Research by Cambridge University shows that it is not increased pollution but a side effect of climate change that is making ozone depletion worse. At high altitudes, 50 percent of the protective layer has been destroyed.

The research has dashed hopes that the ozone layer was on the mend. Since the winter of 1999-2000, when depletion was almost as bad, scientists had believed an improvement was under way as pollution was reduced. But they now believe it could be an additional 50 years before the problem is solved.

What appears to have caused the further loss of ozone is the increasing number of stratospheric clouds in the winter, 15 miles above the Earth. These clouds, in the middle of the ozone layer, provide a platform that makes it easier for rapid chemical reactions that destroy ozone to take place. This year, for three months from the end of November, there were more clouds for longer periods than ever previously recorded.

Cambridge University scientists said Tuesday that, in late March, when ozone depletion was at its worst, Arctic air masses drifted over the U.K. and the rest of Europe as far south as northern Italy, creating significantly higher doses of ultraviolet radiation and sunburn risk.

The results, which were announced at a Geophysical Union meeting in Vienna, Austria, Tuesday, are part of a European venture coordinated by Cambridge University’s chemistry department, which has been studying the relationship between the ozone layer and climate change since May 2004.

Tuesday, professor John Pyle of the university said: “These were the lowest levels of ozone recorded since measurements began 40 years ago. We thought things would start to get better because of the phasing out of CFCs and other chemicals because of the Montreal protocol, but this has not happened. The pollution levels have leveled off, but changes in the atmosphere have made it easier for the chemical reactions to take place that allow pollutants to destroy ozone. With these changes likely to continue and get worse as global warming increases, ozone will be further depleted even if the level of pollution is going down.”

The relationship between the depletion of the ozone layer and climate change is so complex that the European Union is investing 11 million pounds in a five-year project to try to understand and predict what is happening. Reporting the results of the first year, scientists told the meeting in Vienna Tuesday that “the atmospheric lifetime of these [ozone-depleting] compounds is extremely long, and the concentrations will remain at dangerously high levels for another half century.”

Increased greenhouse gases in the air trap more heat in the lower atmosphere, but the stratosphere far above the Earth is getting colder. As a result, ice clouds form between 14 and 26 kilometers above the Earth, exactly in the region where the protective ozone is found.

The European scientists reported the first signs of ozone loss in January. As sunlight returned to northern latitudes, the rate of ozone depletion increased, and rapid destruction of ozone occurred throughout February and March. In the altitude range where the ozone layer usually reaches its maximum concentration, more than half of the ozone was lost. In the lower atmosphere losses were not so great.

“Overall, about 30 percent of the ozone layer was destroyed,” said Markus Rex, from the Alfred Wegener Institute in Potsdam, Germany, another member of the team. He said the cold conditions that created polar stratospheric clouds were four times more extensive in 2005 than in the 1960s and 1970s.

Professor Pyle said overall the mixing of the air in the Northern Hemisphere was far more rapid than in the Antarctic, so a “hole” in the ozone layer did not occur. Instead, as the air mixed in spring, there was a general thinning of the protective ozone over the whole of the Northern Hemisphere. “It just means we have less natural protection than we should have and we are used to. It means that we should be careful about exposing ourselves to the sun, but that is already the case — this just makes things slightly worse,” he said.

The mayors who have signed up represent 29.3 million people. Although most are Democrats, some 12 big cities with Republican mayors, representing 8 million people, have joined, including New York.

Nickels is a Democrat, but he said his campaign is nonpartisan. “This campaign has clearly touched a nerve with the American people,” he said. “The climate affects Democrats and Republicans alike. Here in Seattle we rely on the winter snow for our drinking water and hydroelectricity, but it is disappearing; in Florida they have had hurricanes; in California they have had unseasonable heavy rain. Our weather patterns are changing.”

He said each city had a tough target of cutting its emissions by 7 percent and each mayor would choose “a different path.” “Conditions in Hurst, Texas, are different [from] here in Seattle,” he said, “but we both think we can do it.”

He said the fight to prevent climate change would not be expensive. “There are changes we will have to make, but there are many opportunities to create employment and make for a better life. In any event, the costs of doing nothing are greater than doing nothing. Climate change is happening and causing a lot of problems already. This can only get worse, and we have to start doing something about it now. Lots of other Americans appear to agree.”

Among the proposals are running municipal vehicles on gas or electricity, investment in renewable energy, planting trees, promoting car pooling, improving public transport and providing bicycle lanes.

Each city has signed up to produce a greenhouse gas inventory and a plan on how to reduce it.

In Seattle cruise ships are required to turn off their diesel engines in dock and hook up to the city’s renewable-energy supplies to cut emissions, a move that has caused some ships to refit their electricity systems. Salt Lake City has become Utah’s biggest buyer of wind power in order to meet its target.

New York’s mayor, Michael Bloomberg, who signed up last week, is trying to reduce emissions from the municipal vehicle fleet by buying hybrid-powered vehicles.

Ray Nagin, the mayor of low-lying New Orleans and a Democrat, told the New York Times that he joined the coalition because a projected rise in sea levels “threatens the very existence of New Orleans.”

In Hawaii, the mayor of Maui County, Alan Arakawa, a Republican, said he joined because he was frustrated by Washington’s failure to recognize the scientific consensus that climate change is happening because of human activity.

Seattle’s move is the latest in the groundswell of concern about the Bush administration’s failure to take action on climate. The White House has poured money into research and believes technology will solve the problem while at the same time maintaining that taking action now would lead to higher energy prices and the loss of 5 million jobs.

In a separate alliance, a number of states, including New York, have signed up to a carbon trading deal that would cut the emissions of fossil-fuel-burning power stations by exchanging carbon credits for cash.

The Canadian government claims the cull will protect fish stocks and bring in much-needed revenue and employment for those who live on Canada’s vast northern coastline.

Phyllis Campbell-McCrae, from the U.K. office of the International Fund for Animal Welfare, observed the start of the cull Tuesday. “There was lots of rain mixing with the blood of the seals, and we were wading through pools of blood with the odd carcass floating by,” she said. “It was simply awful. They club the seal and turn it over and slice it down the middle to get the skin off. They are supposed to do the eye test — that is, touch the eye of the seal they have clubbed to make sure it does not blink. That way they know it is dead before they skin it. They work at such a frenetic pace they often do not bother.”

She said tests carried out in previous seasons on cubs that had been clubbed and skinned showed that 42 percent were still conscious when skinned.

The pups are fed for two weeks by their mothers before being abandoned to fend for themselves. They are called “beaters” at this stage because they are unable to swim, and when the ice melts they fall into the water and beat it with their flippers to keep afloat. Left alone they learn to swim and begin looking for food, but before the ice melts they make an easy target for sealers and have no means of escape.

The killing of seals has been controversial for years, with the IFAW saying it is unacceptably cruel. Campaigners also say that the Canadian government’s claim that the seals had been eating too many fish and had to be controlled was making the seals a scapegoat for their own failure to control overfishing.

Opposition to the slaughter in the 1980s led the European Union to ban the import of white-coated seals less than three weeks old and the trade went into decline. In the 1990s overfishing of cod stocks off Newfoundland led to mass unemployment of fishermen and renewed claims that the seals, which were increasing in numbers again, were responsible.

This led to pressure for increased quotas for seal hunting to provide an income for unemployed fishermen. The Canadian government has progressively increased the quota and decided to cull 1 million pups in three years — the largest quota since 1957. The IFAW claims this level of killing led to a serious decline in harp seals.

The trade in the slightly older, soft gray skins of the beaters is now thriving, and the Canadians have decided to kill 320,000 in the next four weeks. Beater skins are worth around 20 pounds (about $38), and a high-fashion sealskin coat made of beater skins sells for 1,200 pounds (more than $2,200).

The Canadian government denies that the killing of the pups is cruel and says it is closely regulated. The government’s latest estimate of harp seal numbers is 5.2 million. Geoff Regan, the fisheries and oceans minister for the federal government, has said he would be happy to see the numbers reduced by one-third.

In an interview with the Ottawa Citizen, he said that while seals had been traditionally hunted for meat and skins, the fatty layer under their skin could be processed into oil to produce omega 3 fatty acids used for treating hypertension, diabetes and arthritis. “Seal oil is great in terms of omega 3 acids,” Regan is quoted as saying. “I’m anxious to see that the whole animal is used as much as possible, and encouraging more ways for that to happen is important.” Regan said his department is not only ignoring calls for the seal hunt to be outlawed but encouraging it to expand.

He accused animal welfare groups of using images of seal pups being slaughtered “to pull at people’s heartstrings.” Regan said that the seal cull is economically viable and is not subsidized.

The largest importer of seal pelts is Norway, which last year paid 1 million pounds for raw pelts. Denmark, Poland and China also import large quantities. Italy imports finished pelts to be made into fashion items.

So sensitive is the Canadian government to criticism of cruelty that Wednesday it put out a fact sheet to journalists. It said that seals may appear to be moving after they are killed because of a reflex reaction similar to that in chickens. The government claimed that quotas are set as “a sustainable, commercially viable fishery based on sound conservation principles. It is a market-driven harvest.”

Measurements of carbon dioxide in the atmosphere have been continuous for almost 50 years at Mauna Loa Observatory, 12,000 feet up a mountain in Hawaii, regarded as far enough away from any carbon dioxide source to be a reliable measuring point.

In recent decades carbon dioxide increased on average by 1.5 parts per million (ppm) a year because of the amount of oil, coal and gas burned, but has now jumped to more than 2 ppm in 2002 and 2003.

Above or below average rises in carbon dioxide levels in the atmosphere have been explained in the past by natural events. When the Pacific warms up during El Niño — a disruptive weather pattern caused by weakening trade winds — the amount of carbon dioxide rises dramatically because warm oceans emit carbon dioxide rather than absorb it.

But scientists are puzzled because over the past two years, when the increases have been 2.08 ppm and 2.54 ppm respectively, there has been no El Niño.

Charles Keeling, the man who began the observations in 1958 as a young climate scientist, is now 74 and still working in the field. He said yesterday: “The rise in the annual rate to above two parts per million for two consecutive years is a real phenomenon. It is possible that this is merely a reflection of natural events like previous peaks in the rate, but it is also possible that it is the beginning of a natural process unprecedented in the record.”

Analysts stress that it is too early to draw any long-term conclusions. But the fear held by some scientists is that the greater than normal rises in carbon dioxide emissions mean that instead of decades to bring global warming under control we may have only a few years. At worst, the figures could be the first sign of the breakdown in the Earth’s natural systems for absorbing the gas. That would herald the so-called runaway greenhouse effect, where the planet’s soaring temperature becomes impossible to contain. As the icecaps melt, less sunlight is reflected back into space from ice and snow, and bare rocks begin to absorb more heat. This is already happening.

One of the predictions made by climate scientists in the Intergovernmental Panel on Climate Change is that as the Earth warms, the absorption of carbon dioxide by vegetation — known as “carbon sink” — is reduced.

Keeling said since there was no sign of a dramatic increase in the amount of fossil fuels being burned in 2002 and 2003, the rise “could be a weakening of the Earth’s carbon sinks, associated with the world warming, as part of a climate change feedback mechanism. It is a cause for concern.”

Tom Burke, visiting professor at Imperial College London, and a former special advisor to former Tory Environment Minister John Gummer, warned: “We’re watching the clock and the clock is beginning to tick faster, like it seems to before a bomb goes off.”

Peter Cox, head of the Carbon Cycle Group at the Met Office’s Hadley Centre for Climate Change, said the increase in carbon dioxide was not uniform across the globe. Measurements of carbon dioxide levels in Australia and at the South Pole were slightly lower, he said, so it looked as though something unusual had occurred in the northern hemisphere.

“My guess is that there were extra forest fires in the northern hemisphere, and particularly a very hot summer in Europe,” Cox said. “This led to a die-back in vegetation and an increase in release of carbon from the soil, rather than more growing plants taking carbon out of the atmosphere, which is usually the case in summer.”

Scientists are have dubbed the two-year carbon dioxide rise the Mauna Loa anomaly. Cox said one of its most interesting aspects was that the carbon dioxide rises did not take place in El Niño years. Previously the only figures that climbed higher than 2 ppm were El Niqo years — 1973, 1988, 1994 and 1998.

The heat wave of last year that is now believed to have claimed at least 30,000 lives across the world was so out of the ordinary that many scientists believe it could only have been caused by global warming. But Cox, like other scientists, is concerned that too much might be read into two years’ figures. “Five or six years on the trot would be very difficult to explain,” he said.

Piers Forster, senior research fellow of the University of Reading’s Department of Meteorology, said: “If this is a rate change, of course it will be very significant. It will be of enormous concern, because it will imply that all our global warming predictions for the next 100 years or so will have to be redone.”

David Hofmann of the U.S. National Oceanic and Atmospheric Administration center, which also studies carbon dioxide, was more cautious. “I don’t think an increase of 2 ppm for two years in a row is highly significant — there are climatic perturbations that can make this occur,” he said. “But the absence of a known climatic event does make these years unusual. Based on those two years alone I would say it was too soon to say that a new trend has been established, but it warrants close scrutiny.”