European environment leaders have said the US and Australia must alter their stance on climate change, as talks opened in Brussels on a major report.
Demonstrators built a symbolic "ark" outside the meeting venue
EU Environment Commissioner Stavros Dimas said a change of the US "negative attitude" to international climate treaties was "absolutely necessary".
The report from the Intergovernmental Panel on Climate Change (IPCC) will detail projections of climate impacts.
It is expected to forecast problems with supplies of food and water.
The final wording of the IPCC's summary conclusions will be finalised during a week of discussions involving scientists and government representatives, and will be unveiled on Friday.
Already, draft versions have been leaked to a number of news outlets; and they suggest the IPCC will outline major issues for concern, particularly in Asia and Africa.
More than one billion people who receive fresh water from glaciers in mountain ranges including the Himalayas, Alps, Andes and Rockies will see supplies dwindle, it is expected to say.
It is likely to project an increased risk of serious inundation for some low-lying cities, both on the coast or on the estuaries of major rivers.
Agricultural output is thought likely to decrease in tropical regions, but to increase in high latitudes.
Mr Dimas contrasted the approach of the European Union to mitigating these impacts through agreed international reductions in greenhouse gas emissions with the stance of the US and Australia, which have both left the Kyoto Protocol un-ratified having initially signed it in 1997.
"(The US) approach doesn't help in reaching international agreement, and doesn't help reduce (US) emissions," he said.
"We expect the US to come closer and not to continue with a negative attitude in international negotiations... it's absolutely necessary that they move."
Mr Dimas also said he could not understand why Australia did not ratify the Kyoto Protocol, saying it would cost the nation less in the long term.
Since 1990, the baseline year for the most important gases dealt with by the Kyoto treaty, US greenhouse gas output has risen by about 20% while emissions for the pre-expansion EU have declined by about 2%.
US Energy Secretary Samuel Bodman said recently that his administration acknowledged human emissions of greenhouse gases were contributing to a changing climate, adding that cutting his country's emissions might damage the economy and send industry overseas.
Belgian Prime Minister Guy Verhofstadt added that "unpopular measures are unavoidable", and said it was up to politicians of all countries to take those measures.