It is being described as the last chance to save the Kyoto Protocol on climate change.
A UN conference has opened in Durban, South Africa, with delegates representing almost 200 countries.
There has been discord for years over how to strike a long-lasting deal on cutting greenhouse gas emissions.
And time is running out to meet a 2012 deadline to reach agreement on new targets.
Tasneem Essop from the World Wide Fund for Nature in South Africa said:
“We do run the risk of losing the Kyoto Protocol, the only rules based system that we have. And what would be devastating is that the sum total of all of that could once again break down the task that we have in the multi-lateral process, the only process that is inclusive and gives a voice to the most vulnerable.”
Climate campaigners have also gathered in Durban, urging quick action from political leaders.
Two UN reports out this month warn that greenhouse gases have reached record levels in the atmosphere.