L'AQUILA, Italy — Canada joined other members of the group of major industrialized countries Wednesday in promising an "ambitious" plan to slash greenhouse-gas emissions over the next four decades.

The agreement on the first day of the G8 summit in Italy acknowledged for the first time that humankind faces catastrophic consequences if average global temperatures, already 0.8 C above levels during the pre-industrial age, rise more than two degrees above levels from the pre-industrial age.

The leaders of the U.S., Russia, Japan, Canada, Great Britain, France, Germany and Italy say they are trying to lead by example in advance of United Nations-sponsored December talks in Copenhagen on a post-2012 plan to deal with the global warming crisis.

Developed countries will take the lead with cuts of "80 per cent or more," according to the G8 pledge that one environmental group said will put enormous pressure on Canada to develop more aggressive targets.

U.S. President Barack Obama was praised for showing leadership after eight years of climate change foot-dragging under the George W. Bush administration.

"There is a much stronger statement this year than in past years," said Prime Minister Stephen Harper. "That's due, principally, in fairness, to the new administration, which has brought commitment to dealing with this problem."

Harper said he hopes the G8 declaration will convince major emitters such as China, India and Brazil to make similar promises at G8 meetings here Thursday.

"I hope it will be the basis for the emerging economies to join us in a consensus, because we are all agreed that we have to make progress on this issue, and can only make it if all major emitters do their part," he said.

"You know, the emerging economies will be hit as hard, if not harder, by climate change than anyone, so it's important for them to also realize the necessity of making commitments."

The statement included some wiggle room to let countries such as Canada stick to their more modest ambitions. It said the planned 80 per cent cut will use 1990 "or later years" as the baseline for reduction comparisons.

While European countries have used 1990 as a baseline for measuring emission cuts, the Canadian government has selected 2006 as the comparison year.

For example, the Harper government has vowed to cut emissions 20 per cent below 2006 levels by 2020. If those cuts were compared against a 1990 baseline, the total reduction over the 20-year period would be three per cent rather than 20 per cent, according to the Pembina Institute, an environmental think-tank.

The federal government has also promised cuts of 60 to 70 per cent below 2006 levels by 2050, which would equal a 51-64 per cent cut if 1990 was used as a baseline.

The Pembina Institute said Wednesday's communique will put pressure on Harper, because Canada's current commitment falls short of the 80 per cent target.

"Canada needs to live up to today's G8 declaration by adopting stronger climate targets as soon as possible, well before this December's Copenhagen negotiations," said Clare Demerse, a policy analyst with Pembina.

She said stronger targets would be "a crucial step in restoring Canada's credibility at the UN climate talks and as the host" of the 2010 G8 summit in Huntsville, Ont.

Demerse said it was important for the G8 to recognize the science-based position that temperatures can't rise more than two degrees, although she criticized leaders for failing to come up with medium-term targets.

The G8 declaration also had some cautiously optimistic words on the economy.

"While there are signs of stabilization, including a recovery of stock markets, a decline in interest-rate spreads, and improved business and consumer confidence, the situation remains uncertain, and significant risks remain to economic and financial stability."

Obama has said he'd consider more stimulus measures, although Harper suggested earlier Wednesday that leaders show greater caution before initiating more spending plans and tax breaks.

"Before there's talk of additional stimulus, I would urge all leaders to focus first on making sure the stimulus that's been announced actually gets delivered," Harper said after making a $5-million contribution to the host city here, which was devastated by an April earthquake that caused about 300 deaths.

"That's been our focus in Canada, and I would encourage the same priority elsewhere."

Harper made the announcement at a podium in front of a ruined government building in central L'Aquila, a town of 70,000 that was left partly in ruins by the April quake. An estimated 65,000 people were left homeless and live in tent communities scattered throughout the region about 95 kilometres northeast of Rome.

The money will help construct a quake-resistant youth centre that will include a gym, a library, a computer room and several multi-purpose rooms.

"Although the epicentre was thousands of kilometres away, the earthquake caused tremendous worry and grief in Canada, home to nearly one and a half million citizens of Italian descent," Harper said.

"Your tears, ladies and gentlemen, were our tears, too."