The delegates from 172 countries began gathering specific proposals on how to slow global warming by curbing carbon emissions, how to help poor countries adapt to climate change and how to raise the hundreds of billions of dollars needed annually.
But participants said not enough ideas were put on the table, and environmental organizations accused the United States, Canada and Australia of obstructing progress.
The talks aim to produce a successor accord to the 1997 Kyoto Protocol, which set targets for 37 industrial countries to cut emissions of greenhouse gases by an average 5 percent by 2012.
A new accord should be in place within 18 months to allow time for ratification and a smooth transition to a new regime in 2012.
"In view of the little negotiating time available up to 2009, proposals need to become much more focused," said Yvo de Boer, the U.N.'s top climate change official.
Harald Dovland, the Norwegian chairman of a key working group on updating Kyoto's terms, acknowledged frustration at the lack of conclusions.
"We need a completely new spirit of cooperation," he said. "If we continue in this mode and speed of work, I fear we will not succeed in achieving the goals set in our work program."
The chairman of a second key working group on multilateral cooperation said, however, that he was encouraged by numerous ideas on financing and technology transfers.
"Parties are making the all-important transition from broader discussions to the negotiating phase," said Luis Figueiredo Machado.
Bill Hare, of the Greenpeace environmental group, said wealthy countries had failed to offer concrete suggestions or creative solutions, and nearly all the innovative ideas came from developing countries.
"Unless the pace accelerates, unless political roadblocks are removed quickly and urgently, there is a major risk of failure in these negotiations," he said.
Delegates will reconvene in August in Accra, Ghana, and again in Poznan, Poland, in December. At least four more major conferences were scheduled for 2009, concluding in Copenhagen, Denmark, in December.
Scientists say the emissions of gases that trap heat and cause the Earth's temperature to rise must level off in the next 10-15 years, then decline sharply by mid-century.
They warn that an increase of more than 2 degrees Celsius could lead to catastrophic results, including a rise in sea levels threatening coastal areas and the extinction of up to 30 percent of species.
Among the proposals receiving the widest attention in Bonn was a plan by Mexico to create a new global fund into which all countries would contribute, possibly based on GDP or their rate of carbon emissions. Poorer countries could then draw on the fund to adapt to climate change.
China also generated buzz by suggesting a protocol to govern the transfer of technology.
It supported a notion to compensate companies for selling patented technologies cheaply to developing countries, in the same way pharmaceutical firms sell HIV/AIDS drugs at a reduced price to African and other AIDS-afflicted poor countries.
Li Yan, of Greenpeace-China, said the once-reluctant Chinese approach to climate change negotiations had turned around in the last two years.
She praised some of its domestic policies on renewable energy and energy efficiency, and its new cooperative attitude toward an international agreement. At the same time, China is building a new coal-fired power plant every six days to feed its fast-growing economy.
China's escalating emissions were highlighted in the report issued Friday by the Netherlands Environmental Assessment Agency.
Global emissions grew 3.1 percent last year, largely because of an 8 percent increase in China, said the report by the Dutch government-financed agency.
That figure will continue to rise, especially because of the huge reconstruction required after the earthquake in Sichuan. Cement production already accounts for one-fifth of China's emissions, the report said.
Though China is emitting more than the United States, U.S. emissions per capita were nearly four times greater than China's, the report said. China now has a 24 percent share of global emissions, compared with 21 percent for the U.S., it said.