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|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
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
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
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
“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
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
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
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
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
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
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
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.”