Leaders of the Group of Eight were close on Thursday to agreeing a need for "substantial" cuts in world greenhouse gas emissions, falling short of European calls to halve emissions by 2050.
"I think it's possible that we leave this summit with a commitment on the part of everyone to a substantial reduction in greenhouse gas emissions by 2050 as a global target," Blair told reporters after a meeting with Us President George W. Bush.
Bush, who unveiled his own plan for cutting emissions beyond 2012 last week, says it is too early to set numerical targets at the June 6-8 summit of the G8 in Heiligendamm, Germany.
Standing alongside Blair, Bush said: "We are deadly earnest about getting something done.
"The US will be actively involved if not taking the lead in a post-Kyoto framework." The United States is the only G8 nation outside the UN's Kyoto Protocol, which sets cuts in greenhouse gases running to 2012.
Bush plans to call together the leading 15 greenhouse emitters -- led by the United States, China, Russia and India -- to agree on cuts beyond 2012 by the end of 2008.
German Chancellor Angela Merkel has pushed hard to include a goal in the G8 text that global emissions of greenhouse gases, mainly from burning fossil fuels, have to be cut by 50 per cent below 1990 levels by 2050.
She says such cuts are needed to ensure that global temperatures do not rise more than 2 Celsius (3.6 Fahrenheit) above pre-industrial levels, a threshold the European Union says will trigger "dangerous" changes in the climate system
You need to keep the 50 percent target or otherwise world temperatures will rise more than 2C," said Tobias Muenchmeyer of environmental group Greenpeace.
Blair said it had been unrealistic to expect a deal on a 50 per cent cut by G8 industrial countries and five major developing nations in a group known as the G8+5, due to meet in Germany on Friday.
"What you won't get, and there was never any question of this, here and now, amongst the G8+5, is the 50 per cent," he said. "What's important is to get an agreement that there should be such a target and that's the sort of ballpark that we are talking about."