Global leaders meeting in Italy Wednesday are being urged to endorse a limit on human-caused increases in global temperatures of 2 degrees Celsius, the threshold above which damage to the planet is expected to be "catastrophic."
The European Union has formally adopted the 2-degree target in its climate-change plans, but the United States and Canada have not, amid concerns that such a target would increase the pressure on them to make deeper emission cuts than they are now planning.
Group of Eight leaders will also be debating international financing proposals that would see developed countries provide as much as $100-billion a year to poorer nations to reduce greenhouse-gas emissions and adapt to changing weather patterns.
The G8 heads of government will also meet with a larger group of leaders from the Major Economies Forum - including China, India, Brazil and South Africa - and hope to win support from large emerging economies for a climate-change treaty that would require them to rein in emissions.
While China has taken a more aggressive stand on global warming over the past year, India is refusing to sign any deal that would limit its development, while Brazil says rich nations must lead the way with deep emission cuts and massive financial transfers to newly emerging economies.
The 2-degree Celsius threshold is seen as a rallying point for aggressive action.
Canada's Environment Minister Jim Prentice has warned that, should temperatures climb more than 2 to 2.5 degrees above pre-industrial levels, "extreme climate" would result in many parts of the world that would be "catastrophic" for huge numbers of people.
However, heading into this week's meeting, Canada has not endorsed the call to adopt the 2-degree target as the basis for a global emissions treaty to be hammered out in Copenhagen in December.
"What the G8 [countries]can do, because they are the largest economies and the historical polluters, is to give some really critical political momentum to the Copenhagen process," said Clare Demerse, a climate-change expert with the Pembina Institute, a Calgary-based environmental group.
She said it is critical for global leaders to agree on a science-based threshold for temperature increases in order to have a treaty that will prevent droughts, flooding and other catastrophic weather changes.
"When you've got a limit like that - basically saying above a certain level is just not somewhere humanity wants to go - then you can work back from that to set the right [emission]targets for 2050 and 2020."
Many environmental groups argue that the industrialized world needs to slash emissions by up to 40 per cent from 1990 levels by 2020 to limit temperature increases, and by 80 per cent from 1990 levels by 2050.
In contrast, the Canadian government has committed to reduce emissions by 20 per cent from 2006 levels by that year, while legislation that passed the U.S. House of Representatives and is now being debated in the Senate would cut U.S. emissions by 17 per cent from 2005 levels by 2020.
In the United States, 47 environmental and religious groups wrote to President Barack Obama last month urging him to endorse the 2-degree target at this week's G8 meeting.
"Our domestic global warming policies, as well as the international agreements we sign, should be aimed at protecting our population and broader national interests, as well as protecting the world's most vulnerable communities from the worst impacts of climate change," said the groups, including the Sierra Club, World Wildlife Federation and Natural Resources Defense Council.
In fact, the House of Representatives legislation does endorse the 2-degree threshold, even though many environmentalists - particularly outside the United States - were disappointed with the emission-reduction targets.
"Having the U.S. takes this first step and driving energy innovation is the first and best way to get to 2 degrees Celsius," said Jake Schmidt, Washington-based climate change director with the NRDC. He added there may have to be "course corrections" in the future to bring current climate-change legislation in line with the target.