The floods battered New England, then Nashville, then Arkansas, then Oklahoma — and were followed by a deluge in Pakistan that has upended the lives of 20 million people.
The summer’s heat waves baked the eastern United States, parts of Africa and eastern Asia, and above all Russia, which lost millions of acres of wheat and thousands of lives in a drought worse than any other in the historical record.
Seemingly disconnected, these far-flung disasters are reviving the question of whether global warming is causing more weather extremes.
The collective answer of the scientific community can be boiled down to a single word: probably.
“The climate is changing,” said Jay Lawrimore, chief of climate analysis at the National Climatic Data Center in Asheville, N.C. “Extreme events are occurring with greater frequency, and in many cases with greater intensity.”
He described excessive heat, in particular, as “consistent with our understanding of how the climate responds to increasing greenhouse gases.”
Theory suggests that a world warming up because of those gases will feature heavier rainstorms in summer, bigger snowstorms in winter, more intense droughts in at least some places and more record-breaking heat waves. Scientists and government reports say the statistical evidence shows that much of this is starting to happen.
But the averages do not necessarily make it easier to link specific weather events, like a given flood or hurricane or heat wave, to climate change. Most climate scientists are reluctant to go that far, noting that weather was characterized by remarkable variability long before humans began burning fossil fuels and releasing greenhouse gases into the atmosphere.
“If you ask me as a person, do I think the Russian heat wave has to do with climate change, the answer is yes,” said Gavin Schmidt, a climate researcher with NASA in New York. “If you ask me as a scientist whether I have proved it, the answer is no — at least not yet.”
In Russia, that kind of scientific caution might once have been embraced. Russia has long played a reluctant, and sometimes obstructionist, role in global negotiations over limiting climate change, perhaps in part because it expected economic benefits from the warming of its vast Siberian hinterland.
But the extreme heat wave, and accompanying drought and wildfires, in normally cool central Russia seems to be prompting a shift in thinking.
“Everyone is talking about climate change now,” President Dmitri A. Medvedev told the Russian Security Council this month. “Unfortunately, what is happening now in our central regions is evidence of this global climate change, because we have never in our history faced such weather conditions in the past.”
Thermometer measurements show that the earth has warmed by about 1.4 degrees Fahrenheit since the Industrial Revolution, when humans began pumping enormous amounts of carbon dioxide, a heat-trapping greenhouse gas, into the atmosphere. For this January through July, average temperatures were the warmest on record, the National Oceanic and Atmospheric Administration reported Friday.
The warming has moved in fits and starts, and the cumulative increase may sound modest. But it is an average over the entire planet, representing an immense amount of added heat, and is only the beginning of a trend that most experts believe will worsen substantially.
If the earth were not warming, random variations in the weather should cause about the same number of record-breaking high temperatures and record-breaking low temperatures over a given period. But climatologists have long theorized that in a warming world, the added heat would cause more record highs and fewer record lows.
The statistics suggest that is exactly what is happening. In the United States these days, about two record highs are being set for every record low, telltale evidence that amid all the random variation of weather, the trend is toward a warmer climate.
Climate-change skeptics dispute such statistical arguments, contending that climatologists do not know enough about long-range patterns to draw definitive links between global warming and weather extremes. They cite events like the heat and drought of the 1930s as evidence that extreme weather is nothing new. Those were indeed dire heat waves, contributing to the Dust Bowl, which dislocated millions of Americans and changed the population structure of the United States.
But most researchers trained in climate analysis, while acknowledging that weather data in parts of the world are not as good as they would like, offer evidence to show that weather extremes are getting worse.
A United States government report published in 2008 noted that “in recent decades, most of North America has been experiencing more unusually hot days and nights, fewer unusually cold days and nights, and fewer frost days. Heavy downpours have become more frequent and intense.”
The statistics suggest that the Eastern United States may be getting wetter as the arid West dries out further. Places that depend on the runoff from spring snow melt appear particularly vulnerable to climate change, because higher temperatures are making the snow melt earlier, leaving the ground parched by midsummer. That can worsen any drought that develops.
“Global warming, ironically, can actually increase the amount of snow you get,” said Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colo. “But it also means the snow season is shorter.”
In general, the research suggests that global warming will worsen climate extremes across much of the planet. As in the United States, wet areas will get wetter, the scientists say, while dry areas get drier.
But the patterns are not uniform; changes in wind and ocean circulation could cause unexpected effects, with some areas even cooling down in a warmer world. And long-established weather patterns, like the periodic variations in the Pacific Ocean known as El Niño, will still contribute to unusual events, like heavy rains and cool temperatures in normally arid parts of California.
Scientists say they expect stronger storms, in winter and summer, largely because of the physical principle that warmer air can hold more water vapor.
Typically, a storm of the sort that inundated parts of Tennessee in May, dumping as much as 19 inches of rain over two days, draws moisture from an area much larger than the storm itself. With temperatures rising and more water vapor in the air, such storms can pull in more moisture and thus rain or snow more heavily than storms of old.
It will be a year or two before climate scientists publish definitive analyses of the Russian heat wave and the Pakistani floods, which might shed light on the role of climate change, if any. Some scientists suspect that they were caused or worsened by an unusual kink in the jet stream, the high-altitude flow of air that helps determine weather patterns, though that itself might be linked to climate change. Certain recent weather events were so extreme that a few scientists are shedding their traditional reluctance to ascribe specific disasters to global warming.
After a heat wave in Europe in 2003 that killed an estimated 50,000 people, the worst such catastrophe for that region in the historical record, scientists published detailed analyses suggesting that it would not have been as severe in a climate uninfluenced by greenhouse gases.
And Dr. Trenberth has published work suggesting that Hurricane Katrina dumped at least somewhat more rain on the Gulf Coast because the storm was intensified by global warming.
“It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability,” Dr. Trenberth said. “Nowadays, there’s always an element of both.”