The endorsement from Andrew Weaver comes as Mulcair unveils a new approach to tackling greenhouse gas emissions.
Mulcair's plan goes further than the cap and trade proposal advanced by late NDP leader Jack Layton in the party's election platform last spring.
It would apply the "polluter-pay" principle not just to the 700 largest industrial emitters but to all major sources of greenhouse gases.
Mulcair says Canada "can no longer afford to focus only on the worst of the worst."
The plan and Mulcair's candidacy have been endorsed by Weaver, lead author of the United Nations Intergovernmental Panel on Climate Change and co-recipient of the 2007 Nobel Peace Prize.
"Canada needs a prime minister who recognizes that a healthy economy does not have to come at the expense of a healthy environment," Weaver said in a written statement Thursday.
"In my considered assessment, Thomas Mulcair is ideally suited for the task."
Mulcair released his cap and trade plan just as Canada is being roasted by environmentalists -- and some other nations -- for planning to opt out of the international Kyoto treaty on greenhouse gas emissions and for allegedly obstructing efforts to craft a post-Kyoto deal.
"(Prime Minister) Stephen Harper has let the climate change crisis get worse," said Mulcair, a former Quebec environment minister.
"His inaction is leaving a huge ecological debt in the backpacks of future generations. With every day that we wait, the cost of our failure to address climate change grows greater."
Cap and trade is a system of financial penalties and rewards aimed at encouraging polluters to clean up their act.
Under the NDP's plan in the last election, the largest emitters would have faced a government-imposed limit on greenhouse gas emissions. Those exceeding the limit would have been forced to buy carbon credits, at a rate of about $45 for every tonne of emissions over the limit.
Those that managed to reduce emissions below their limit would have been able to sell carbon credits to other emitters.
The NDP estimated its proposed system would generate $21.5 billion in revenue for the federal government.
However, critics maintained the financial penalties on emitters such as oil and gas companies and coal-powered hydro plants would simply be passed on to consumers.
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