Roger Highfield, The Telegraph 7 Aug 08;
Predictions that our warming world will become a wetter place have been confirmed by a study which suggests that extreme rainstorms will rise by more than 10 per cent by the year 2050.
For some time, computer models of the global climate have predicted that global warming will increase the intensity of rainfall and it is these extremes, linked with landslides and flooding, that are among the major impacts.
Today, in the journal Science, a study conducted at the University of Reading and the University of Miami provides the first hard evidence to confirm the link between a warmer climate and more powerful rainstorms, suggesting that extreme events may be even more common than predicted.
To understand how rainfall responds to a warmer climate, the researchers used a natural long term climate pattern called El Niño, a warming originating in the Pacific off South America that influences weather patterns around the globe.
Based on nearly 20 years of satellite observations the team could examine the relationship between tropical rainfall and changes in surface temperature as well as in atmospheric moisture.
In this way, they found a distinct link between tropical rainfall extremes and temperature, with heavy rain events increasing during warm periods and decreasing during cold periods.
"A warmer atmosphere contains larger amounts of moisture which boosts the intensity of heavy downpours," said Dr Brian Soden, at the University of Miami.
Changes in heavy rainfall seem to keep pace with atmospheric moisture which rises by around 7 per cent for each ºC of warming. Based on computer models, this could mean an increase in the intensity of heavy rainfall of around 10 per cent by 2050.
However, the observed increase in extreme downpours appears to be larger than the increases predicted by current computer simulations, suggesting that predicted changes in rainfall due to global warming may be underestimated, either because of flawed measurements or because computer models lack some key understanding, for instance of the action of aerosol particles in the atmosphere.
The researchers say it is difficult to be precise about the underestimate and add that it is crucial to determine the cause for this discrepancy as soon as possible in order to accurately understand the implications of global warming and its effects on the water cycle.
"Comparing observations with results from computer models improves understanding of how rainfall responds to a warming world" said Dr Richard Allan, NERC advanced fellow at the University of Reading's Environmental Systems Science Centre. "Differences can relate to deficiencies in the measurements, or the models used to predict future climatic change.
"Heavy rain in the tropics is often associated with thunderstorms and the same processes will also apply in the UK, particularly in summer where intense downpours are fueled by the additional moisture carried by warm, humid air.
"One of the most serious challenges that humanity will face in response to climate change is adapting to changes inextreme weather events. There is a major concern that heavy rainstorms will become more common and more intense in a warmer climate.
"Floods can completely devastate areas and people's livelihoods and so this knowledge could have massive implications on how we plan for our changing climate in the future."
Tropical downpours worsening, say scientists
Alister Doyle, Reuters 7 Aug 08;
OSLO (Reuters) - Tropical downpours are becoming more frequent and the trend seems worse than expected, bringing greater risks of flash floods, scientists said on Thursday.
"As the tropics warm are seeing an increased frequency in the heaviest rainfall," said Richard Allan of the University of Reading in England, who co-authored a study of tropical rains with Brian Soden of the University of Miami.
The satellite review of tropical rainstorms since the 1980s gave the first observational evidence to confirm computer models that predict more intense cloudbursts because of global warming stoked by human activities, they said.
Writing in the journal Science, they also said the trend to extreme soakings was stronger than predicted by computer models "implying that projections of future changes in rainfall extremes ... may be underestimated".
The findings were based on a study of the tropical oceans, where satellites can more easily record rainfall. Allan said the trends were likely to be matched over land.
The U.N. Climate Panel, drawing on the work of 2,500 scientists, said last year that rainfall was likely to get more intense in many tropical regions, raising risks of flash floods, erosion and mudslides.
The satellite data showed 2-3 times more intense downpours than predicted by the climate models, stoked by rising emissions of greenhouse gases from burning fossil fuels in cars, factories and power plants.
But Allan told Reuters that the finding that the models were too cautious was less certain than the conclusion that tropical rainfall had become more intense. Checks might be made on land by examining rain gauges or river flows, he said.
FARMING THREATENED
The U.N. Climate Panel says that shifts in rainfall patterns are likely to disrupt farming in many regions, affecting the livelihoods of hundreds of millions of people especially in Asia, Africa and Latin America.
"Flash flooding can cause damage to settlements and societies. It can contaminate ground water, drinking supplies, with potential health effects," Allan said.
"The spread of disease can be impacted by heavy rainfall," he added. "Very intense rainfall can destroy crops. There are also possibilities of enhanced erosion, degradation of soil".
Scientists say it is hard to link climate change to one-off events such as floods in the U.S. Midwest that damaged millions of acres of cropland in June.
The U.N. Panel predicts more rain overall this century in many parts of the tropics and towards the polar areas with declines in middle latitudes such as the Mediterranean basin, the Western United States and southern Africa.
(Editing by Andrew Roche)
Extreme Rains to Be Supercharged by Warming, Study Says
Mason Inman, National Geographic News 7 Aug 08;
Global warming could make extreme rains stronger and more frequent than previously forecast, a new study suggests.
Such a scenario could make floods fiercer, damage more crops, and worsen the spread of diseases such as malaria, scientists say.
Rainfall patterns are already shifting as Earth warms under a blanket of humanmade greenhouse gases, experts say.
Study co-author Richard P. Allan, an atmospheric scientist at the University of Reading in Berkshire, United Kingdom, said previous studies have shown that "wet regions are becoming wetter, and dry regions drier."
The study team analyzed satellite images of rainfall over tropical oceans over nearly two decades, from 1988 to 2004.
The researchers found that during El Niño years, which tend to be warmer, rain fell in heavier showers. An El Niño is a climate event where the flow of abnormally warm surface Pacific waters temporarily changes global weather patterns.
"This is something that climate models had predicted," Allan said. "But getting the data from observations is very important."
Many previous rainfall pattern studies have relied on measurements from rain gauges. Such gauges are sparsely distributed across land, Allan said, whereas satellites can see large areas as a whole.
Global Warming Forecast
Although our planet is warming overall, Earth's climate still varies between warmer and wetter El Niño years and cooler and drier La Niña years.
Looking at these changes in rainfall can give scientists a good estimate of what will happen with continued global warming, according to Allan and his co-author, Brian Soder of the University of Florida.
With continued global warming, the changes in Earth's rainfall patterns will be worse than previously forecast, Allan and Soder write.
"The models seem to underestimate the response in extreme rainfall with warming," Allan said.
For every 1.8 degree Fahrenheit (1 degree Celsius) rise in global temperature, heavy rain showers became more common, with most intense category jumping 60 percent, says the study, which will be published tomorrow in the journal Science.
During the 20th century, Earth's average global temperature rose about 1.2 degrees Fahrenheit (0.7 degrees Celsius). But researchers predict that bump will be dwarfed by the warming to come.
The latest UN Intergovernmental Panel on Climate Change report predicts at least three times as much warming—about 3.2 to 7.1 degrees Fahrenheit (2 to 4 degrees Celsius)—by the end of the 21st century. (See [April 6, 2007].)
The uptick could drive a big jump in intense rainfall events, Allan and Soden argue.
Human Impact
Warmer air can hold more moisture. "So if the air is more moist, you get more heavy rainfall," Allan noted, adding that such extreme weather takes a toll on people.
With intense rains, "you can get flash flooding, and heavy rainfall can destroy crops," he said. "Those are the most immediate impacts."
Coupled with rising global temperatures, more frequent and intense rainfall has "major implications for infectious diseases," said Paul Epstein, a tropical disease expert at Harvard Medical School in Boston, Massachusetts.
"After floods one often sees clusters of vector-borne diseases—malaria, dengue fever, Japanese B encephalitis," Epstein said.
Floods often cause a jump in cholera and other water-borne diseases, as well as plague and other rodent-borne diseases, he added.
David Neelin, a climate scientist at the University of California in Los Angeles, takes a more cautious view of the study results.
"Rainfall changes remain among the hardest impacts of global warming to predict precisely," he said.
But, Neelin added, "Allan and Soden's results add fuel to the growing concern from a number of research groups [that] the extremes of rainfall may increase under global warming."
