Capping a tortuous four-year effort, negotiators from more than 150 countries are heading to the Moroccan city of Marrakech to put the final touches on an accord aimed at limiting humanity's influence on the climate.
The big question is how much difference it can make without the backing of a skeptical United States.
A two-week conference beginning today hopes to complete a set of rules that will affect the way countries produce energy, will influence heavy industry like car making, will encourage forestry projects and new farming techniques, and will pay rich countries to help poor ones.
The objective is to write the legal language governing the Kyoto Protocol, the 1997 treaty drafted in Japan that would oblige industrial countries to reduce emissions of the greenhouse gases blamed for warming the Earth.
Cabinet ministers and policy-makers from most countries will arrive in Marrakech for the last three days, Nov. 7-9, to approve the final document.
Described by diplomats as the most complex international agreement ever negotiated, the value of the protocol has been cast in doubt since it was rejected by the United States, source of 24 percent of the world's greenhouse gases.
President Bush calls the treaty unfair and harmful to the U.S. economy, and says the United States won't be bound by it.
Last July in Bonn, Germany, negotiators spent sleepless nights battling over rules that would affect their economies for decades to come. Now they must settle the technical questions of how to carry those decisions through.
Environmentalists are watching Australia, Canada, Russia and Japan, which they fear could try to weaken or undermine the Bonn decisions, or reopen debate on key issues that were only loosely tied up in Bonn.
"I will have my backtrack radar on," said Jennifer Morgan, the climate director for the World Wildlife Fund.
The protocol is the foundation of a grand coalition of countries pledged to limit their artificial contribution to a natural atmospheric warming that has changed the planet in a host of ways raising sea levels, melting ice caps and changing rainfall patterns.
The treaty would require industrialized countries to slash emissions of gases that trap heat in the atmosphere, primarily carbon dioxide from power stations and vehicles, demanding a costly shift away from coal and dirty fossil fuels toward cleaner energy.
But the agreement reached in Bonn offered various ways to ease the pain and delay industrial action.
Countries can offset part of their quota of reductions through afforestation, buying credits from countries that exceed their targets, or helping developing countries control their emissions.
The experts in Marrakech must translate those agreements into U.N. legal language, and resolve such issues as how to report and verify emissions so that countries cannot cheat.
Perhaps the toughest problem is setting penalties for the noncompliant.
Michael Zammit Cutajar, the top U.N. official on climate issues, says once the rules are in place, the Kyoto partners can focus on how to re-engage the United States in a process it once championed, then abandoned.