Prime Minister Stephen Harper has left the G8 meetings in Japan heralding a declaration from the leaders of the world's industrialized nations to halve greenhouse gas emissions by 2050.

However, Harper also said in an afternoon news conference that developing countries would need to make some serious changes in order to help lower the level of greenhouse gas emissions in the air.

"By 2050, the developed world will probably represent no more than 20 per cent of emissions," he told reporters on Wednesday. "So, when we say we need participation by developing countries, this is not a philosophical position. This is a mathematical certainty.

"You can't get a 50 per cent cut from 20 per cent of emissions."

However, the G8 declaration leaves it up to individual countries to make their own emissions cuts.

The call to lower emissions got a significant boost when both Russia and the U.S. decided to endorse the long-term plan at the summit.

The hope was to find a consensus between emerging and leading countries over how to lower the harmful emissions. However, developing countries recoiled from the numerical targets set by the G8 leaders.

India, China, Mexico, Brazil and South Africa -who now call themselves the G5 - have all rejected the idea that they have a shared responsibility in reducing emissions by 50 per cent.

The G5 issued a statement urging G8 leaders to take more action.

"It is essential that developed countries take the lead in achieving ambitious and absolute greenhouse gas emissions reductions," the statement read.

However, Harper told reporters developing countries can help the cause by slowing the rate by which their emissions grow. He said they wouldn't necessarily have to cut down on their carbon output.

The American environment adviser to U.S. President George Bush told The Canadian Press that the G5 were being cautious.

"They want to be sure they can take steps that won't wreck their economy," said James Connaughton in an interview from Toyako, Japan. "We are not in complete convergence yet."

On Thursday, G8 members will be meeting with leaders from the emerging industrialized nations of South Africa, Mexico, Brazil, India and China.

Harper said real progress won't occur until those developing nations also pledge to cutting emissions -- a commitment he will likely be pushing on Thursday.

He pointed out that developing countries are most at risk from the effects of climate change and their contribution to global greenhouse emissions is growing rapidly with the expansion of their economies and populations.

Environmental critics were quick to shrug off the advancement, saying specific short-term and long-term targets need to be put in place before real change can happen.

Jeremy Hobbs, executive director of international agency Oxfam, said in a news release that the summit "did not deliver the breakthroughs so urgently needed."

"The consensus reached was at best shallow, especially on climate," he continued.

Stephane Dion, Canada's opposition leader, called the G8 leaders' declaration "a watered-down agreement" and said Harper "failed" Canadians by leading the climate change talks into the wrong direction.

"How will we get China and India to join the international fight against climate change if countries like Canada don't lead by example?" he said in a news release.

Antonio Hill, also of Oxfam, denounced the announcement as a stalling tactic.

"At this rate, by 2050 the world will be cooked and the G8 leaders will be long forgotten," he said.

The environment isn't the only pressing issue for G8 leaders. A debate over the world's weakening economy which has been depressed by soaring oil prices, food inflation and a credit crunch, will also take up much of the leaders' time.

This week the G8 leaders urged oil-rich countries to boost production to bring down the prices. They also urged countries that depend heavily on oil to improve their energy efficiency and to find alternate sources of fuel for the future.

With a report from The Canadian Press