National Geographic Daily News
An aerial view of flooded buildings in Hungary

The town of Vac, in Hungary, is flooded in 2002 by an overflowing Danube River.

AP Photo/MTI, Sandor H. Szabo

Brian Handwerk

for National Geographic News

Published February 16, 2011

If you were following headlines when Pakistan was underwater last year, you’ve heard that there may be a connection between climate change and increased flooding. Now new studies of severe storms and catastrophic floods add more evidence to the theory that rising greenhouse gas levels actually do increase the odds of such extreme weather events—and perhaps make them stronger.

The recent research is among the earliest that claim to present observable scientific evidence for a human role in altering these natural phenomena, though climate models and observations have long suggested such a link exists in a warming world.

Francis Zwiers and colleagues studied half a century’s worth of rainfall data (1951 to 1999) from a large swath of the Northern Hemisphere, including the United States, Europe, and Asia. In about two-thirds of the weather stations represented, greenhouse gases—which have risen over the same period—correlate with intensification of heavy precipitation events.

“What we see in these observations over a large area of the Northern Hemisphere is that the largest 24-hour precipitation event (single-day storm total) in a year is becoming larger over time,“ said Zwiers, who heads the Pacific Climate Impacts Consortium at the University of Victoria, in Canada. His study appears this week in the journal Nature.

(Read more about the relationship between climate and water.)

Sandra Postel, director of the Global Water Policy Project and National Geographic’s freshwater fellow, said such research provides valuable scientific observations and adds evidence to what climate models had already been predicting.

“We’re told by climate scientists to expect more of these extreme weather events, flooding in some areas and drought in others,” Postel added. “And as Australia has shown us they can even occur in the same place, when biblical-scale flooding followed a decade of historic drought.”

(Read Postel’s blog post, “History's Water Wake-up Call for the Greenhouse Century.”)

You might think this additional rain would help in drought areas, especially when 80 percent of global water supplies are at risk, but not if we can’t capture it.

“Water is only beneficial for irrigation, manufacturing, or supplying cities if we can store it and control it,” Postel said. “Massive flooding is mostly going to do damage to both cities and agricultural lands, as we saw in Pakistan and Australia.

(Related: “Photos: Unprecedented, ‘Biblical’ Floods Inundate Australia” and “How to Stem a Global Food Crisis? Store More Water.”)

Zwiers compared the real observed data with climate models that account for the effects of carbon dioxide (CO2) and other atmospheric greenhouse gases. An intriguing underlying signal was revealed.

“These dramatic precipitation changes can’t be explained by natural forces,” Zwiers said. Natural changes typically follow patterns of increase and decrease, like El Niño, and the changes Zwiers documented did not. Zwiers explained that in general a warming atmosphere will be able to hold more water, and therefore will produce more precipitation.

When It Rains, It Pours

Top climate models predict the wet will get wetter and the dry drier with respect to precipitation in a warming world, but that extremes will increase just about everywhere. His observations showing that real precipitation extremes rose during the 20th century’s second half seem to bear that out.

“And if our projections into the future are to be believed,” he added, “those changes could be substantially larger than they have been already.”

For example, Canadian climate models suggest that the late 20th century’s 100-year extreme precipitation events could become 50-year events by the late 21st century, Zwiers said. Storms of 100-year severity have a one percent chance of occurring in any given year, while a 50-year storm is twice as likely.

“If you think about the town that you live in, the storm sewer and water system may have been developed with a 100-year event in mind. If it stays the size it is, and greenhouse gas concentrations continue to increase in the future as they have in the past, you might expect damages twice as frequently.”

(See an interactive on the greenhouse effect and global warming.)

Growing Odds of Rising Floodwaters

In separate research, Pardeep Pall and colleagues studied the historic and extremely destructive floods that inundated England and Wales in the autumn of 2000, the wettest autumn in a record dating back to 1766. Their goal was determining how greenhouse gas-driven warming might have changed the odds of this specific type of damaging weather event.

Pall, of the Institute for Atmospheric and Climate Science in Zurich, Switzerland, tied human-influenced warming patterns to elevated odds of floods caused by a specific displacement of the North Atlantic jet stream—the same type of event that caused the fall 2000 floods.

His team created and ran thousands of computer climate simulations models—even tapping a large network of off-hours home computers volunteered by weather enthusiasts through the distributed computing project climateprediction.net.

Their seasonal forecasts compared real world conditions with those of a parallel fall 2000 in which the 20th century’s greenhouse gas emissions and related warming had not occurred, to arrive at an estimate revealing how the odds of a similar event might have changed over time.

“When we fed all these weather simulations into a flood model from Risk Management Solutions (RMS) we found that the odds of these floods occurring in autumn 2000 likely increased by about double or more,” he told reporters.

Ninety percent of the time, in fact, the model showed that the gases humans sent skyward over the past century raised the risk of flooding in England and Wales by more than 20 percent. In two out of every three models such floods were determined to be 90 percent more likely.

(Read Freshwater 101: Climate Change, Droughts, and Floods.)

Where Water Is Wanted—And Where It’s Not

Myles Allen of the University of Oxford, in the United Kingdom, a co-author of the study of Britain’s Fall 2000 floods, said predicting the ways in which warming might impact different types of extreme weather events, in different locations around the world, is both daunting and vitally important.

“There will of course, for some time to come, be e­­­­­­­xtreme weather events happening that have been made less likely by human influence on climate,” he said. “You can’t assume just because the climate is changing and you see an extreme weather event that that particular weather event has been made more likely by climate change.

“Disentangling these things, and identifying—of the many extreme weather events that happen—which ones really have been made substantially more likely by human influence on climate, is a difficult but very interesting scientific endeavor.”

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