As the Group of Eight summit wrapped up in northern Japan on Wednesday, Prime Minister Stephen Harper said it's a "mathematical certainty" that developing countries will do the brunt of the work in lowering global greenhouse gas emissions.

His comments to reporters in the resort town of Toyako came as several developing countries reportedly balked at climate change targets proposed by the G8 countries the previous day.

The major industrial countries represented by the G8 set a goal Tuesday to halve emissions that contribute to global warming by 2050, though no international baseline year was set and the plan lacked midterm goals.

Harper said that by 2050, developed nations will likely account for no more than 20 per cent of global carbon emissions.

"So, when we say we need participation by developing countries, this is not a philosophical position. This is a mathematical certainty," he told Canadian reporters at a news conference Wednesday.

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

Slowing rate might suffice: PM

The prime minister said that doesn't mean emerging economies will need to cut their carbon output; slowing the rate by which their emissions grow might suffice.

"I don't think the argument that we should do more if others are not doing anything at all will be a very credible argument that will get anybody very far," he said.

"It's an interesting argument, but it can't be made by those who aren't doing anything. So I think the pressure will be on them to do something."

But developing countries appear uneasy with proposed targets set out by G8 leaders.

A Japanese official told reporters that five developing nations — China, India, Brazil, Mexico and South Africa — rejected targets at a U.S.-brokered meeting between developed and developing countries on Wednesday.

3 others back G8 targets

Three other nations invited to the meeting — Australia, Indonesia and South Korea — reportedly backed the G8 approach.

In a statement issued after the meeting, there was no mention of the 2050 targets endorsed earlier in the summit. Instead, it made reference to a vague pledge to back a UN effort to conclude a new climate pact by 2009.

Yvo de Boer, who leads the UN's negotiations to forge a new climate change treaty, said Wednesday that despite endorsing a vague goal to halve pollution by 2050, the major industrial countries failed to set out a clear aim to curb carbon emissions.

He said the target for a 50-per-cent reduction of greenhouse gas emissions by 2050 mentioned no base line, and was open to vastly different interpretations.

An aide to U.S. President George W. Bush conceded that no consensus had been reached at the meeting, saying countries cited economic concerns.

"They want to be sure they can take steps that won't wreck their economy," James Connaughton, Bush's environment adviser, said in Toyako.

However, the U.S. president described the three-day summit as making "significant progress" on climate change.

"We made progress, significant progress, toward a comprehensive approach," Bush said, according to the text of a speech made at the conclusion of the summit.

Among the successes Bush cited was an agreement to increase oil and production-refining capacity to help ease skyrocketing prices.

The G8 leaders also threatened to deploy "financial and other measures" against those responsible for violence in the runup to last month's run-off presidential election in Zimbabwe.

With files from the Associated Press