nder the first treaty addressing global warming, 193 countries, including the United States, pledged to avoid "dangerous" human interference with the climate.

There was one small problem with that treaty, enacted 11 years ago. No one defined dangerous. With no clear goal, smokestack and tailpipe emissions of gases linked to rising temperatures relentlessly climbed.

On Feb. 16, a stricter addendum to that treaty, the Kyoto Protocol, enters into force, requiring participating industrialized countries to cut such emissions.

But its targets and timetable were negotiated with no agreement on what amount of cuts would lead the world toward climatic stability. The arbitrary terms were cited by President Bush when he rejected the Kyoto pact in 2001, leaving the world's biggest source of such gases on the sidelines.

After a decade of cautious circling, some scientists and policy makers are now trying to agree on how much warming is too much.

One possible step toward clarity comes today, as 200 experts from around the world meet at the invitation of Prime Minister Tony Blair in Exeter for three days of talks on defining "dangerous climate change" and how to avoid it.

The researcher running the meeting, Dennis A. Tirpak, formerly of the Environmental Protection Agency, said that experts always realized it would take a long time for science's projections to be absorbed by society, but few thought it would take this long.

"I've always been a believer that science and truth will win out in the end," he said. "But I have a sense we might be running out of time."

It has taken this long not just because the "dangerous" question is complicated, but because it holds dangers in and of itself. If scientists offer answers, as some have in recent days, they can be criticized for playing down uncertainties and intruding into the policy arena. If a politician answers, that creates a yardstick for measuring later progress or failure.

It is much easier for everyone simply to call for more research.

But some experts now say that by the time clear evidence is at hand, calamity later in the century will be unavoidable. They say fresh findings show that potentially enormous environmental changes lie ahead.

"I think that the scientific evidence now warrants a new sense of urgency," said Dr. James E. Hansen, a climate scientist and director of NASA's Goddard Institute for Space Studies.

A particular concern is the Arctic. An eight-nation, four-year study concluded in November that accumulating carbon dioxide and other emissions from human activities were contributing to the thawing of tundra and the retreat of sea ice. Recent studies of accelerating flows of ice to the sea in some parts of Antarctica also point to the prospect of a quickening rise in sea levels in a warming world. Other scientists point to the prospect of intensified droughts and floods.

With pressure building for resolution and fresh action, some countries and groups of experts have tried to define a specific rise in earth's average temperature that presents unacceptable risks.

The European Union has set this threshold at 2.5 degrees of additional warming from current conditions. That was also the danger level chosen last week by an international task force of scientists, policy experts, business leaders and elected officials led by Senator Olympia J. Snowe, Republican of Maine, and Stephen Byers, a Labor Party member of the British Parliament.

Some scientists have criticized this approach, saying understanding of the impact of greenhouse gases on the atmosphere remains far too primitive to manage emissions and thus avoid a particular temperature target.

Others say the most logical response to the problem is to make societies more resilient to inherent extremes of climate. "If we just significantly minimize our vulnerabilities to the extremes which occurred during the last 250 years, we'll be O.K. for the next 100," said Dr. John Christy, a climate scientist at the University of Alabama who has long opposed cuts in emissions. As for rising seas, he said, "You've got 100 years to move inland."

Dr. Michael Schlesinger, who directs climate research at the University of Illinois, will contend at the meeting that the persistent uncertainty itself about big climate perils is precisely the reason to invest now in modest mandatory curbs on greenhouse-gas emissions.

Only with such a prod will societies move toward less-polluting choices, even as research continues on energy options that could in a few decades sharply reduce the human contribution to the greenhouse effect.

Without global participation in such emission curbs, though, the shared atmosphere will essentially remain a dump with no gate or tipping fee for countries rejecting action.

Any consensus on climate risks will likely intensify pressure on the Bush administration to shift from its current opposition to any cuts in the gases.

In a speech Wednesday at the World Economic Forum, Mr. Blair pressed the United States to join Britain and other industrialized countries that have agreed to curbs on the gases.

While the risks remained uncertain, Mr. Blair said, "It would be wrong to say that the evidence of danger is not clearly and persuasively advocated by a very large number of entirely independent and compelling voices."

The Exeter meeting will probably set the tone for the next review of climate trends and causes. In 2007, the Intergovernmental Panel on Climate Change, a United Nations body, will issue a report that is expected to be the most comprehensive summation so far of human understanding of global warming.

In three reports to date, that panel has fastidiously avoided defining unacceptable danger, though it has confirmed that humans have contributed to recent warming.

Its current chairman, Dr. Rajendra K. Pachauri, an economist and engineer from India, is to address the conference today.

In an interview, he said it was clear that emissions contributing to warming had to be reduced, but defining what is dangerous remained a "value judgment" that was fundamentally the responsibility of society and its elected officials.

He and several other experts said that everyone in the climate debate, scientists and policy makers, had to get used to the idea that whatever decisions were made, they would be made without scientific clarity.

Efforts to imply a false sense of certainty will backfire, and efforts to use uncertainty as an excuse for doing nothing will simply raise the stakes as more years slide by, and more long-lived emissions accumulate in the air.