Increased flooding driven by climate change: study

Marlowe Hood Yahoo News 16 Feb 11;

PARIS (AFP) – Global warming driven by human activity boosted the intensity of rain, snow and consequent flooding in the northern hemisphere over the last half of the 20th century, research released Wednesday has shown.

Two studies, both published in Nature, are among the first to draw a straight line between climate change and its impact on potentially deadly and damaging extreme weather events.

Australia, Sri Lanka, Brazil and Pakistan have all been recently ravaged by massive flooding, raising questions as to whether global warming was at least partly to blame.

Computer models have long predicted that the observed rise in atmospheric greenhouse gases would magnify episodes of diluvian rainfall.

But up to now, the link has been largely theoretical.

"This paper provides the first specific evidence that this is indeed the case," said Francis Zwiers, a researcher at the University of Victoria in Canada and a co-author of one of the studies.

"Humans influence the intensity of precipitation extremes," he told journalists in a telephone press conference.

Data gathered between 1951 and 2000 from across Europe, Asia and North America showed that, on average, the most extreme 24-hour precipitation event in a given year -- whether rain, snow or sleet -- increased in intensity over the last 50 years of the 20th century.

When this measurable spike was compared with changes simulated by climate models, the fingerprint of human influence on Earth's weather patterns was unmistakable, Zwiers said.

"The observed change cannot be explained by natural, internal fluctuations of the climate system alone."

The main driver was simply more water in the air. "In a warmer world the atmosphere has greater moisture-holding capacity," he explained.

That doesn't necessarily mean that in a place where it doesn't rain very much precipitation will increase, he added. Indeed, some spots on Earth are likely to be drier.

But it does mean that when a hurricane or snowstorm does occur, there is more water available.

Why did it take so long for scientists to begin to make solid connections between global warming and extreme weather events?

One reason is that only in recent decades has the accumulated influx of heat-trapping gases become more obvious. "We are finding it easier and easier to detect that signal in observations," Zwiers said.

Progress has also been hampered by the lack of reliable, long-term data, and the sheer computer power needed to test ever-more complex computer models against reality.

In the second study -- which sought to tease out the impact of global warming on England's wettest autumn on record, in 2000 -- scientists led by Myles Allen of the University of Oxford tapped into the power of Internet-based social networks to overcome this last constraint.

The researchers compared two climate models, one based on detailed historical weather data and the other on a "parallel" autumn 2000 simulating conditions had no greenhouse gases been emitted in the 20th century.

Global warming likely doubled the odds that such an event would occur, they found.

"To really pin down the difference between these two worlds, we needed to repeat the simulation thousands of times," explained lead author Pardeep Pall, who initiated the project as a graduate student in 2003.

"We asked members of the public across the world to run the simulations for us on their own personal computers using their idle time."

Based on the results of the study, Britain national climate and weather office is developing tools to measure the human influence on future extreme weather events.

"This kind of study is going to allow us to quantify how climate change is affecting people now so it ceases to be some hypothetical projection of the future," said Allen.

The tool could also be useful in legitimating requests from developing countries seeking to tap into the hundreds of billions of dollars earmarked for climate adaptation, he said.

Members of the public interested in lending computing power can find information at climateprediction.net, which is currently fueled by 50,000 to 60,000 personal computers at any given time.

Floods Linked To Manmade Climate Change: Studies
Gerard Wynn PlanetArk 17 Feb 11;

Man-made greenhouse gas emissions are linked to more frequent heavy rainfall, two studies published found on Wednesday, portraying a clearer human fingerprint after a spate of floods around the world.

Scientists agree that greenhouse gas emissions are warming the world and expect that in turn would lead in the future to more evaporation of water, more moist air and heavier rainfall.

But the two new papers were the first to pin an increase in heavy rainfall in the second half of the last century directly on climate change, as well as one particular extreme flood in Britain.

"The two studies demonstrate that a human impact upon the intensification of rainfall and associated flooding is already detectable," said Richard Allan at the department of meteorology, University of Reading, in Britain.

A Canadian study published in the journal Nature analyzed a general increase in heavy rainfall globally from 1951-1999, and for the first time matched those observations with climate simulations, where the effects of man-made greenhouse gas emissions were included.

"We were able to establish a link between anthropogenic influence and the changes in precipitation," said co-author Xuebin Zhang, from Environment Canada, a government agency.

"The caveat is we haven't been able to identify, to quantify, how big is that influence. That is difficult because of the limitations in observations."

A separate paper, also published in Nature, went a step further and forged a link between climate change and a particular extreme flood in Britain in 2000.

The authors found that the flood was more likely when greenhouse gases were included in simulations than without.

"In two out of three cases we found around a doubling of risk," said co-author Myles Allen from Oxford University.

Extreme weather caused devastating floods in Pakistan and China in 2010, which was tied as the warmest year since records began, and heavy rains in Australia late last year and early 2011 disrupted coal mines and damaged transport infrastructure.

INFANCY

The British study used several thousand computers to simulate the weather in 2000 across England and Wales, where October and November were the wettest since records began in 1766, before the industrial revolution.

The models ran simulations with and without the effects of man-made greenhouse gases on climate change.

The study was striking in trying to attribute man-made climate change to a particular event, given all the complex, chaotic events that contribute to a given storm.

"The evidence for human influence on climate is now even more compelling," said British energy and climate secretary Chris Huhne, responding to the paper.

But the methods it used were in their infancy, said Roger Pielke at the University of Colorado, and the authors acknowledged more research was needed.

For example, one in 10 simulations showed that greenhouse gas emissions increased the chance of the flood by less than 20 percent. "There's quite a lot of uncertainty around this and we're hoping other centers will try to reproduce this and we'll see what results they get," said Allen.

(Editing by Jon Hemming)

Scientists connect global warming to extreme rain
Seth Borenstein, Associated Press Yahoo News 16 Feb 11;

WASHINGTON – Extreme rainstorms and snowfalls have grown substantially stronger, two studies suggest, with scientists for the first time finding the telltale fingerprints of man-made global warming on downpours that often cause deadly flooding.

Two studies in Wednesday's issue of the journal Nature link heavy rains to increases in greenhouse gases more than ever before.

One group of researchers looked at the strongest rain and snow events of each year from 1951 to 1999 in the Northern Hemisphere and found that the more recent storms were 7 percent wetter. That may not sound like much, but it adds up to be a substantial increase, said the report from a team of researchers from Canada and Scotland.

The study didn't single out specific storms but examined worst-of-each-year events all over the Northern Hemisphere. While the study ended in 1999, the close of the decade when scientists say climate change kicked into a higher gear, the events examined were similar to more recent disasters: deluges that triggered last year's deadly floods in Pakistan and in Nashville, Tenn., and this winter's paralyzing blizzards in parts of the United States.

The change in severity was most apparent in North America, but that could be because that's where the most rain gauges are, scientists said.

Both studies should weaken the argument that climate change is a "victimless crime," said Myles Allen of the University of Oxford. He co-authored the second study, which connected flooding and climate change in the United Kingdom. "Extreme weather is what actually hurts people."

Jonathan Overpeck, a University of Arizona climate scientist, who didn't take part in either study, praised them as sensible and "particularly relevant given the array of extreme weather that we've seen this winter and stretching back over the last few years."

Not all the extreme rain and snow events the scientists studied cause flooding. But since 1950, flooding has killed more than 2.3 million people, according to the World Health Organization's disaster database.

The British study focused on flooding in England and Wales in the fall of 2000. The disaster cost more than $1.7 billion in insured damages and was the wettest autumn for the region in more than 230 years of record-keeping.

Researchers found that global warming more than doubled the likelihood of that flood occurring. Similar studies are now under way to examine whether last year's deadly Russian heat wave and Pakistan floods — which were part of the same weather event — can be scientifically attributed to global warming.

For years scientists, relying on basic physics and climate knowledge, have said global warming would likely cause extremes in temperatures and rainfall. But this is the first time researchers have been able to point to a demonstrable cause-and-effect by using the rigorous and scientifically accepted method of looking for the "fingerprints" of human-caused climate change.

The scientists took all the information that shows an increase in extreme rain and snow events from the 1950s through the 1990s and ran dozens of computer models numerous times. They put in the effects of greenhouse gases — which come from the burning of fossil fuels — and then ran numerous models without those factors. Only when the greenhouse gases are factored in do the models show a similar increase to what actually happened. All other natural effects alone don't produce the jump in extreme rainfall. Essentially, the computer runs show climate change is the only way to explain what's happening.

In fact, the computer models underestimated the increase in extreme rain and snow. That is puzzling and could be even more troubling for our future, said Michael Oppenheimer of Princeton University, who wasn't part of the study.

Similar fingerprinting studies have found human-caused greenhouse gas emissions triggered changes in more than a dozen other ecological ways: temperatures on land, the ocean's surface, heat content in the depths of the oceans, temperature extremes, sea level pressure, humidity at ground level and higher in the air, general rainfall amounts, the extent of Arctic sea ice, snowpack levels and timing of runoff in the western United States, Atlantic Ocean salinity, wildfire damage, and the height of the lower atmosphere.

All those signs say global warming is here, said Xuebin Zhang, a research scientist for the Canadian government and co-author of the Northern Hemisphere study. "It is affecting us in multiple directions."

Most of the 10 outside climate experts who reviewed the papers for The Associated Press called the research sound and strong.

However, climate scientist Jerry North of Texas A&M University, while praising the work, said he worried that the studies were making too firm a connection based on weather data that could be poor in some locations. But Francis Zwiers of the University of Victoria, a lead author of the study with Zhang, said the data was from National Weather Service gauges and is reliable.

"Put the two papers together and we start to see an emerging pattern," said Andrew Weaver of the University of Victoria, who wasn't part of either study. "We should continue to expect increased flooding associated with increased extreme precipitation because of increasing atmospheric greenhouse gas. And we have no one to blame but ourselves."

Climate change raises flood risk, researchers say
Richard Black BBC News 16 Feb 11;

Greenhouse gas emissions are making extreme rainfall events more common, scientists say - and in the UK, have increased the risk of flooding.

Two research groups present their findings in the journal Nature.

Using real-world data and computer models, one team says it has proven the link between greenhouse emissions and the observed increase in extreme rains in the Northern Hemisphere.

The other says greenhouse warming made the UK floods of 2000 more likely.

That autumn saw the highest rains in England and Wales since records began in 1766.

The Hampshire village of Hambledon was underwater for six weeks, and insurers put the final cost to the country at more than £1bn.

A research team led from Oxford University ran computer models of the atmosphere as it actually was, and parallel models of the atmosphere as it would have been without the carbon dioxide and other greenhouse gases that had accumulated from humanity's emissions.

This produced projections of rainfall patterns, which were then fed into a further model that translated rainfall into the impact on river basins across England and Wales.

The 2000 floods occurred when river basins filled up rapidly.

"We looked at how greenhouse gas emissions affected the odds of a flood," related Pardeep Pall, the Oxford researcher who led the study.

"We found that the emissions substantially increased the odds of a flood occurring in 2000, with about a doubling of the likelihood."

Global influence

In the second study, researchers from Canada and the UK looked at the increase in the frequency of extreme rainfall events documented across much of the Northern Hemisphere between 1950 and 2000.

There are variations from year to year and from place to place; but across the piece, intense downpours have become more common over the period.

The researchers suggest there is nothing that can explain this trend except the slow steady increase in temperatures caused by greenhouse gas emissions.

"In North America, precipitation extremes correspond to the El Nino effect in pretty characteristic ways, where some regions get heavy rainfall while others receive less extreme precipitation," said Francis Zwiers from the University of Victoria in Canada.

"But we don't see these spatial variations in our study, and our models don't generate that kind of spatial structure either.

"The evidence is leading us in another direction, to a phenomena that influences precipitations in a global scale - and the only thing we can think of is the changing composition of the atmosphere."

For decades scientists have believed that on a global scale, a warmer world should be a wetter one, as warm air holds more moisture than cold air.

But the researchers say this is the first "formal identification" of the link between emissions and intense rains.

They also found that a number of the computer models commonly used in research currently underestimate the extent to which heavy rains have increased.
Model concerns

Both research groups were at pains to emphasise that these two papers are not the end of the road.

More research needs to be done, they say, with better records needed from regions of the world where data is scarce, improvements in models, and an extension to other types of weather event.

"There will be for some time to come extreme weather events happening that have been made less likely by human influence on climate," said Myles Allen, one of the Oxford group behind the year 2000 analysis.

"Disentangling these things and deciding which events have been made more likely and which less likely is a difficult but a very interesting scientific endeavour."

This was a point taken up by Sir Brian Hoskins from the Grantham Institute for Climate Change Research, who was not involved with either of these studies.

"Both studies depend heavily on the accuracy of their computer models," he told reporters.

"We need to understand better the actual physics of different flooding events and make sure that the models are able to capture this. Studies like these should be repeated as models continue to improve."

Researchers said they also needed to include other changes to the atmosphere in future analyses, such as the tiny aerosol particles of dust that reflect sunlight back into space.
'Alarming' picture

If the risk of floods is increasing, policymakers will have to ask themselves how to respond; and in the UK, the government has come under fire in recent weeks having cut the flood defence budget by 8% earlier this month.

But a spokesman for the Department for Environment, Food and Rural Affairs (Defra) said the implications of the research - which Defra partially funded - were being taken on board.

"This work reinforces the scientific evidence that the UK needs to tackle climate change and become more resilient to the likelihood of extreme weather events caused by climate change," he said.

"Defra will spend more than £2.1billion over the next four years to provide greater protection to at least 145,000 homes. Defra is also exploring innovative new ways of managing flood risk, such as supporting three land management demonstration projects to see if the rate at which rainfall enters watercourses can be slowed."

Bjorn Lomborg, the Danish "sceptical environmentalist" who was in London for a seminar, told BBC News that society had to look at where and how people lived.

"Some people criticise computer models and of course they are only computer models, but it's the only way we can talk about the future, through models - so I don't disagree with using models," he said.

"But is the right way to handle future flooding by focusing on climate change? The answer is no - that's an incredibly expensive way of making extreme flooding very slightly less likely in 100 years.

"We should focus on the simple ways - making better protection, making sure people don't settle on flood plains, and that we have some places where rivers can naturally flood as they did in the past."

As Professor Lomborg was speaking to the BBC in London, Christiana Figueres, executive secretary of the UN climate convention (UNFCCC), was telling the Spanish parliament that tackling emissions was becoming ever more of a priority.

"It is alarming to admit that if the community of nations is unable to fully stabilise climate change, it will threaten where we can live, where and how we grow food and where we can find water - the very stability on which humanity has built its existence.

"On a global level, increasingly unpredictable weather patterns will lead to falling agricultural production and higher food prices, leading to food insecurity.

"Recent experiences around the world clearly show how such situations can cause political instability and undermine the performance of already fragile states."