UN Secretary-General Mr Kofi Annan has pleaded with more than 100 world leaders at the world summit for them to work together.
He told the leaders that failure to take decisive action to help the poor and rescue the world's environment will have too high a cost.
The summit in Johannesburg aims to agree on a plan to follow through on promises made at the 1992 Earth Summit.
British prime minister Mr Tony Blair repeated his challenge to world leaders to summon the political will to tackle the problems of poverty and climate change.
In a five-minute speech to the World Summit in Johannesburg he said the world faces "catastrophe" unless greenhouse gas emissions are curbed.
He also repeated his claim that the poverty of Africa was a scar on the conscience of the world that must be healed.
He told fellow heads of state and government: "We know the problems. A child in Africa dies every three seconds from famine, disease or conflict. We know that if climate change is not stopped all parts of the world will suffer, some will even be destroyed. And we know the solution - sustainable development, so the issue for this summit is the political will."
Mr Blair went on: "We know one other thing: the key characteristic of this world is its interdependence. Your problem becomes my problem, your country's war becomes my country's asylum seekers."
He said the last Earth Summit in Rio "did not deliver everything, neither will Johannesburg, no summit can." But he insisted: "This summit can and will make our world change for the better."
Mr Annan said: "Let us not be deceived when we look at a clear blue sky into thinking that all is well. All is not well." "Let us stop being economically defensive and start being politically courageous," he said.
Head of the US delegation Ms Paula Dobriansky said: "The process is not just about approving text. It is about working with developing countries that look to us for concrete action. Failure is not an option".
Diplomats said negotiators settled on wording over the weekend to address the Kyoto Protocol on climate change, which the US has refused to sign. The agreed text says nations that have ratified Kyoto "strongly urge" states that have not done so to ratify it in "a timely manner".
But Greenpeace climate director Mr Steve Sawyer called it "a tremendous achievement in this process because basically it does not go backwards. It is about the only thing in this text that doesn't".