Harper said world leaders must now set a world standard that includes a range of individual national targets that reflects the different circumstances of each country.
This could take time, he said. The declaration called for global targets set under the United Nations by 2009.
"There will obviously be plenty of discussion – for months and maybe even years," the prime minister told a news conference at the summit.
"But I think we are all on the same path now toward a consensus of industrialized countries, and I think that's necessary to attain a consensus with developing countries."
The language of the G8 declaration cited a desire to halve greenhouse-gas emissions by 2050.
But it appeared to be well short of a full commitment, saying all members should "seriously consider" following the European Union, Japan and Canada in cutting emissions by 50 per cent.
Some of the thousands of protesters kept well away from the summit site have been pressing for clear goals for cutting emission.
The German summit hosts had lobbied their counterparts for months to accept binding emissions cuts but appeared to have failed in bringing the United States and Russia on side.
Still, German Chancellor Angela Merkel declared the gathering a success.
"We agree that we need reduction goals – and obligatory reduction goals," she said.
"No one can escape this political declaration. It is an enormous step forward."
British Prime Minister Tony Blair called the declaration a major step forward leading up to UN climate-change negotiations in Bali, Indonesia, at the end of the year.
But Harper said it would be optimistic to expect a deal to be reached there.
The prime minister said developed countries would obviously need to shoulder a major share of the burden because they can afford it, but an effective deal would include everyone.
He said getting such a deal could take months and years. The important thing is getting it right this time, he said.
"We know the history of Canada – we committed to targets without thinking those targets through 10 years ago, and then are unable to reach them," he said of the Kyoto accord.
"When others say we want a fulsome discussion before we actually determine what reasonable targets are, I think that's something we have to be flexible on."
The United States declared the go-slow approach in the agreement is a vindication of its position.
Back in Canada, the Liberals accused Harper of helping U.S. President George W. Bush delay progress in the climate-change fight.
"Canada... contributed to sinking the process, contributed to backing President Bush," Liberal leader Stéphane Dion said. "We are now less advanced than we were in December 2005 at the United Nations conference on climate change."
Bush resisted a binding deal at the summit, arguing that world leaders should have until the end of 2008 to bring economic powers like China, India and Brazil on side.
His national security adviser, Stephen Hadley, said the deal reflected his call to have the top 15 polluters meet to set a long-term goal and decide for themselves how much to do toward meeting it.
Harper agreed that targets should vary depending on national circumstances.
He used as an example the European Union, which has various targets for member countries that add up to its overall goal of reducing emissions 20 per cent by 2020.
"There should be a global target. And then obviously there need to be national targets," Harper said.
"Those targets are going to have to be very different, depending on different circumstances."
He suggested developed countries should carry a heavier share of the burden. People with a higher standard of living should "accept some of the economic pain," he said.
That expressed willingness to shoulder extra responsibility stood out as a striking reminder of how much Harper's position on climate change had evolved from his time in opposition.
At the time, he called the Kyoto accord a money-sucking socialist scheme while pointing out that carbon – a key greenhouse gas – was also a necessary element for life on Earth.
He hailed the international declaration of war on carbon emissions and stressed the need for a global deal.
"We're a long way to getting there but I think we've made a very important first step," Harper said.
"We have to come towards real, mandatory, enforceable targets. That's certainly my understanding of where this needs to go."