It has been a summer of extremes. Rains have deluged Europe and Asia, swamping cities and villages and killing some 2,000 people, while drought and heat have seared the American West and Eastern cities. What is going on?

The floods and droughts could simply be flickers in the inherently chaotic weather system, some experts say. But many warn that such extremes will be increasingly common as the world grows warmer.

Such a shift could pose big problems in places where water is already a strained resource, they say.

''Their water use is already finely balanced, and based on hydrology they think they're going to get, and climate change is telling us they're going to get something different,'' said Dr. Peter H. Gleick, the director of the Pacific Institute, a private environmental research center in Oakland, Calif.

A warmer world is more likely to be a wetter one, experts warn, with more evaporation resulting in more rain, in heavy and destructive downpours.

But in a troublesome twist, that world may also include more intense droughts, as the increased evaporation parches soils between occasional storms.

''In a hotter climate, your chances of being caught with either too much or too little are higher,'' said Dr. John M. Wallace, a professor of atmospheric sciences at the University of Washington.

And the globe is getting warmer. The last several decades of global temperature readings curve up sharply on graphs.

Climate experts concluded for the first time last year that humans were causing most of the warming trend by burning coal and oil, which release carbon dioxide and other heat-trapping greenhouse gases.

The main way that warming is likely to manifest itself, scientists say, is through changes in the balance of water as liquid, vapor and ice.

Still, a scientific debate persists. Some experts say the earth has built-in buffering mechanisms that can limit extremes. But many others say that past records, current trends and computer models all point to big changes ahead. One new study this summer found evidence that the Asian monsoon, as part of the warming trend, has already intensified.

Generally, agriculture is expected to falter in arid subtropical areas like the eastern Mediterranean and southern Africa, while flourishing in northern climes -- like the North American wheat belt -- as more precipitation and longer growing seasons boost yields.

But climate experts say that even there, rain is more likely to fall as field-scouring torrents. Government scientists have already measured a significant rise in downpour-style storms in the United States over the last 100 years.

Long-term planners in the western United States are already trying to adjust. Next year, California will for the first time incorporate climate change into its five-year water-management plan.

Water supplies there are already squeezed by growing populations, said Jonas Minton, the deputy director of the California Department of Water Resources. A warming climate is intensifying the problem, he said.

Over the past 50 years, he said, winter precipitation in the Sierra Nevadas has been falling more and more in the form of rain, increasing flood risks, instead of as snow, which supplies farmers and taps alike as it melts in the spring.

One of the clearest climate shifts can be seen in mountain glaciers, said Dr. Lonnie G. Thompson, the director of the Byrd Polar Research Center at Ohio State University.

Some glaciers, like those in the Alps, were already retreating before the 20th-century burst of greenhouse emissions. But the rate of melting in the last decade in Peru, Tibet and many other places has quickened far beyond the pre-industrial pace, he said.

Referring to the glaciers, Dr. Thompson said: ''It doesn't matter if you're in the Himalayas, South America, Africa. They are all speaking with the same voice. The system is changing.''

The shriveling of mountain glaciers is likely to eventually disrupt water and hydroelectric power from Cuzco, Peru, to New Delhi, even as populations in such cities continue to grow.

In the short run, the melting could unleash sudden floods and avalanches as it overwhelms reservoirs and stream beds, experts say.

The United Nations Environment Program recently identified 44 glacier-fed lakes in Bhutan and Nepal that are swelling rapidly and could pose a risk of disastrous flash floods this decade.

In the long run, though, these long-frozen sources of water will run dry, said Cesar Portocarrero, a Peruvian engineer who worked for Electroperu, the government-owned power company, for 25 years monitoring the country's glacial water supply. He now serves as a consultant to Dr. Thompson.

Mr. Portocarrero said some Peruvian planners had explored the notion of supplying the thirsty cities along the Pacific coast with water from the Amazon River basin by building pipelines through the Andes. The costs of such a project would be enormous, he said.

In the meantime, the shifting climate is reflected in other ways in his hometown, Huaraz, a small city perched 10,000 feet up the Andes.

''I was doing work in my house the other day and saw mosquitoes,'' Mr. Portocarrero said. ''Mosquitoes at more than 3,000 meters. I never saw that before. It means really we have here the evidence and consequences of global warming.''