Mira Oberman Yahoo News 31 Jan 08;
Climate change has already dramatically altered the water cycle and these changes signal a looming water supply crisis, according to a prominent group of hydrologists and climatologists writing Thursday in Science magazine.
They argue that radical water cycle changes will be widespread and that past trends can no longer be relied upon when planning future water management.
"Our best current estimates are that water availability will increase substantially in northern Eurasia, Alaska, Canada and some tropical regions, and decrease substantially in southern Europe, the Middle East, southern Africa and southwestern North America," said lead author Christopher Milly, a research hydrologist with the US Geological Survey.
More frequent droughts can also be expected in drying areas, he added.
"Even with aggressive mitigation, continued warming is very likely given the residence time of atmospheric carbon dioxide and the thermal inertia of the Earth system," the authors concluded.
The article says that new models must be used to prepare for floods or droughts, determine the size of water reservoirs and decide how to allocate for residential, industrial and agricultural uses.
This is a massive undertaking seeing as annual global investment in water infrastructure is more than 500 billion dollars a year and these are made under outdated assumptions that the water cycle will fluctuate within a relatively narrow historical band.
"Historically, looking back at past observations has been a good way to estimate future conditions," Milly said.
"But climate change magnifies the possibility that the future will bring droughts or floods you never saw in your old measurements."
Climate change has already resulted in changes to rainfall patterns and river flows and created a greater risk of flooding in some areas, the authors wrote.
Rising sea levels will "heighten risk of contamination of coastal freshwater supplies" while a "poleward expansion of the subtropic dry zone" is reducing water runoff levels.
In a separate article in the same journal, researchers linked sharp changes in water supplies in the western United States to global warming.
They studied why more winter precipitation was falling as rain instead of snow and why snow was melting earlier in the US west, an arid region with a growing population.
These changes resulted in heavier river flows in the spring and lighter flows in the summer, which has coincided with a warming in temperatures over most of the region.
The study concluded that "up to 60 percent of the climate-related trends of river flow, winter air temperature and snow pack ... from 1950-1999 are due to human-caused climate change from greenhouse gases and aerosols."
The results "portend, in conjunction with previous work, a coming crisis in water supply for the western United States," wrote lead author Tim Barnett of the University of California San Diego.
This analysis "foretells of water shortages, lack of storage capability to meet seasonally changing river flow, transfers of water from agriculture to urban uses and other critical impacts."
