By the time Hurricane Irene hit New York City on August 27, climate experts, talk-show hosts, bloggers, politicians, and environmental activists had already staked out their usual positions. Irene was either another sign of human-induced climate change or just another troublesome storm. Or maybe it was impossible to say which it was.
There has been enough weird weather lately to supply combatants in the war over climate change with ample ammunition. Connecticut, for example, saw a record-smashing five feet of snow in the month of January. Tornadoes killed record numbers of people from Missouri to Alabama this spring. An unprecedented plague of intense heat and drought has gripped Texas, Oklahoma, and Kansas for months. Meanwhile, record rainfall and snowmelt in the north-central states swelled the Missouri River with 10 million acre-feet of extra runoff water in July alone.
In the political debate over what’s behind our increasingly unpredictable weather, the standards of logic and evidence tend to be looser than they are in scientific debates. In July 2010, for instance, in the midst of what was to end up being the hottest year on record worldwide, Sen. Jim Inhofe of Oklahoma announced, “We’re in a cycle now that all the scientists agree is going into a cooling period.”
It hasn’t helped that Mother Nature insists on barraging us with mixed messages. This July was the hottest month ever recorded in Oklahoma, while just five months before, some locations in Inhofe’s state had set all-time records for low temperature (-31 degrees F) and snowfall (27 inches in 24 hours).
In 2009, fifteen climate scientists took such fluctuations into account when they concluded in a paper in the Proceedings of the National Academy of Sciences that “it is now more likely than not that human activity has contributed to observed increases in heat waves, intense precipitation events, and the intensity of tropical cyclones.” That statement, based on the most recent report by the Intergovernmental Panel on Climate Change (IPCC), suggests that greenhouse emissions have not only helped barbecue the Great Plains this summer but were a factor in Hurricane Irene’s size and strength as well.
Writer and activist Bill McKibben took a lot of heat from adversaries and a from few allies when he quipped in a recent column that “Irene’s got a middle name, and it’s Global Warming.” But even the tenacious McKibben may be getting tired of having to argue repeatedly, whenever an extreme weather event comes along, that it’s an indicator of human-induced climate change. He ended his article on Irene by noting that this hurricane “will be a distraction in the short run from our efforts, but in the long run it underlines what the fight is all about.”
Roll of the dice
Given the complexity of climate science, it’s not surprising that the best way to get the attention of the media and the public is to talk about wild weather that’s happening right now rather than the bigger threat of long-term climate disruption. But that makes life difficult for those who study climate for a living.
One such researcher is Katharine Hayhoe, an associate professor at Texas Tech University. She sees a clear necessity to come back hard against fatuous arguments that, she says, go something like this: “Well, you know, temperatures are cooling in the month of September in Erie, Pennsylvania, so how can the planet be warming?” But, she warns, climate scientists have to be careful themselves not to go beyond the data: “It is very tempting to seize on a single dramatic event, but we have to stay true to what we know, to stick to terms like ‘consistent with’ and ‘risk of…”
In public statements, most climate scientists are indeed careful to stress that we cannot draw conclusions from individual extreme weather events. But Michael Mann, director of the Earth System Science Center at Pennsylvania State University, is now thinking that such caution may have gone too far.
He says many of his colleagues, weary of being attacked by climate skeptics, tend to “jump too soon”, starting their responses to reporters by discounting the relevance of individual weather events. But by understating the links, he believes, they are “erring in the opposite direction”, which in itself can be misleading. “Statistics show very much that these events really are part of bigger trends,” he stresses.
To illustrate, Mann uses a metaphor popular among climate researchers these days: “Suppose you’re betting on dice, but that someone had replaced the ‘five’ on this particular die with a second ‘six’. If you don’t know this, you might get cheated out of a lot of money. But when you demand your money back, can you point to any one roll of that die that you can prove lost you money? No, you can’t.” But, he says, it’s a fact that the size of your losses is a direct result of the change in the die.
Similarly with the Earth’s atmosphere, he says, statistics tell us that shifts in climate have contributed to extreme weather: “As the numbers start piling up, you can say that they have been shifted by climate change. A thousand-year event becomes a thirty-year event.”
Daily weather data that have been recorded for fifty or a hundred or more years in many places can tell us a lot about extremes. Says Mann, “If we were seeing a lot of longer, more intense cold periods, we’d all be scratching our heads. But when you confirm what the hypothesis proposed, you have an increased degree of confidence.”
(Mann’s confidence that we are seeing a real impact from greenhouse emissions has made him a popular target for climate skeptics. In 2009, email hackers charged that he had engaged in scientific misconduct; however, repeated inquiries, including two by his university and a more recent one by the National Science Foundation, cleared him of any wrongdoing. Meanwhile, conservative Virginia Attorney General Kenneth Cuccinelli’s search for evidence that could be used to prosecute Mann has turned up nothing.)
But if climate models project with some assurance that present and future emissions of greenhouse gases will lead to rapid warming of the atmosphere and more extreme weather, is it even necessary to continue digging into past climate and weather records for evidence of change? Do historical studies add any useful information as we plan for the future?
A 2008 report published by the U.S. Climate Change Science Program stressed that while the past does not hold all the answers, it is important to learn what history can tell us. The first step, they explain, is “detection”—establishing that changes have occurred in some type of extreme, say heat waves, over time.
Then comes the second step, “attribution”, in which those observed changes are compared with changes that would have been predicted by climate models. If real-world weather events are consistent with events that are expected to occur in a greenhouse world but not in a world of exclusively natural forces, that is evidence for a human impact on extreme weather.
When extreme heat becomes routine
If the brain-cooking heat that gripped much of the country these past two summers has seemed to be unusually persistent, it’s not just our fevered imaginations. Weather records show that multiple-day runs of exceptionally high temperatures have become more common since 1960. A half-century ago, record high and record low temperatures occurred at approximately the same rate in the United States. Now record highs are happening at least twice as often as record lows, and the ratio might be as high as four-to-one.
One link between heat waves and human-induced warming of the atmosphere is simply a matter of statistics. Daily temperatures are distributed like most phenomena, in a bell-shaped curve, with most readings heaped up in the middle—that is, near the average for the date—and the rarer extremes tapering away in both directions as “tails”. As the earth warms, that curve tends to shift to the right, toward higher temperatures, with its right tail leading the way.
Even if the bell curve stays exactly the same shape as it moves, a small shift can lead to many more heat waves. Notes Michael Mann, “The one-degree Celsius increase we have seen in average temperature, for example, appears to be leading to a doubling of the rate at which record-breaking temperatures occur.” That happens because as the curve moves right (a phenomenon firmly linked to greenhouse emissions), the “fatter” part of the tail moves into “extreme” territory.
But there may be more to the story. The monster heat wave that killed tens of thousands of Europeans in 2003 was off the charts—impossible to explain by a simple shift in the bell curve, according to a Swiss climate team. The group reported a few months after the disaster that even considering the increase in average temperatures in Europe from 1990 to 2003–but assuming no change in the shape of the curve—a heat wave like that of 2003 could be expected to occur only once every 46,000 years.
The fact that the 2003 event actually happened led the group to search for other explanations in greenhouse climate models. Those models, they discovered, predict a large increase not only in average temperature but in variability as well—a flattening of the bell curve that would make killer heat waves much more common in, say, Switzerland.
Indeed, the Swiss scientists’ models suggest that in Central Europe “towards the end of the century—under the given scenario assumptions—about every second summer could be as warm or warmer (and as dry or drier) than 2003.” And while not as wildly unpredictable as Europe ‘03, the 2010 killer heat wave in Russia went well beyond anything else yet experienced and might also be an indicator of a flattening bell curve.
“The heat in Russia and the floods in Pakistan in the past year were not just weather flukes,” adds Mann. Greenhouse models, he points out, projected that sinking dry air would migrate from northern Africa and southern Europe toward Central Europe and Russia in summer, and that moist air would move north from the tropical Indian Ocean toward subtropical Pakistan. “Those events were part of a larger circulation pattern,” he says—a pattern that greenhouse-climate models had predicted.
Extreme rains, deep snow
In recent years, precipitation patterns appear to have gone haywire, not just in Pakistan but on every continent. Katharine Hayhoe has seen this up close: “In the five years I’ve lived in West Texas, we’ve had a 111-day rainless stretch—the longest ever recorded—and two ‘hundred year’ rainfalls”—ones so heavy that such an event occurs only once per century on average.
“But,” she says, “it all makes sense from a basic physics perspective. The atmosphere is holding more water vapor. Storm systems, when they come, have more to work with.”
Warmer air is capable of holding more water vapor than is cooler air. As a consequence, the concentration of moisture in the atmosphere also has been increasing since the 1960s, both in the United States and across the globe. A comprehensive 2007 study led by scientists at Yale University concluded that the increases in humidity observed planet-wide can be attributed to human influence and that natural forces alone cannot explain the change.
With more moisture in the air, an increasing proportion of precipitation is coming in the form of more intense rainstorms around the world. Over the past thirty years, the southeastern United States has seen simultaneous increases in droughts, wet years, and strong rainstorms. According to a 2010 report by researchers at the National Oceanic and Atmospheric Administration’s Climate Prediction Center in Maryland, the University of Texas, and Duke University, these big swings in precipitation are related to the continuing rise in Atlantic Ocean surface temperatures and the increasing variability of those temperatures.
Even on a warming planet, regions with traditionally cold winters will still have plenty of below-freezing weather; when that cold air combines with moist air masses (in a generally warmer atmosphere that’s able to carry more water vapor than it used to), a lot of moisture can suddenly be dumped in the form of snow. There has been a slight upward trend in strong snowstorms over the past century in the United States. What part of that trend you see depends on where you live. Warmer areas of the country are seeing fewer big snowstorms, but the upper Midwest and the Northeast are getting hit with more of them.
A June 2009 report published by the U.S. Global Change Research Program found that the share of precipitation falling as snow rather than rain is increasing in the northeastern United States. Furthermore, said the report, “Heavy snowfall and snowstorm frequency have increased in many northern parts of the United States.” Six months after publication of that report, with the northern and middle Atlantic coast paralyzed by record snowfall, the authors could be even more confident that the trend they had observed was not a mirage.
And as we have seen once again, those most media-friendly of all extreme weather phenomena—hurricanes—are also among the most controversial subjects in the climate debate. A group of experts published a paper last year in Nature Geoscience examining all existing evidence of links between greenhouse emissions and Atlantic hurricanes. In contrast to the IPCC report that had concluded it is “more likely than not” that humanity’s emissions have influenced tropical cyclone activity, this study found that “despite some suggestive observational studies, we cannot at this time conclusively identify” a human fingerprint on the increasing intensity of tropical cyclones. However, the group concluded, “a substantial human influence on future tropical cyclone activity cannot be ruled out.”
No news is bad news
Whatever happens on the ground, in the sea, and in the atmosphere in coming decades, it is very likely that public discussion of the climate will tend to focus on events like heat waves, floods and storms more than on the invisible, and ultimately more important, transformation of the planet-wide climate.
In a 2007 essay, Andrew Revkin of the New York Times explained why storms make headlines but climatic disruption does “not constitute news as we know it.” He predicted that “the incremental nature of climate research and its uncertain scenarios will continue to make the issue of global warming incompatible with the news process. Indeed, global warming remains the antithesis of what is traditionally defined as news… Journalism craves the concrete, the known, the here and now and is repelled by conditionality, distance, and the future.”
But could it be that energetic public discussion of meteorological media events like Irene can actually help remedy the situation, by introducing more people to the complex forces that are taking our climate on this wild ride? Do more people now know, for example, that if there is extraordinary weather again this winter, it can be entirely consistent with what we’d expect when living in a warmer, moister atmosphere? Will more of us see in next summer’s heat waves and hurricanes the roll of loaded dice?
When I asked Michael Mann those questions, he chuckled. “Well, yes, I hope the past year has provided a learning opportunity” for Americans. But will we actually learn from it? On that question, Mann—who makes his living estimating statistical confidence—did not seem very confident at all.
