By JUSTIN GILLIS


The floods that have battered wide swaths of America 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 hectares of wheat and thousands of lives in a drought worse than any other on 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 construed in a single word: probably.

“The climate is changing,” said Jay Lawrimore, chief of climate analysis at the National Climatic Data Center in Asheville, North Carolina. “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 , bigger snowstorms , more intense droughts and more record-breaking heat waves. Scientists and government reports say that much of this is starting to happen.

But the averages do not necessarily make it easier to link specific weather events to climate change. Most scientists are reluctant to go that far, noting that weather was characterized by remarkable variability long before humans began burning fossil fuels .

“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 . “If you ask me as a scientist whether I have proved it, the answer is no - at least not yet.”

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 0.8 degrees Celsius 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 this month.

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.

But 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 , scientists published detailed analyses suggesting that it would not have been as severe in a climate uninfluenced by greenhouse gases.

And Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colorado, 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.”


John Collins Rudolf contributed reporting.


PAULA BRONSTEIN/GETTY IMAGES
Pakistan’s worst floods in 80 years are but one weather-related
disaster this season. A rescue in Sukkur.