OTTAWA - A prominent environmental scientist and author says Canada has been a laggard in talks to forge a new deal to fight climate change.

Australian Tim Flannery says the world faces a watershed moment at the United Nations climate-change conference in Copenhagen in December.

The best-selling author of The Weather Makers says Canada has been "singularly unhelpful" at negotiations to broker a draft deal in time for Copenhagen.

He took the government to task for "not engaging at the sort of level that you would expect of a developed country of this influence and this economic heft."

However, Canada's top climate-change envoy says recent talks in Thailand -- where many developing countries walked out after Canada's address -- were actually constructive.

Michael Martin says he's optimistic countries will hammer out a deal by the end of the Copenhagen talks.