example, that imprudent societal decisions encouraging more construction in areas highly vulnerable to hurricane damage have driven up the economic losses from the storms far more than any increase in their number or severity has.
It's an excellent insight in many ways, and it suggests actions that planners would be foolish not to take in any case. But I take issue with how the argument is often used: to dismiss focusing on climate change as an appropriate or efficient goal for policy. Why bother with the painful business of carbon emissions reduction, after all, when we can blunt any effect of climate changes by making better development decisions?
The problem with that attitude is that it ignores climate's still significant effect. Better development policies might indeed reduce the economic consequences of more severe storms, shifts in rainfall, rising sea levels and so on -- in fact, policy makers will have to make sure that they do. Climate change will foreclose options the policy makers might have otherwise had. Where storms causing coastal flooding will be more severe or commonplace, for example, some kinds of coastal development now allowed will likely be forbidden.
Critics of climate policy reform often argue that spending on carbon emissions control will come at the expense of other, better things that money could be spent on. But they neglect to mention that living in a rapidly changing climate may impose considerable opportunity costs, too.
Moreover, a strategy based on boosting society's resilience to climate change, or reducing its vulnerability to it, might make sense if we know precisely how much and how quickly the climate will shift. We still don't, however. Given that recent global carbon emissions have been exceeding what was once considered to be a worst-case scenario, and that the increase in global temperatures could be 5 degrees Celsius or more, the costs and difficulties of adapting to the climate are mounting. And if the climate should change faster or differently than these adapters have planned, the toll could be heavy.
Global warming's role in causing individual, specific disasters isn't yet as great as the bad economy's is in destroying people's livelihoods. Yet the unrelenting rise in carbon dioxide levels from human activities is only increasing global warming's influence. If we wait until global warming would be the major, undeniable determinant of weather-related damage before trying seriously to curb it, it will be too late.
Photo: Hurricane Katrina, courtesy NASA Goddard Space Flight Center