Strike a deal for world diplomacy - Obama
Leaders risk missing a golden opportunity to show the world that they can work together for the greater good.
The President's appearance scotched rumours flying round the conference that he would not attend as a successful conclusion was looking increasingly unlikely.
Obama said he had not come for the photo opportunity, but to take action.
He reiterated Hilary Clinton's warnings of the day before that climate change was not just about the environment but also about the economy and national security.
The key sticking points that have dogged the conference still remain.
Developing countries want binding targets for wealthy states of around a 25% in emissions by 2020.
America is committed to its calls for 'transparency' - effectively a demand for all countries to open their books to show how climate change mitigation is being achieved and how international funding is being spent.
This is a barb particularly aimed at China, seen by the US as a major competitor and reluctant to allow any openness that may 'infringe on its sovereignty'.
Even the ray of hope in terms of a likely international agreement - an annual fund of $100bn to help the poorest nations adapt to and reduce climate change - raises its own set of questions.
Industrialised states want some of this revenue to be raised through carbon trading and offsetting agreements, while the developing world wants straightforward public funding.
As talks enter their final hours here in Copenhagen, the mood is not optimistic, but it is hard to see through the fog of rhetoric and there is always an outside chance that world leaders will pull something out of the conjurer's hat in the eleventh hour.
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