The meeting gathers more than 200 government officials and experts and is meant to pave the way for a new pact to replace the Kyoto Protocol on harmful emissions. Kyoto binds 35 nations and expires in 2012.
The delegates are expected to mull ways to bring more countries into a new agreement, notably nations like China, India and the United States that have snubbed Kyoto.
The new protocol is meant to be thrashed out at the next major environmental meeting in December on the Indonesian island of Bali.
Damning reports on climate time-bomb
The Bonn conference is the first UN meeting of its kind since the world body published a series of damning reports this year about the dangers of global warming.
The latest, published in Bangkok last Friday, concluded that efforts to stabilize the level of greenhouse gases over the next 20 to 30 years will be crucial in the fight against the phenomenon that is threatening the planet with environmental and economic disaster.
The conference runs until May 18.