"Britain had to be repopulated over and over again. Completely new people had to come back, sometimes with a gap of 100,000 years between these occupations," said Professor Chris Stringer, the head of human origins at the Natural History Museum in London.

"From now on, as these cities and areas keep on growing we're going to have to start using more expensive sources of water and we're going to have to pay for it."

"Early Britons had to cope with these changes of climate. Often they couldn't and they died out completely. Britain and the British people today are new arrivals. We're products of only the past 12,000 years," Professor Stringer said.

Scientists worry about a global warming vicious cycle that was not part of their already gloomy climate forecast: Warming already under way thaws permafrost, soil that has been continuously frozen for thousands of years. Thawed permafrost releases methane and carbon dioxide. Those gases reach the atmosphere and help trap heat on Earth in the greenhouse effect. The trapped heat thaws more permafrost and so on.

"The higher the temperature gets, the more permafrost we melt, the more tendency it is to become a more vicious cycle," said Chris Field, director of global ecology at the Carnegie Institution of Washington, who was not part of the study. "That's the thing that is scary about this whole thing. There are lots of mechanisms that tend to be self-perpetuating and relatively few that tend to shut it off."

Some scientists say this vicious cycle is already under way, but others disagree.

Most of the methane-releasing permafrost is in Siberia. Another study earlier this summer in the journal Science found that the amount of carbon trapped in this type of permafrost - called yedoma - is much more prevalent than originally thought and may be 100 times the amount of carbon released into the air each year by the burning of fossil fuels.

It won't all come out at once or even over several decades, but if temperatures increase, then the methane and carbon dioxide will escape the soil, scientists say.

The permafrost issue has caused a quiet buzz of concern among climate scientists and geologists. Specialists in Arctic climate are coming up with research plans to study the permafrost effect, which is not well understood or observed, said Robert Corell, chairman of the Arctic Climate Impact Assessment, a study group of 300 scientists.

"It's kind of like a slow-motion time bomb," said Ted Schuur, a professor of ecosystem ecology at the University of Florida and co-author of the study in Science.

Most of the yedoma is in little-studied areas of northern and eastern Siberia. What makes that permafrost special is that much of it lies under lakes; the carbon below gets released as methane. Carbon beneath dry permafrost is released as carbon dioxide.

Using special underwater bubble traps, Walter and her colleagues found giant hot spots of bubbling methane that were never measured before because they were hard to reach.

"I don't think it can be easily stopped; we'd really have to have major cooling for it to stop," Walter said.

Scientists aren't quite sure whether methane or carbon dioxide is worse. Methane is far more powerful in trapping heat, but only lasts about a decade before it dissipates into carbon dioxide and other chemicals. Carbon dioxide traps heat for about a century.

"The bottom line is it's better if it stays frozen in the ground," Schuur said. "But we're getting to the point where it's going more and more into the atmosphere."

Vladimir Romanovsky, geophysics professor at the University of Alaska at Fairbanks, said he thinks the big methane or carbon dioxide release hasn't started yet, but it's coming. In Alaska and Canada - which have far less permafrost than Siberia - it's closer to happening, he said. Already, the Alaskan permafrost is reaching the thawing point in many areas.

Methane trapped in a special type of permafrost is bubbling up at a rate five times faster than originally measured, the journal said.

Scientists are fretting about a global warming vicious cycle that had not been part of their already gloomy climate forecasts: Warming already under way thaws permafrost, soil that had been continuously frozen for thousands of years.

Thawed permafrost releases methane and carbon dioxide. Those gases reach the atmosphere and help trap heat on Earth in the greenhouse effect. The trapped heat thaws more permafrost, and so on.

"The higher the temperature gets, the more permafrost we melt, the more tendency it is to become a more vicious cycle," said Chris Field, director of global ecology at the Carnegie Institution of Washington. "That's the thing that is scary about this whole thing. There are lots of mechanisms that tend to be self-perpetuating and relatively few that tends to shut it off."

The effect reported in Nature is seen mostly in Siberia, but also elsewhere, in a type of carbon-rich permafrost, flash frozen about 40,000 years ago. A new more accurate measuring technique was used on the bubbling methane, which is 23 times more powerful a greenhouse gas than the more prevalent carbon dioxide.

"The effects can be huge," said lead author Katey Walter of the University of Alaska at Fairbanks. "It's coming out a lot and there's a lot more to come out."

Another study earlier this summer in the journal Science found that the amount of carbon trapped in this type of permafrost - called yedoma - is much more prevalent than originally thought and may be 100 times the amount of carbon released into the air each year by the burning of fossil fuels.

It won't all come out at once or even over several decades, but the methane and carbon dioxide will escape the soil if temperatures increase, scientists say.

The issue of methane and carbon dioxide released from permafrost has caused concern this summer among climate scientists and geologists. Specialists in Arctic climate are coming up with research plans to study the effect, which is not well understood or observed, said Robert Corell, chairman of the Arctic Climate Impact Assessment, a group of 300 scientists.

"It's kind of like a slow-motion time bomb," said Ted Schuur, a professor of ecosystem ecology at the University of Florida and co-author of the Science study. "There's these big surprises out there that we don't even know about."

Most of this yedoma is in north and eastern Siberia, areas that until recently had not been studied at length by scientists.

What makes this permafrost special is that during a rapid onset ice age, carbon-rich plants were trapped in the permafrost. As the permafrost thaws, the carbon is released as methane if it's underwater in lakes, like much of the parts of Siberia that Walter studied. If it's dry, it's released into the air as carbon dioxide.

Scientists aren't quite sure which is worse. Methane is far more powerful in trapping heat, but only lasts about a decade before it dissipates into carbon dioxide and other chemicals. Carbon dioxide traps heat for about a century.

"The bottom line is it's better if it stays frozen in the ground," Schuur said. "But we're getting to the point where it's going more and more into the atmosphere."

Vladimir Romanovsky, geophysics professor at the University of Alaska at Fairbanks, said he thinks the big methane or carbon dioxide release hasn't started yet, but it's coming. It's closer in Alaska and Canada, which only has a few hundred square kilometres of yedoma, he said.

In Siberia, the many lakes of melted water make matters worse because the water, although cold, helps warm and thaw the permafrost, Walter said.

As the magma deep underground rises to the surface, it crystallises with the reduction in pressure and the water forms bubbles.The magma releases heat, and its temperature rises from 900C to 1000C.

When the molten rock reaches the surface, the combination of water bubbles and temperature increase blows out the magma with explosive force, much like champagne spurting from a bottle when it is uncorked. "By looking in great detail at the erupting rock, we've been able to show that, as the magma ascends beneath the volcano prior to an eruption, it crystallises in response to the drop in pressure and it also gets hotter," said Professor Jon Blundy of Bristol University, who led the work, published today in Nature.

"This work is now being used to gauge the direction of current volcanic activity at Mount St Helens in the US and could be applied to any active volcano for which monitoring and petrological records are available." The magma can increase in temperature by about 100C in just a few years. This provides a trigger for an eruption without the need for an outside heat source, such as hotter magma below.

Scientists normally monitor active volcanoes by measuring how the ground around them deforms and how the gases they release change over time. Local earthquakes can also provide clues on how magma is moving under a volcano. Explosive volcanic eruptions are caused by the escape of gases from magma stored in underground reservoirs. Monitoring magma has been difficult because it is so far below the Earth's surface. "What we've done is study little droplets of volcanic liquid that are trapped inside the magma as it rises to the surface," said Madeleine Humphreys of Cambridge University, speaking yesterday at the British Association festival of science in Norwich.

By measuring the water and chemical content of these droplets, called melt inclusions, the researchers were able to tell how the magma was moving underneath the volcano and what condition it was in.

If the magma is at high pressure under the volcano, it will contain a lot of water and can form a lot of bubbles. "The sort of material that's ejected by a volcano during its precursory activity could hold the clues to what's going on underground and might indicate what might happen in the future," said Prof Blundy. "More bubbles, more explosive eruptions. If the magma is stored at a low pressure, [there is] potential to form bubbles and less explosive potential."

In their study, the researchers looked at the lava thrown out of Mount St Helens in the US and Shiveluch in Kamchatka, Russia. Mount St Helens erupted in May 1980 because of sudden decompression of the magma under the volcano. Prof Blundy said the release of pressure was like smashing the top off a bottle of champagne after giving it a vigorous shake.

Mount St Helens lay dormant until 2004, when large columns of magma started erupting from the top of the volcano. Analysis of current eruptions shows that it is unlikely to explode soon, Prof Blundy said.

Combining the analysis of melt inclusions with traditional observations of volcanoes was the "holy grail of modern volcanology," he said.

It posed no immediate threat to land, though its large size - tropical storm force winds extended 290 miles from its center - could be a concern later in the week.

"The concern would be Bermuda at this point, how close the destructive force winds will move toward it," said Dave Roberts, a Navy forecaster at the hurricane center.

Florence developed in the peak of hurricane season in warm Atlantic waters, the source of energy for storm development this time of year. While warm enough to spur storm intensification, forecasters said those waters are not as warm as last year's storm season, which boasted a record 28 named storms and 15 hurricanes, including Katrina.

The 2006 Atlantic hurricane season has not been as rough as initially feared. The National Hurricane Center lowered its forecast in August to between 12 and 15 named storms and seven to nine hurricanes.

Florence follows on the heels of Tropical Storm Ernesto, which was briefly the season's first hurricane before weakening and blowing up the East Coast last week. The storm delayed the launch of the space shuttle Atlantis and blacked out thousands of homes and businesses from North Carolina to New York.

At least nine deaths in the United States and two in Haiti were blamed on Ernesto. Authorities in northwest North Carolina searched Wednesday for two young brothers missing from a home near a fast-moving stream swollen by rain and runoff from Ernesto's passage over the state. At least 140 people, including six people on Wednesday, have been evacuated since the Cape Fear River rose out of its banks Friday in Ernesto's wake.

In some Potomac tributaries, nearly all of the male smallmouth bass caught in last year's survey were the abnormal fish. In the Potomac itself, seven of 13 largemouth bass exhibited female characteristics, including three that were producing eggs.

Although the frequency discovered was surprisingly high, Blazer cautioned that the sample size was relatively small, with about 10 male and 10 female fish taken from each of eight locations in Maryland, Virginia and the District of Columbia.

Researchers were reluctant to remove large numbers of bass from the rivers because of conservation concerns, she said.

Female fish caught in the survey did not develop any unusual sex traits, though fish of both sexes exhibited lesions and other pollution-related problems, said Blazer, who coordinated the survey.

Smallmouth bass appear to be more susceptible to intersex development than largemouth bass, Blazer said.

Blazer said researchers are still waiting on data that would help them determine the water quality at the time the fish were caught, but preliminary data taken from the Potomac found a variety of chemical pollutants.

It is not exactly clear what is causing the changes, though it is likely a combination of pollutants, scientists say.

Certain chemicals and pesticides are believed to stimulate estrogen production. Also, estrogen from birth control pills and human waste can make its way from sewage treatment plants to the waterways.

The Environmental Protection Agency has been studying the issue of so-called "endocrine disruptors" since 1996, but currently does not issue guidelines to water treatment plants for allowable levels of estrogenic compounds.

Jeanne Bailey, a spokeswoman for Fairfax Water, said the findings are a concern.

The water authority, which draws from the Potomac and Occoquan rivers to provide service to roughly 1.5 million people, is working with USGS and other agencies to research and develop ways to improve water treatment to eliminate potentially harmful compounds.

The water treatments used by Fairfax Water, including ozone and activated charcoal, have been shown to reduce levels of estrogenic compounds, she said.

Bailey cautioned against drawing dire conclusions about the impact on human health. She said, "Fish are a great indicator of the health of our waters, but they are not a great indicator of what may translate to humans."