As UN climate chief Christiana Figueres urged all states to turn political pledges into concrete action to save the planet, observers and developing states insisted the rich world should commit to tougher reduction goals.
Figueres cited new research which predicted Earth's temperature rising by as much as five degrees Celsius, instead of the 2 degrees estimate being reckoned with.
"We still have a gap remaining between intent and effort," Figueres told the press, as experts and diplomats from some 170 countries met to start laying the groundwork for a new global warming pact to be finalised by 2015.
These are the first formal talks since UN member states agreed in Durban, South Africa, last December to bring all major greenhouse-gas emitting countries under a single legal roof from 2020.
Officials started Monday the process of drawing up amendments to the Kyoto Protocol on climate change for adoption at the next UN climate conference in Doha in November and December.
There has been much debate about how much proportional responsiblity the rich and poor world should bear for curbing greenhouse gases.
"All countries have a responsibility to do their fair share, particularly those with the largest historical emissions," the Alliance of Small Island States said in a statement.
A grouping of least developed countries accused developed nations of "trying to renegotiate pledges and decisions made," instead of delivering on financial promises made to help curb climate change in the developing world.
And environmental body Greenpeace International urged the European Union to boost its commitment to reduce Earth-warming gas emissions by 20 percent.
But EU official Christian Pilgaard Zinglersen told a press conference that he did not think “the EU would change its stand" on the emissions target in Bonn.
Figueres conceded the political process was "incredibly challenging."
"We need to temper our excitement with realism," she told a press conference broadcast live on the Internet, but added a solution was "technically attainable and economically feasible."