One sticking point could be demands by some countries, led by the French President, Jacques Chirac, for tougher wording on the scientific evidence that man-made climate change is occurring.
The gloom began to set in as scientists published new evidence on the damage being done to the seas by global warming. Seawater is turning acid, with potentially dire effects for marine life, as a result of the carbon dioxide emissions from industry and transport, a report from the Royal Society warned.
By the end of the century there could be wide-reaching and harmful changes in the ocean food chain, directly affecting a range of vital organisms from plankton to coral, and having a knock-on effect on larger marine animals, said the report from a working group of senior UK scientists.
About half of the CO2 produced remains in the atmosphere while the rest dissolves in the oceans – and when it does so, it reacts with seawater to produce carbonic acid.
In the latest wrangling, the US upset campaigners against global warming by seeking to remove the phrase "Our world is warming" from the opening of a leaked draft of the Gleneagles summit communiqué. President Chirac is putting to one side the EU row with Mr Blair over the Common Agricultural Policy and is backing Mr Blair's aims for the communiqué to call for urgency on climate change, setting out an action plan, with further measures beyond the Kyoto targets for 2012.
Margaret Beckett, the Secretary of State for Environment, said: "The UK is in no doubt about the strength of scientific evidence on climate change. But the theology is less important than action. It would be really worthwhile if we could get an action plan that moves us all towards a shared goal of a low carbon economy."
Officials are arrving today in London for exhaustive talks to thrash out a deal. British ministers were playing down their hopes of a breakthrough on climate change and said Mr Blair was being ambitious by seeking a joint deal on global warming and aid to Africa.
"I would not at all be surprised if the negotiations go down to the wire. But he will keep trying," said a cabinet source. "I genuinely applaud what Tony Blair is doing. My anxiety has been that we could get to this stage and come out with something decent on development but bugger-all on climate change."