The two-week conference, drawing some 2,000 delegates from more than 160 countries and dozens of agencies, begins Monday.
The Bali conference agreed to conclude a new climate change treaty by December 2009. Another conference four months later in Bangkok adopted a negotiating timetable.
In Bonn, "the real work is now only beginning," says Yvo de Boer, the U.N.'s top climate change official.
Scientists say the world's carbon emissions must peak within the next 10 to 15 years and then fall by half by mid-century to avoid potentially catastrophic changes in weather patterns, a rise in sea levels that would threaten coastal cities and the mass extinction of plants and animals.
The new climate change pact will succeed the first phase of the 1997 Kyoto Protocol, which requires 37 industrialized nations to reduce greenhouse gas emissions an average of 5 percent below 1990 levels by 2012.
The United States is the only industrialized nation not to have ratified Kyoto. Negotiators hope Washington's consent to the Bali "action plan" marked the end of its hostility toward working with other countries to contain global warming.
"Their attitude, their activity, has changed very much in the recent year. It's really a big change," said Andrej Kranjc of Slovenia, the head of the European Union delegation.
Still, the Bush administration rejects specific and mandatory targets to reduce emissions over the next dozen years. And countries such as India and China question why they should accept limits on their development without commitments from the U.S., the world's largest per-capita polluter by far.
Delegates say such major decisions must wait for the new U.S. administration next January.
"It's unlikely we are going to make lot of progress this year because we need strong signals from the U.S., and that's not going happen until the election," said Ian Fry, the delegate from the tiny Pacific nation of Tuvalu.
But time is pressing.
The basic outline of the post-Kyoto agreement should be ready by next summer to prepare for the critical December conference in Copenhagen, Denmark, where the new pact should be adopted, de Boer says.
That allows negotiators just six months after the next U.S. president takes office to negotiate a complex, multifaceted and hugely expensive treaty.
"Everyone refers to the U.S. as the elephant in the room," said Angela Anderson, the director of the Global Warming Campaign for the Pew Environment Group. She said the presidential candidates already should be formulating policies and making them part of their campaigns.
"The new administration will have a challenge, but perhaps a welcome challenge. They can mend the U.S. reputation abroad by engaging constructively in the climate talks," she said.
Delegates in Bonn will begin work on how to help developing countries adapt to anticipated changes in their climate, on transferring new technologies to help them avoid hefty carbon emissions as they expand their economies, and on how to raise the trillions of dollars required over the next decades to curb climate change.
Each objective faces a multitude of obstacles.
Governments cannot commit to transferring technologies that belong to private companies, which are protected by intellectual property rights, for example. Small countries need satellite monitoring, especially of deforestation, which they cannot afford without help. The costs of installing carbon-storage facilities on power stations, once it becomes technically feasible, will be out of reach to all but the richest.
Proposed "adaptation funds" for developing countries are plagued by questions of how money will be generated, who will control it and how it will be allocated.
Some countries favor a levy on airplane tickets and maritime transport to be deposited in a special account earmarked for developing countries.
No one expects answers by the end of the Bonn conference. But de Boer said in an earlier interview he hoped the meeting would "take things to the next level," with the discussions crystalizing ideas and leading governments to submit written proposals for the Copenhagen accord.
"We don't expect a breakthrough (in Bonn), of course. We have a long road ahead of us," said Kranjc, the EU's chief delegate. He cautioned against trying to move too fast. "Things are not ripe."
