OTTAWA - Prime Minister Stephen Harper announced his support for a UN summit on global warming Wednesday as past musings about climate change came back to haunt him.

His explicit acknowledgement of the urgent need to tackle climate change was made as his political foes pummelled him with his own words for the second straight day.

Harper vowed Canada would attend a global summit that hasn't even been called yet -- a conference on climate change that some in the United Nations are demanding.

"I have not received an invitation from the United Nations Secretary General. However, if we did, we would accept,'' Harper said.

"We all recognize this is a serious environmental problem that needs immediate action. Canada's decision to do nothing over the past decade was a mistake, and we want to do better.''

The sincerity of his convictions was called into question by Liberals waving a five-year-old letter written by Harper that downplayed the reality of climate change and heaped scorn on the Kyoto accord.

Liberal Leader Stephane Dion said Canadians find themselves in the unfortunate circumstance of having a "climate-change denier'' as a prime minister.

"What I'm saying is he hasn't changed,'' Dion said.

Throughout the day Liberals relentlessly tried to use the letter to sow doubt over Harper's conversion to green.

"The reality is people are prisoners of their past beliefs,'' said Michael Ignatieff, the party's deputy leader.

"This is a man with ideological opposition to the scientific reality of climate change. Now if that's the case, it's very difficult for this old leopard to change his spots.''

Several Conservative MPs tried to draw a distinction between the Kyoto accord and the need to tackle climate change -- a need that the government has now made a priority.

"The Kyoto accord itself doesn't do a whole lot -- as we've seen with the Liberal record -- to reduce greenhouse gases,'' Saskatchewan MP Andrew Scheer said.

"(It's) a trading system, a transfer of wealth from one part of the world to another.''

The Conservatives announced in the last election campaign that they considered Kyoto's greenhouse gas-reduction targets unattainable.

But with polls suggesting the environment has become the top priority for voters, most Conservatives stick to criticizing the Liberals' failure to cut emissions rather than directly attacking the Kyoto protocol.

Canada's emission levels have risen 27 per cent since 1990, while the accord calls for a six per cent cut by 2012.

Harper used that statistic to poke fun at Dion -- who has named his dog Kyoto. He said the Liberal leader is in denial about his own party's track record.

"I suggest he should rename that dog for all his various denials,'' Harper said.

"Perhaps he could call the dog Clean Air, or perhaps he could call it Fiscal Imbalance. Or maybe he could even call his dog The Sponsorship Scandal.''

The Conservatives have agreed to rewrite their own Clean Air Act, which came under fire when it failed to set greenhouse-gas reduction targets for decades.

The original legislation has been sent to a special committee, which is scheduled to produce a new draft by April.

Meanwhile, word of Harper's 2002 letter has made it on to some U.S. news sites -- including the hugely influential Drudge Report website.

That site, which claims to receive over 15 million visits a day, ran in bold red type the headline: Canadian Prime Minister's Letter Dismisses Kyoto as `Socialist Scheme.'