Friday, April 29, 2011

2010 extreme weather cost lives, health, economy

Climate Progress, Posted: 29 Apr 2011

"April is the cruelest month." — T. S. Eliot

CAP's Daniel J. Weiss, Valeri Vasquez, and Ben Kaldunski have written an excellent 34–page report, "The Year of Living Dangerously."  Its conclusion begins, "The extreme weather in 2010 could be a preview of a not-too-distant future should we fail to reduce carbon dioxide pollution."

Here is an overview of that report by the authors.

April 2011 has been a cruel month indeed for Americans due to extreme weather. The Weather Channel observed that:

It's been a truly awful, record-setting, tornadic April. We've had eleven major severe weather events, some lasting multiple days.

These extreme events included "supercell thunderstorms" in Iowa, severe drought and record wildfires in Texas, and heavy rains across the United States. The recent southeastern storms and tornados took at least 297 lives across eight states. And heavy rains in the Mississippi River valleycould cause the most severe, damaging floods there in nearly a century.

This extreme weather, though record setting in some places, may be the new normal. Last year, unprecedented extreme weather led to a record number of disaster declarations by the Federal Emergency Management Agency. The United States and the world were swept by flooding, severe winter storms, heat waves, droughts, hurricanes, and tornadoes.

The extreme weather of 2010 exacted a huge human and economic toll as wellMore than 380 people died and 1,700 were injured due to weather events in the United States throughout the year. And the magnitude of these events forced the Federal Emergency Management Agency, or FEMA, to declare 81 disasters last year. For nearly 60 years, the annual average has been 33. In 2010, total damages exceeded a whopping $6.7 billion. As of April 2011, FEMA had dedicated more than $2 billion in financial assistance to those harmed by extreme weather in 2010.

A February 2011 special report from Reuters noted that it's been rough going for the $500 billion U.S. property insurance business, explaining that "storms are happening in places they never happened before, at intensities they have never reached before and at times of year when they didn't used to happen."

It is precisely this uncertainty "associated with climate change that substantiates the risks to the economy and society," says George Backus, D.Engr., of the Discrete Mathematics and Complex Systems Department at Sandia National Laboratories. This is bad news for a nation just emerging from the grips of the Great Recession. Per Backus, a 2010 report from Sandia estimates that "the climate uncertainty as it pertains to rainfall alone [puts] the U.S. economy is at risk of losing between $600 billion and $2 trillion, and between 4 million and 13 million U.S. jobs over the next 40 years."

Dr. Evan Mills, a scientist in the Environmental Energy Technologies Division at the Lawrence Berkeley National Laboratory confirms that in the United States, "insured weather-related losses in recent years have been trending upward much faster than population, inflation, or insurance penetration, and far outpace losses for non-weather-related events."

It is difficult, of course, to link or "attribute" individual extreme weather events in a single year to global warming. Climate factors—including human influences—shape weather patterns. According to Munich Re, one of the world's largest reinsurers, "the only plausible explanation for the rise in weather-related catastrophes is climate change." And as Kevin Trenberth, Sc.D., head of the Climate Analysis Section at the National Center for Atmospheric Researchexplained at the American Meteorological Society's January 2011 meeting, "Given that global warming is unequivocal, the null hypothesis should be that all weather events are affected by global warming rather than the inane statements along the lines of 'of course we cannot attribute any particular weather event to global warming.'"

In other words, says Trenberth, "it's not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there's always an element of both."

Atmospheric concentrations of carbon dioxide and other greenhouse gas pollutants are turning up the heat on our planet. Scientists agree that the string of disastrous weather extremes this past year are the types of severe weather that will become more frequent or ferocious as the planet continues to warm. For instance, in the "first major paper of its kind" tracking global climatic trends from 1951 to 1999, Scottish and Canadian researchers used sophisticated computer models to confirm a human contribution to more intense precipitation extremes with very high confidence.

This analysis is supported by a 2010 Duke University-led study that found, "Global warming is driving increased frequency of extreme wet or dry summer weather in southeast, so droughts and deluges are likely to get worse."

study published in the 2011 Journal of Climate presents "evidence of a significant human influence on the increasing severity of extremely warm nights and decreasing severity of extremely cold days and nights."

Likewise, a report by the National Center for Atmospheric Research, Climate Central, The Weather Channel, and the National Oceanic and Atmospheric Administration shows that "if temperatures were not warming, the number of record daily highs and lows being set each year would be approximately even. Instead … record high temperatures far outpace record lows across the U.S."

The recent extreme weather should not be a surprise. In 1999, Trenberth projected that global warming would lead to severe precipitation.

An increase in heavy precipitation events should be a primary manifestation of the climate change that accompanies increases in greenhouse gases in the atmosphere.

Nine years later, the U.S. Climate Change Program under President George W. Bush came to a very similar conclusion. "Heavy downpours have become more frequent and intense. Droughts are becoming more severe in some regions." These are some of the extreme weather events we experienced this April, and in 2010.

Because we have not brought carbon pollution under control, the weather events of 2010 will continue to revisit us—with a vengeance. We must act quickly and unequivocally to address climate change before the threat becomes insurmountable. This includes recognizing that global warming is already affecting us both domestically and internationally.

The purpose of this report is to gather, condense, and synthesize some of the massive amount of data about extreme weather and its links to global warming. This summary of climate science can help provide context to the recent surge in extreme weather events. In this report we will catalogue the extreme U.S. weather in 2010 and then examine the consequences on our health and economy.

As we note in the conclusion, conservatives remain eager to dismiss these weather extremes by claiming they are solely due to natural variability. What's more, the House of Representatives voted to defund federal science programs that gather and analyze the data essential to understand changes in global weather patterns and other climate impacts. But all this denial cannot make this threat disappear. We must act before cruel Aprils occur every month.

Download this report (pdf)

Download the introduction and summary (pdf)

Daniel J. Weiss is a Senior Fellow and the Director of Climate Strategy, Valeri Vasquez is a Special Assistant for Energy Policy, and Ben Kaldunski is a former intern with the Energy Team at American Progress.  The authors thank Dr. Heidi Cullen, CAPAF Senior Fellow Dr. Joseph Romm, and CAPAF Think Progress Climate Editor Brad Johnson.

See also:

Related Posts:

Saturday, April 23, 2011

What’s With the Weather? Is Climate Change to Blame?

One of the thorniest questions facing climate scientists is whether human-induced climate change is leading to more heat waves, floods, and extreme weather events. Now, employing increasingly sophisticated methods of studying weather extremes, climatologists say they are closer to answering that key question.

Environment 360, 21 APR 2011: ANALYSIS

by alyson kenward

On July 29, 2010, temperatures in Moscow climbed above 100 degrees F for the first time in recorded history, soaring to 102 degrees F. During a 35-day run when the daytime temperatures reached at least 86 degrees F, more than 10,000 people died in Moscow alone because of the sweltering conditions, Russian officials said. Throughout the summer, wildfires burned more than 1.6 million acres in western Russia and the heat-induced drought in July and August destroyed 40 percent of the country's annual grain yield.

Almost from the beginning, Kevin Trenberth's phone began ringing off the hook. Trenberth leads the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado, where he studies how different manifestations of global climate connect with each other. That makes him one of the usual suspects when extreme weather events — including floods, heat waves, and intense storms — strike and the public wants to know: Is this happening because of climate change?

"We get requests from the media as these events are unfolding," says Trenberth, "but it isn't necessarily dealt with properly by anyone in the [climate science] community." What happens all too often, he says, is that scientists end up giving the same hand-waving explanation that while climate change may well have contributed to whatever is going on, it can't be definitively fingered as the sole cause.

Finding trends in extreme climate events is tricky because extreme events are rare, by definition. It's instinctive to compare heat waves, droughts, or floods to average weather conditions. So for the Russian heat wave, say, you would look to compare it to an average summer month. But climate scientists who specialize in statistics say this doesn't capture the true nature of an extreme event. Instead, they argue, you need to compare the most extreme events to "ordinary" extreme events.

"When you're looking at extreme heat, for example, you need to compare the five hottest days in every year, or the highest overnight temperature in a month," says Gabriele Hegerl, a professor of climate science at the University of Edinburgh in Scotland. She says this so-called "extreme value approach" is a better way to see the long-term variations in even the most unusual kind of event.

Using the extreme value approach, Hegerl and others have confirmed that the number of extremely hot nights around the world has been increasing over the past few decades. And when they've looked to computer models to see if the same sort of thing might have happened in an emissions-free world, they don't see it. It seems, says Hegerl, that the increased frequency of heat waves may well have been caused by human behavior.

"It is really interesting because for years we've just laid back and said, 'You can't say anything about a single climate event,'" says Hegerl. "But now people are showing you can say something about the probability of that kind of event occurring."

In a broad sense, this is what detection and attribution — D&A, to insiders — is all about. It involves searching for both changes in climate trends and for what, exactly, has caused these changes. Basically, D&A is broken down into two questions: Is the climate changing? And if so, are humans causing it?

The simplest and oldest question in D&A is whether rising atmospheric concentrations of carbon dioxide (CO2) caused by human activities, such asfossil-fuel burning and deforestation, are driving up Earth's average temperature. The increase in CO2 was first confirmed in the 1960s, and rising temperatures in the late 1980s. But simply spotting the two trends wasn't proof there was a cause-and-effect relationship. In order to show that, scientists had to rule out other possible explanations.

In recent years, climate scientists have run thousands of computerized simulations to compare the climate with only so-called natural "forcings" — from the sun, volcanic activity, and other non-human influences on temperatures — and also with greenhouse gas emissions exactly as they have been in real life. Time and again, the simulations run with added CO2 have shown a warming climate, which allowed the 2007 Fourth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) to conclude that humans are the overwhelming cause of increasing global temperatures.

Climate models also show that global temperatures should continue to rise as concentrations of CO2 and other greenhouse gases increase, and suggest that heat waves, severe floods, and powerful storms will also become more frequent. But most people want to know: Is it already happening?

"When an extreme climate event occurs, then people find out what we are not adapted to," says Francis Zwiers, a statistician who directs the Pacific Climate Impacts Consortium in Victoria, British Columbia, and was the lead author of the chapter on climate attribution for the IPCC's Fourth Assessment Report. "These events cause a lot of damage, so people are particularly interested in them."

The best that 2007 report could do was to say that such extreme events asheat waves and heavy rainstorms had "likely" become more frequent over the past 50 years, and that extreme heat waves in particular were at best "more likely than not" to have been caused by human activity.

Because these kinds of events happen so infrequently, it's been difficult for scientists to confirm any difference in their long-term trends. Extreme heat waves might hit a region only once every 10 or 20 years, and mega-droughts may only happen a couple of times a century.

This is where the extreme value approach becomes key. To confirm that temperature extremes have been rising globally over the past few decades,D&A scientists have used as a benchmark the hottest days and nights of every month and every year. To see if heat waves — defined as three or more days of excessively high temperatures — are increasing, they've looked for the hottest three-day periods for each year going back through the last century's weather records. In both cases, it's clear that what used to be highly unusual events are now becoming more common.

That was the detection part. For attribution to human influences, climate scientists have looked once again to climate models. And once again, models in which only natural forces are at work don't show any clear trend, but those with human emissions match the rise in heat waves that scientists have detected.

More recently, Hegerl and Zwiers have also looked at how heavy-rainfall events are changing. It's tougher here than it is with heat waves. That's because, unlike heat events, really heavy rainfall — the kind that can lead to vicious floods — usually lasts for hours, not days or weeks, and often happens over just a few hundred square miles. So far, Zwiers says, scientists have some good evidence that precipitation extremes are also on the rise worldwide, but it's too early to say anything with complete confidence. "We're at the beginning stages on extremes and on figuring out what the human influence is," says Zwiers.

In terms of finding a human fingerprint in more frequent extreme climate events, he says the detection and attribution community is at a similar place today as it was a few decades ago when scientists were trying to prove, beyond a reasonable doubt, that greenhouse gas emissions had led to global warming. "We are ready to make a cautious statement that humans have contributed to more climate extremes because most of the evidence suggests this," says Zwiers. "But it's not a sure thing yet."

For most people, that's not good enough. They don't care about statistics; they care about actual weather. When events like last summer's heat wave strike, researchers still have to field questions about what role global warming played in that particular situation.

Answering those questions is one way scientists think they can help people better understand how climate change will affect them in the future. It will always be true, as researchers have cautioned, that no single climate event can be caused entirely by climate change. "There have always been extreme events," says Peter Stott, a climatologist from the UK's Met Office. "Natural variability does play a role, but now so does climate change. It is about changing the odds of the event happening."

What Stott and others have started to do, therefore, is to use climate models to compare how often specific extreme events having the characteristics of, say, the 2010 Russian heat wave, would happen without man-made CO2, and how often they should happen with it. Then they canwork backwards and apply the extreme value approach to the most unusual events the climate models have simulated.

For example, after the blistering 2003 summer heat wave in Europe, Stott's group calculated that greenhouse gas emissions had more than doubled the likelihood that such a heat wave would have occurred. More recently, they found that man-made climate change had also increased the odds in favor of the kind of major flood that struck the United Kingdom in 2000. Their analysis doesn't rule out the possibility that the events were strictly natural, but they do show that greenhouse gas emissions have made those particular extremes much more likely.

According to Trenberth, however, while the idea of calculating the changing odds is a good one, in practice the computer models aren't good enough to capture how the entire global climate might influence a particular region.

"To get a really good answer, you need a perfect climate model," he says. "And to the extent that climate models aren't perfect, then where does the error go?" All too often, according to Trenberth, scientists err in the direction of saying extreme climate events are still largely caused by natural variations.

The question of how well global climate models can be used to zoom in on regional climate has already emerged as scientists search for the causes of last summer's Russian heat wave. Recent reports from the National Oceanic and Atmospheric Administration (NOAA) suggest that the climate conditions leading to the heat wave were primarily natural and that any human influence was drowned out by climatic variations that are inherently part of the natural system, rather than triggered by human influences.

What concerns Trenberth is that NOAA's analysis doesn't take into account the abnormally high sea surface temperatures in the Indian Ocean at the time of the heat wave, and how that could have influenced what happened in Europe. Even though the Indian Ocean and Western Russia are thousands of miles apart, the climate systems over them are interwoven. The Indian Ocean temperatures can affect air pressure across Asia, for example, which can influence the flow of the jet stream — and the influx of warmer air masses from the south — across Europe. "They haven't answered the question of what caused the sea surface temperatures to get so high," he says, which means that scientists can't really be certain yet if global warming was a major cause of last year's extreme heat in Russia.

Nevertheless, Trenberth and others in the D&A community say that the science of climate extremes is vital. Tracking how extreme events are changing, and searching for a human fingerprint in them, is an important step in helping the public figure out what they can expect from future climate.

"For extreme events, the question isn't, 'Is it global warming or natural variability?'" says Trenberth. "It is always both. The question is just how much each is contributing." 


Monday, April 11, 2011

Plants’ earlier bloom times hurting some creatures

By Brigid Schulte

Cristol Fleming has gone out hunting for the first wildflower blooms of spring for close to four decades. She knows where every tiny bluish clump of rare phacelia can be found, where every fragile yellow trout lily grows.

And in the definitive guide she co-authored on finding Washington area wildflowers, she writes that mid- to late April is the best time to see the forests and riverbanks carpeted with a riot of these delicate blooms.

So it was with some consternation that the local field botanist found two of her favorite early flowers — sprigs of white and purple "harbinger of spring" no higher than an inch and graceful white twinleaf — in full bloom in the chill of late March.

Fleming was expecting to see some "spring beauty," one of the earliest bloomers of the area, and perhaps a few of the weedier species. But she found, among others, "Dutchman's breeches," the funny little white flower that looks like long underwear hanging upside down, and cut-leaved toothwort.

"I was surprised to see that," she said. "That's something I would have expected two weeks later."

Bloom hunters like Fleming, who for 40 years have been tramping through the woods, roaming along riverbanks and scrambling over rocky outcrops to document the first blooms of spring in the Washington area, worry that what they have been seeing is nothing less than the slow, inexorable shift of global warming.

They even have a name for it: season creep. And it's happening all over the world.

Warning signs

For 1,000 years, the Koreans have recorded the first cherry blossoms of spring, so central is that flower to their cultural identity. For 300 years, Europeans have meticulously tracked when grapevines bloom to time planting and harvest. On both continents, botanists are finding earlier and earlier blooming.

In Washington, chronicling blooms began as sort of a rite of spring for botanists and amateur flower lovers eager to see the first signs of life after a long, barren winter. Initially, they wrote down their findings for more than 600 species in an enormous log book at the Smithsonian Institution's National Herbarium year after year.

But over time, they began to notice the native blooms coming earlier and earlier. In a 2005 analysis of 100 of the most popular flowers they hunted, Smithsonian botanists found that 90 species bloomed two to 44 days earlier than they had 20 years ago; only 10 species, on average, bloomed later. Even the famous cherry blossoms along the Tidal Basin, they found, were blooming six to nine days earlier than they had in the 1970s.

"There is always variation from year-to-year in nature. And I don't want to sound alarmist that spring is coming earlier and earlier," said Fleming, who is in her 70s. "But, boy, every year, we do feel it."

Botanists poring over the Asian and European blooming records, the Smithsonian log, and the accounts of American observers such as Henry David Thoreau and explorers Meriwether Lewis and William Clark, are finding the same phenomenon.

"When you gather together all the scientific studies that have documented this, we can see that about 80 percent of the species are changing earlier in the spring," said Jake Weltzin, an ecologist with the U.S. Geological Survey.

Weltzin was in town this week trying to drum up support for the new National Phenology Network that he runs. Its aim is to do for the entire United States what bloom hunters like Fleming have done for the Washington area: track the patterns of season creep and explain why that matters to humans.

It's already begun to matter for the plant and animal world. Some species in the complex chain of interdependence have been unable to keep up.

In Europe, the leaves of the English oak are coming out earlier, Weltzin explained, which means the winter moth caterpillar that feeds on them are also coming out earlier. But the pied flycatcher birds that eat those caterpillars are still migrating north at that time, so when they do arrive, the caterpillars have already turned into moths and are gone. That has decimated the bird population in recent years.

Likewise in San Francisco, some populations of the Edith's Checkerspot butterfly are simply gone. With the gradual warming of Earth and ocean temperatures that have also shifted rainfall patterns, the leaves of the plantago plant come out earlier. The leaves, which the checkerspot caterpillar depends on for food, are already dried and withered by the time the larvae emerge.

And these failures to adapt, or adapt in time, are what worry Cris Fleming.

A search for spring

On a blustery April evening this week, Fleming, bundled up in a blue fleece jacket and warm hat and gloves, joined about a dozen other bloom hunters at the Carderock Recreation Area in Maryland along the Potomac River. For three decades, Fleming taught plant identification for the U.S. Department of Agriculture and led blossom hunting trips for the Smithsonian and the Audubon Naturalist Society. For her, the search for early spring blooms started out as an almost metaphysical pursuit.

"This is something you can count on year after year — the return of spring, the thrill of seeing that life returns, beauty goes on," she said, poking at the dead brown leaves on the ground with a stick to better see the minuscule white blooms of early saxifrage. "In a world where there's so much you can't count on, it's nice to have something that you can."

She made her way down a steep, muddy set of stairs leading to the Potomac. The banks were awash in swaths of Virginia bluebells. She trod among delicate yellow buttercups, pungent wild spring onion leaves and the innocent white petals of bloodroot, whose red sap is toxic to people.

She ducked under two trees that have fallen in recent windstorms and came to a rocky slope.

She scanned the rock face for signs of the butterfly-shaped leaves of the twinleaf, Jeffersonia diphylla, named for Thomas Jefferson, that she found in such abundance in late March. She spotted one or two. The rest had already disappeared for the season.

That's why plants like harbinger of spring and twinleaf are called ephemerals, Fleming explained: The blooms last barely a week. "Neither of them will be here in the next few days," she sighed.

What if, she worries, these plant life cycles are speeding up, but their insect pollinators' life cycles are not? And what if the warming Earth changes the habitat?

"Unlike animals, plants can't just get up and move," she said. "If they end up in a climate that's too warm, well, they'll just die."

On April 15, Fleming will go on another of her early spring bloom walks along Turkey Run. "But I may already be too late."

Friday, April 8, 2011

Climate beliefs change with the weather

By Genelle Weule for ABC Science Online

ABC News Online, Fri Apr 8, 2011 
US researchers have found people's climate beliefs blow hot and cold depending upon the weather of the day.
When people think the day's temperature is hotter than usual they are more likely to believe in and feel concerned about global warming.
Likewise, when the day's temperature is lower than usual, people's belief in global warming plummets.
These are the findings of a new study from Columbia University's Centre for Research on Environmental Decisions published in Psychological Science.
"This myopic focus on their immediate experience suggests that people's beliefs can be as mercurial as the weather," the researchers wrote.
Using an online survey, the researchers asked a group of 582 people from the United States to report how convinced they were that global warming is happening and whether they were concerned about global warming.
They were also asked whether they thought the day's temperature was warmer or colder than usual for that time of year.
Wanting to gather data during summer as well as winter, the researchers asked a group of 290 Australians the same questions a week later.
To test whether people's perceptions translated into action, the researchers asked another 251 people whether they would donate a small amount of money to an environmental charity after they answered the survey.
The researchers found people's perception of daily temperature also influenced whether or not they would donate to the charity.
They say their results raise the question of why beliefs in global warming are affected by daily temperatures.
"Global warming is so complex, it appears that some people are ready to be persuaded by whether their own day is warmer or cooler than usual, rather than think about whether the entire world is becoming warmer or cooler," lead author Dr Ye Li said.

'Scientifically irrational'

Professor Andy Pitman, co-director of the Climate Change Research Centre at the University of New South Wales, says people's beliefs about global warming are heavily influenced by recent events.
But he says people are also quick to discount disasters quite rapidly.
"It's a very complex area of human psychology," said Professor Pitman, who co-authored a paper on the psychology of global warming that appeared last year in the Bulletin of the American Meterological Society.
"There is no relationship whatsoever between individual weather days and climate trends. Climate is a process operating on timescales of decades not days.
"It is utterly scientifically irrational for there to be a relationship the weather on the day you answer that kind of questionnaire and your belief around global warming."
Professor Pitman believes this latest paper will help climate scientists understand how humans take on board information around climate change.
"We're rather desperate to understand the relationship between how people assimilate information in decision making and climate change in the hope that we can learn to communicate the science around climate change in ways that allow people to make informed decisions," he said.
"As this paper suggests, people are personally embedded in something they think is climate in day-to-day weather and [climate scientists] haven't managed to convince them that what they see on a day-to-day basis is not relevant to the problem."

Sunday, April 3, 2011

Expert warns of reef climate change deadline

 By Kirsty Nancarrow 

ABC Online, Posted April 4, 2011      

See also CSIRO boss backs carbon price

A Queensland climate change scientist says the world has only another decade to reduce greenhouse gasses to save the Great Barrier Reef. 

The director of the Global Change Institute at the University of Queensland, Professor Ove Hoegh Guldberg, is addressing a climate change conference in Cairns in the far north today. 

Professor Hoegh says coral bleaching events are becoming more frequent because of rising sea temperatures and levels. 

He says good management and the low population along the Great Barrier Reef have helped it bounce back in the past, but it could be gone in 40 years if carbon emissions are not reduced. 

"If we actually act today we can save the Great Barrier Reef and reefs around the world," he said. "What it'll take is a very concerted global effort to remove these dangerous gasses from the atmosphere." 

He says climate modelling shows sea temperatures and ocean acidification will soon rise to levels that cannot sustain coral reefs. 

"We're really right at the crossroads right now," he said. "If we go another 10 years of pumping two parts per million or more CO2 into the atmosphere, we'll pass a point at which we won't be able to constrain further temperature increases and greenhouse gas concentrations that will allow reefs to persist."

Friday, April 1, 2011

Flood of floods: Here comes the rain

 by Stephen Battersby and Michael Le Page 

New Scientist Magazine issue 2804, 24 March 2011

Torrential downpours are on the increase, but is climate change to blame?

IT WAS a monstrous monsoon. Over just a few days in late July last year, more than 300 millimetres of rain fell on northern Pakistan. As the water swept down the river Indus, it killed close to 2000 people and affected 20 million more.

Pakistan was not the only place to suffer. Australia, China, Thailand, Brazil, the Balkans, Bangladesh, Indonesia, Colombia, the Philippines, Sri Lanka and Tennessee all experienced devastating floods in the past year. What's more, there were unusually heavy snowfalls in many regions, severely disrupting transport systems. Globally, 2010 was the wettest year ever recorded (see chart).

Inevitably after such freakish weather, people ask whether climate change is to blame. Is this flood of floods our fault? And is there worse to come?

There is no doubt that temperatures are rising, and basic physics suggests that warmer means wetter, because warmer air can hold more moisture. Observations confirm that the lower atmosphere holds about 5 per cent more water than a century ago, giving it that much more ammunition to unleash in a downpour or blizzard.

So does this explain the recent floods? "I don't think it is legitimate to assume that climate change played a role in these events until we've done the work," says Myles Allen of the University of Oxford. There are many types of floods, he says, and while climate change is making some extreme events more likely, others - such as floods caused by melting snow - may become less likely.

Others think we can say more. "The pervasive increase in water vapour changes the intensity of precipitation events with no doubt whatsoever," Kevin Trenberth of the US National Center for Atmospheric Research told a meeting in January. "Yes, all events. Even if temperatures or sea surface temperatures are below normal, they are still higher than they would have been, and so too is the atmospheric water vapour amount and thus the moisture available for storms."

For instance, El Niño's sister phenomenon, La Niña, which brings warmer surface waters - and thus moister air - to the western Pacific, has been blamed for some of the recent floods in Asia and Australia. Trenberth thinks high sea surface temperatures due to global warming also played a role. "The La Niña in place favours these sorts of things but the extra high sea surface temperatures make them record breaking," he says.

It is clear is that rainfall patterns are changing. In the US, for instance, Kenneth Kunkel of the National Climatic Data Center in Asheville, North Carolina, is analysing data from more than 1000 rain gauges across the country. Over the past century, the intensity of extreme rainstorms that occur once per year on average has risen 1.4 per cent per decade, he has found.

Put another way, in 2000, a "once a year" storm was dumping 14 per cent more rain on average than it would have done in 1900. "It would be wise for society to take this into account when building a dam to last 50 or 75 years, or a new housing development," says Kunkel. "That's not really done right now."

At the top end of the scale, the intensity of "once in 20 years" extreme rainstorms is increasing even faster, by about 3 per cent per decade, although for these rare events the statistics are more patchy. The overall increase also seems to be accelerating. Most of it happened in the last three decades, and Kunkel hasn't even included 2010 in his analysis yet. "I'm guessing that when the data come in, they are going to be high," he says.

Bucketing down

Similar studies show that heavy rainfall has been increasing in intensity in most other parts of the world too, just as climate models predict. These results can't be put down to changing methods or issues with scientific instruments, as almost all the figures come from the humble rain gauge, unchanged over the decades. "It is basically an 8-inch bucket - so simple that you almost can't call it technology," says Kunkel.

Insurers are also seeing a rise in claims for flooding. Much of that increase can be put down to demographic factors such as population growth, but not all, says Gerd Henghuber, a spokesman for insurance firm Munich Re. Claims for flood damage are rising faster than claims for other natural disasters such as earthquakes and eruptions. "The growing number of weather-related catastrophes most probably cannot be fully explained without climate change," he says.

So physics says that climate change should lead to more rain, buckets say that more rain is indeed falling and insurers say that that more rain is causing more destruction - a chorus suggesting that we are, indeed, bringing the skies down on our heads. But the case is not quite closed.

In theory, some long-term natural oscillation might be to blame for higher rainfall over the past few decades. "The climatic system has natural periodicities that operate over a whole range of timescales," says Stephen Gale of the University of Sydney in Australia, who studies past flooding. To pin the blame squarely on humans, we need to rerun the past century minus all the added greenhouse gases to see what happens. In the absence of spare Earths or a time machine, that means using computer models.

In 2007, a team including Francis Zwiers, now at the University of Victoria in British Columbia, Canada, compared the results of 14 different models with rainfall data for the 20th century. Simulations that included greenhouse gas emissions matched the overall global pattern seen so far, with increased rainfall in most regions (Nature, vol 448, p 461).

Still, that study looked only at total precipitation. When it comes to flooding, what really matters is whether climate change is making extreme events even more extreme. This is a harder question to tackle, partly because extreme events are rare and so our information on them is limited.

Extremes are also difficult to model, says Elizabeth Kendon of the Met Office Hadley Centre in Exeter, UK. One reason for this is that small-scale processes are important. The internal convection of storm clouds has to be approximated, for example, as it is on far too fine a scale to be modelled.

Despite these difficulties, Zwiers's team has now done a study comparing records of extreme rain and snowfall in the northern hemisphere between 1951 and 1999 with the simulations produced by eight models (Nature, vol 470, p 378). "Our research indicates that humans have contributed to the observed intensification of extreme precipitation, but we're not in a position to quantify the magnitude of that contribution," says Zwiers.

In another recent study, a team led by Allen looked at one specific event - the floods that occurred in the UK in autumn 2000, causing £1.3 billion worth of damage. With the help of computer time loaned by volunteers via, they did thousands of model runs simulating the weather in 2000, with initial conditions both as they were and as they could have been without global warming. They found that climate change had nearly doubled the likelihood of this kind of flood (Nature, vol 470, p 382).

The limited predictive ability of the model does not undermine this finding, Allen says. The work is comparable to proving dice are loaded: you don't need to be able to predict the outcome of each throw, you just need to show sixes occur more often than expected.

Put it all together, and the case that global warming is leading not only to more rainfall but also to more extreme rainfall, and is thus making severe floods more likely, starts to look very convincing. The big question is, what is going to happen next?

For every 1 °C rise in temperature, the water content of the lower atmosphere is expected to rise by 7 per cent, and temperatures could rise by 4 °C as early as the 2060s. That is a lot more water in the air.

Climate models suggest that precipitation will become more intense pretty much everywhere: when it rains or snows, it will rain and snow harder. However, there could be longer dry intervals in between, and in some parts of the subtropics less rain than usual may fall overall. "You can have a situation where mean rainfall decreases but the extremes increase," says Kendon. In other words, droughts interspersed with floods.

Models produce a wide range of results when it comes to extreme events in the future. Take winter rainfall in the UK, for example. Based on a scenario in which global temperature rises just 3 °C, some models suggest that rainfall on the worst 5 per cent of rainy days will increase by 50 per cent, while others suggest there will be no change.

What's more, how much rain falls is only part of the story. Warmer weather will dry out soils faster, for instance, which reduces the risk of flooding. "The relationship between precipitation and flooding is immensely complex," says Gale. "Variation in a single factor, such as soil moisture, might mean that the same precipitation event could remain in channel or could generate a significant flood."

On top of this, we are changing the physical nature of river valleys enormously. "What we do is take all the agricultural land around cities, pave it, build houses and put in storm-water sewers, all of which stops water from infiltrating the ground where it can be stored and percolate slowly into rivers," says Gale. Instead, rainwater is funnelled rapidly into main rivers. So part of the problem with forecasting future flooding is not knowing how the landscape will change.

Flooded out

Another big problem is that we do not even know what has happened in the past. Records of flooding in most countries are surprisingly brief and full of holes. "We don't have the data to predict big events," says Gale.

Most government planners and engineers, for instance, rely on river gauging stations to record the height that floodwaters reach. These are a relatively recent tool. "If you have flood data that go back 50 years, you are doing well," says Gale.

What's more, gauging stations can be overwhelmed by a big flood. "Before 2010, the last big floods in Australia were back in 1990," says Gale. "More than 220,000 square kilometres was under water; the town of Nyngan was evacuated, entirely flooded out, and we have no idea how high the flood there was. Hydrologists went in after the event and estimated the discharge indirectly, but there is no official record, the flood was just too big."

So predicting the extent of a 100-year flood means extrapolating using statistical methods, and the results depend heavily on the assumptions used. "Predictions for the flow rate of a 100-year flood might vary by a factor of 4 or more," he says.

Zwiers's latest study suggests there could be yet another problem with flood forecasts. The team's "best guess" is that the observed increase in extreme precipitation is two to three times larger than global climate models predict, though the uncertainties are huge. It is possible that natural variability is to blame, but the most likely explanation is that the models are underestimating the extent of change. If so, extreme precipitation events may strengthen more quickly in the future than projected and have more severe impacts.

The upshot of all this is that while all the evidence points to an increased risk of flooding in large parts of the world as a result of climate change, no one can say for sure exactly which parts of the world or how bad it will be. This is not much help to planners.

Nevertheless, there are many measures we could be taking, from banning development in flood-prone areas to using permeable paving to reduce run-off. "There are lots of sensible things we could do," says Allen. He points out that these would benefit us whatever happens, come rain or shine.

Stephen Battersby is a consultant for New Scientist based in London. Michael Le Page is a features editor