Developed countries must contribute collectively to a $100 billion annual fund to fight climate change in developing countries by 2020, UK Prime Minister Gordon Brown said on Friday, Reuters reported.

Global talks to agree in December in Denmark a new treaty to fight climate change have struggled to determine how rich countries pay for climate action in poorer nations.

"I propose we take a working figure for this purpose of around $100 billion per annum by 2020," Brown said in London.
"If we are to achieve an agreement in Copenhagen I believe we must move the debate from a stand-off over hypothetical figures."