Farmers want a special deal in global climate negotiations to make sure they are not saddled with excessive costs for curbing Greenhouse gas emissions, said the head of the world's leading farming lobby.

Agriculture is responsible for up to 20 per cent of global Greenhouse gas emissions, blamed for global warming, but farmers were limited in how much they could cut emissions, International Federation of Agricultural Producers president Ajay Vashee said.

Mr Vashee, who's organisation represents 600 million farmers and 115 farmer organisations in 82 countries, cautioned against imposing financial penalties on food production in order to cut Greenhouse emissions.

"There is a certain threshold beyond which we cannot go below. It is important to realise the realities of the situation," Mr Vashee told Australia's National Press Club in Canberra.

Environment ministers will meet in Copenhagen in late next year to start negotiations on a global agreement to curb Greenhouse gas emissions, to replace the Kyoto Protocol agreement which ends in 2012.