Government ministers meeting for the closing days of the United Nations climate talks were urged to be "courageous" in efforts to make progress towards a new international deal on tackling global warming.
The UN talks in Cancun, Mexico, are the latest attempt to make headway on a new global agreement to cut emissions and provide finance for poor countries to cope with climate change after efforts to secure an international deal ground to a halt at last year's conference in Copenhagen, Denmark.
A handful of presidents and prime ministers have joined environment, energy and climate ministers to take over from officials for the "high level" segment of the two-week conference.
It had been expected that the Cancun talks would concentrate on issues where it was thought progress could be made, including providing finance for poor countries to cope with climate change and tackling deforestation.
But the major stumbling blocks, which halted progress in Copenhagen, including the future of the existing climate change treaty, the Kyoto protocol, and ways of monitoring whether countries were keeping their pledges on action on global warming, have once again been central to the talks.
According to Sol Oyuela, senior adviser on climate justice at Christian Aid, negotiators have taken discussions as far as they can - and ministers now faced clear political choices on emissions cuts and how to ensure countries were being transparent on action.
"Now we need ministers to make up their minds, on the basis of the texts on which everyone has worked so hard.
"And we need them to be courageous, if these talks are to achieve anything for poor people struggling with the changing climate. Indecision will cost lives," she said.
She warned that a failure to make decisions on major issues such as cutting emissions and transparency will prevent progress on other areas such as providing money to poor countries and preventing deforestation.
Oxfam's international climate change policy adviser Tim Gore said: "It is important that ministers build on signs of progress so far and use their political leadership so a repeat of the Copenhagen debacle is avoided and a deal is done on Friday, which the whole world can be proud of."
Mr Gore said steady progress had been made towards setting up a "climate fund" to provide money to poor countries to develop green economies and tackle problems such as floods, droughts and storms brought on by global warming - but more work was needed in the next "crucial" days.
He added: "With four days to go, ministers must elevate the level of vision and ambition for these negotiations."
Ministers are also being provided with a visual message of the need for them to tackle action, as the whole world is threatened by climate change.
Greenpeace is submerging replicas of famous buildings including Big Ben, the Eiffel Tower, the Taj Mahal and the Statue of Liberty in the sea.
The UK's Climate Change Secretary Chris Huhne, amid uncertainty over whether he would have to return home to vote on tuition fees, has had a key role in leading talks between countries on the thorny issue of the future of the Kyoto protocol
Developing countries do not want to see Kyoto abandoned, as it legally commits rich nations to emissions cuts - but the US has not signed up to it and the treaty does not include major emitters such as China.