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  Yahoo News 23 Jul 07
Global warming has already changed world's rainfall patterns: study

PlanetArk 24 Jul 07
Humans to Blame for Global Changes in Rain - Study
Story by Deborah Zabarenko, Environment Correspondent

National Geographic News 23 Jul 07
Humans Changing Rainfall Patterns, Study Says
John Roach

BBC 23 Jul 07
Humans 'affect global rainfall'

Human-induced climate change has affected global rainfall patterns over the 20th Century, a study suggests.

Researchers said changes to the climate had led to an increase in annual average rainfall in the mid-latitudes of the Northern Hemisphere. But while countries such as Canada, Russia and northern Europe had become wetter, areas including India and parts of Africa had become drier, they added. The findings will be published in the scientific journal Nature on Thursday.

Climate models have, for a number of years, suggested that human activity has led to changes to the distribution of rain and snow across the globe. However, the computer models have been unable to pinpoint the extent of our influence, partly because drying in some regions have cancelled out moistening in others.

Making the link

The team of scientists from Canada, Japan, the UK and US used the patterns of the changes in different latitude bands instead of the global average. They compared monthly precipitation observations from 1925-1999 to those generated by complex computer models to see if they could identify if human activity was affecting rainfall patterns.

"We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands," the researchers wrote in the paper. "These changes cannot be explained by internal climate variability or natural forcing."

They added that natural factors, such as volcanic eruptions, had contributed to shifts in the global rainfall patterns but to a much lesser extent.

One of the paper's co-authors, Nathan Gillett of the Climatic Research Unit at the University of East Anglia, UK, said the team's findings helped clear up any uncertainty.

"This study shows that there has been a significant human effect on global rainfall patterns, with human influence causing a decrease in rainfall in some regions, and an increase in rainfall in others."

However, Dr Gillett said it was not possible to make a direct link between the recent floods in the UK and human-induced climate change.

"While our study shows a human influence on rainfall at the global scale, the role of human influence in the UK flooding remains uncertain.

"Climate models generally predict that the UK will become wetter in winter and drier in summer," he explained. "In the UK we have seen a trend towards more extreme rainfall in the winter but no clear trend in summer extreme rainfall."

PlanetArk 24 Jul 07
Humans to Blame for Global Changes in Rain - Study
Story by Deborah Zabarenko, Environment Correspondent

WASHINGTON - Human activities that spur global warming are largely to blame for changes in rainfall patterns over the last century, climate researchers reported on Monday.

The report was released as record rains caused deadly flooding in Britain and China. Human-caused climate change has been responsible for higher air temperatures and hotter seas and is widely expected to lead to more droughts, wildfires and floods, but the authors say this is the first study to specifically link it to precipitation changes.

"For the first time, climate scientists have clearly detected the human fingerprint on changing global precipitation patterns over the past century," researchers from Environment Canada said in a statement.

The scientists, writing in the journal Nature, found humans contributed significantly to these changes, which include more rain and snow in northern regions that include Canada, Russia and Europe, drier conditions in the northern tropics and more rainfall in the southern tropics.

So-called anthropogenic climate change has had a "detectable influence" on changes in average precipitation in these areas, and it cannot be explained by normal climate variations, they wrote.

LIVING WITH MORE FLOODS

Weather experts in Britain raised the possibility that the current rains there may be related to climate change.

"The global climate models indicate a future for the UK with drier summers and wetter winters, but storm events in the summer are predicted to be more frequent and more intense," David Butler of the University of Exeter said in a statement. "So it may well be the case that we will have to learn to live with more flooding.

Nick Reeves, executive director of the Chartered Institution of Water and Environmental Management in Britain, said, "Extreme events such as we have seen in recent weeks herald the specter of climate change and it would be irresponsible to imagine that they won't become more frequent."

Numerous studies and a report by a panel of scientists convened by the United Nations have reported with increasing certainty that human activities -- notably the burning of fossil fuels that emit greenhouse gases -- have contributed to global warming in the last half-century and that the effects of this are already evident.

The UN's Intergovernmental Panel on Climate Change estimated temperatures would rise 3.2 to 7.3 degrees Fahrenheit (1.8 to 4.0 degrees Celsius) by the year 2100, leading to more hunger, water shortages and extinctions

National Geographic News 23 Jul 07
Humans Changing Rainfall Patterns, Study Says
John Roach for National Geographic News

Humans have caused global precipitation patterns to change substantially over the past century, new research says.

About 1.8 inches (4.5 centimeters) more rain fell annually in Canada, Russia, and Europe in recent years than it did in 1925. In the northern tropics and subtropics, such as Mexico and northern Africa, rainfall has decreased by nearly 2.8 inches (7 centimeters) per year.

And the southern tropics and subtropics such as Peru and Madagascar have seen increased rainfall of about 2.4 inches (6 centimeters).

Altogether humans account for about two-thirds of the precipitation increase in Canada, Russia, and Europe, a third of the drying out in the northern tropics and subtropics, and nearly all of the increase south of the Equator, the study says.

A significant driver behind the altered rainfall is greenhouse gas emissions, primarily from coal and oil burning, that contribute to global warming.

The study, led by climate researcher Xuebin Zhang of Environment Canada in Toronto, is the first to connect human activity with changing precipitation patterns.

A Significant Shift

Human-caused greenhouse gases emissions have previously been linked to several climate events, including rising sea and air temperatures around the world.

A global warming connection to rainfall, however, has proven more difficult to establish—partly because drier conditions in some regions cancel out wetter conditions elsewhere.

In the new study, the authors examined precipitation trends in different sections of land north and south of the Equator, rather than the globe as a whole. This latitudinal approach shows a significant shift in global precipitation patterns over the past century.

The researchers then compared the observations to the changes in rainfall that multiple climate models predict should be attributable to human activity.

"Because we have this large number of simulations, we can average them all together in essence and filter out the effects of internal variability ... to obtain a best estimate," said study co-author Francis Zwiers, a climate scientist with Environment Canada.

The comparison, which appears in this week's journal Nature, shows that most of the changes in precipitation are due to human actions.

"You have this large-scale engine that moves moisture around the planet, and under greenhouse gas forcing, this engine essentially become more intense," Zwiers said.

However, the reasoning fails to fully explain why the region just south of the Equator is wetter. All things being equal, it should be drier, Zwiers noted.

The researchers suspect the increase is the result of a southward shift in the zone where trade winds of the Northern and Southern Hemispheres meet, but why the zone may have shifted remains unresolved.

Future Climate

Looking to the future, the study increases confidence in the prediction of climate models. The models indicated a change should have occurred in the 20th-century rainfall, and the pattern shows up in the observations, Zwiers said.

However, the change predicted by the models was less than observed, which indicates the models, as a group, are not responding to greenhouse gases as much as they should.

Climate models' under-prediction of 20th-century change makes their accuracy for the 21st century a bit worrisome, Zwiers said.

"Maybe the projections for the future are not projecting a big enough change," he said.

The models predict the 20th-century trend should continue in the Northern Hemisphere, but most suggest the wet region just south of the Equator will revert to a drier pattern. Any specific event cannot be pinned on human emissions of greenhouse gases, Zwiers said. Further studies of the climate models should reveal if increased greenhouse gases raise the risk of extreme rainfall events.

Others are not waiting for these scientific studies.

The findings were released as heavy flooding in the United Kingdom and China make headlines around the world.

"Extreme events such as we have seen in recent weeks herald the specter of climate change," Nick Reeves, the executive director of the Chartered Institution of Water and Environmental Management in London, England, told the Reuters news agency today. "And it would be irresponsible to imagine that they won't become more frequent."

Yahoo News 23 Jul 07
Global warming has already changed world's rainfall patterns: study

A study has yielded the first confirmation that global warming is already affecting world's rainfall patterns, bringing more precipitation to northern Europe, Canada and northern Russia but less to swathes of sub-Saharan Africa, southern India and Southeast Asia.

The changes "may have already had significant effects on ecosystems, agriculture and human regions that are sensitive to changes in precipitation, such as the Sahel," warns the paper, released on Monday by Nature, the British science journal.

Scientists have long said that global warming is bound to interfere with snow and rainfall patterns, because air and sea temperatures and sea-level atmospheric pressure -- the underlying forces behind these patterns -- are already changing.

But, until now, evidence for declaring that the interference is already happening existed anecdotally or in computer models, rather than from observation.

One problem for researchers has been a lack of accurate, long-term rainfall data from around the world that would enable them to distinguish between regional or cyclical shifts in rainfall.

Francis Zwiers, a scientists with Environment Canada, Toronto, found a way around these problems by using two data-sets of global rainfall pattern beginning, conservatively, in 1925 and ending in 1999.

They compared these figures with 14 powerful computer models that simulate the world's climate system, and found a remarkably close fit.

Over the 75-year period under study, global warming "contributed significantly" to increases in precipitation in the Northern Hemisphere's mid-latitudes, a region between 40 and 70 degrees north, they say.

In contrast, the Northern Hemisphere's tropics and subtropics, a region spanning from the equator to 30 degrees latitude north became drier. And the Southern Hemisphere's tropics (equator to 30 degrees latitude south) became wetter.

The study looked at annual average rainfall on the land, not at sea. In addition, it did not look at extreme weather events -- episodes of drought and flooding -- whose frequency and severity are also seen as likely to increase as a result of global warming. The investigation is published by Nature on Thursday.

Previous work in the past few years has highlighted the loss of alpine glaciers and snow cover and the retreat of Arctic permafrost.

These were interpreted by the UN's Intergovernmental Panel on Climate Change (IPCC), in a landmark report published this year, as confirmation that global warming has already started to affect parts of the climate system.
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