VANCOUVER - More than 120 of Canada's top climate scientists have signed an open letter criticizing Conservative government policy and urging Canadians to vote "strategically" for the environment in next week's federal election.

"Global warming is the defining issue of our time," said Andrew Weaver, a lead author with the Nobel Peace Prize-winning Intergovernmental Panel on Climate Change.

But Weaver said Tuesday that Stephen Harper's government "has yet to get engaged in the innovative and urgent policies that we need to significantly reduce greenhouse gas emissions in Canada."

This is shaping up to be "the rare election in which the environment is the issue," said the group's John Stone.

Never has attention to the environment been more necessary, Stone said. But the opportunity for an informed national debate on Canada's response to global warming is slipping away, he added.

Campaigning in Vancouver on Tuesday, Liberal Leader Stephane Dion said the Conservatives have still not embraced the concept that climate change is real.

"We have a government that believes that ideology should trump science," Dion told party supporters. "Stephen Harper has denied the reality of climate change."

The letter is signed by a who's who of Canada's top climate experts. It says "it seems people have simply no idea how serious this issue is" even though it's clear the public accepts that global warming is a threat.

"Global warming is a problem that must be dealt with now, before it's too late," says the letter. "Any further delay will only increase the risks of damage and costs of action.

"The world needs to start down a path of greenhouse-gas reduction to avert the most serious consequences of global warming."

Even if atmospheric greenhouse gases are stabilized at current levels, it says, the Arctic would still go ice-free in the summer, between 10 and 25 per cent of the world's species would still be committed to extinction, and weather will continue to become more extreme.

Many of the letter's signatories are research scientists who depend on federal granting agencies for the bulk of their funding.

But David Schindler, who won Canada's top science prize in 2001, says the scientists are confident that granting agencies will continue to support good science on its merits and on the basis of professional peer review rather than politics.

"Regardless, this is not a moment for any Canadian to be timid," Schindler said. "This is an urgent issue and I am proud to side with so many scientists who are willing to stand up for what they believe in."

The letter said Canadians stand at a critical juncture in their country's history, with key international meetings on the environment and climate change coming up over the next year.

But it is critical of Harper's government, saying Canada has obstructed international efforts to address global warming over the past two years:

-- At the 2007 Commonwealth Heads of Government Meeting in Uganda, it says, Canada scuttled attempts aimed at getting consensus on a strongly worded commitment to greenhouse-gas reduction.

-- At a United Nations meeting in Bali in December 2007, it says Canada was "an international embarrassment," doing more than any other country to block progress and resisting attempts to include reduction targets proposed by the European Union and others.

The Conservative government plan focuses on emission intensity, not emissions reductions, it notes.

"Reducing emission intensity means that you continue to pollute, but do so more efficiently," it says.

It cites the oilsands as an example, noting part of the plan will be to require project participants to reduce emissions intensity by 2020.

"But oilsands production could quadruple by that time," the scientists write.

"The overall net effect of the federal regulation would be to allow a tripling or more of greenhouse-gas emissions from the oilsands sector by 2017, and possible continued increases after that. Frankly, no matter how you try to spin it, that is not a reduction."

The only way to deal with global warming is to put a price on emissions, it says, citing a carbon tax like that proposed by the Liberals, or a cap-and-trade system such as the one proposed by the New Democrats -- or both.

"The carbon tax provides price certainty, is easier to implement, more transparent, easy to make revenue-neutral and less open to abuse," the letter says.

"Cap-and-trade systems require self-regulation and reporting, cumbersome bureaucracy to administer and take a long time to implement while details, such as credit for early action, process of awarding emissions permits, and reporting requirements get worked out."

In both cases the price is passed on to consumers through increases in the prices of carbon-intensive products, it says. In the carbon-tax case the consumer sees what price is added, whereas it is obscured in the cap-and-trade system.

"It is disingenuous to claim on the one hand that the carbon tax will cause an economic disaster and on the other hand advocate for a cap-and-trade system," it says. "They are equivalent economic instruments that have the same effect of pricing emissions.

"Ordinary Canadians deserve to be told this and not have the issue obscured in political rhetoric."