When you think of Canada, which qualities come to mind? The world's peacekeeper, the friendly nation, a liberal counterweight to the harsher pieties of its southern neighbour, decent, civilised, fair, well-governed? Think again. This country's government is now behaving with all the sophistication of a chimpanzee's tea party. So amazingly destructive has Canada become, and so insistent have my Canadian friends been that I weigh into this fight, that I've broken my self-imposed ban on flying and come to Toronto.

So here I am, watching the astonishing spectacle of a beautiful, cultured nation turning itself into a corrupt petro-state. Canada is slipping down the development ladder, retreating from a complex, diverse economy towards dependence on a single primary resource, which happens to be the dirtiest commodity known to man. The price of this transition is the brutalisation of the country, and a government campaign against multilateralism as savage as any waged by George Bush.

Until now I believed that the nation that has done most to sabotage a new climate change agreement was the United States. I was wrong. The real villain is Canada. Unless we can stop it, the harm done by Canada in December 2009 will outweigh a century of good works.

In 2006 the new Canadian government announced it was abandoning its targets to cut greenhouse gases under the Kyoto protocol. No other country that had ratified the treaty has done this. Canada was meant to have cut emissions by 6% between 1990 and 2012. Instead they have already risen by 26%.

It is now clear that Canada will refuse to be sanctioned for abandoning its legal obligations. The Kyoto protocol can be enforced only through goodwill: countries must agree to accept punitive future obligations if they miss their current targets. But the future cut Canada has volunteered is smaller than that of any other rich nation. Never mind special measures; it won't accept even an equal share. The Canadian government is testing the international process to destruction and finding that it breaks all too easily. By demonstrating that climate sanctions aren't worth the paper they're written on, it threatens to render any treaty struck at Copenhagen void.

After giving the finger to Kyoto, Canada then set out to prevent the other nations striking a successor agreement. At the end of 2007, it singlehandedly blocked a Commonwealth resolution to support binding targets for industrialised nations. After the climate talks in Poland in December 2008, it won the Fossil of the Year award, presented by environmental groups to the country that had done most to disrupt the talks. The climate change performance index, which assesses the efforts of the world's 60 richest nations, was published in the same month. Saudi Arabia came 60th. Canada came 59th.

In June this year the media obtained Canadian briefing documents which showed the government was scheming to divide the Europeans. During the meeting in Bangkok in October, almost the entire developing world bloc walked out when the Canadian delegate was speaking, as they were so revolted by his bullying. Last week the Commonwealth heads of government battled for hours (and eventually won) against Canada's obstructions. A concerted campaign has now begun to expel Canada from the Commonwealth.

In Copenhagen next week, this country will do everything in its power to wreck the talks. The rest of the world must do everything in its power to stop it. But such is the fragile nature of climate agreements that one rich nation – especially a member of the G8, the Commonwealth and the Kyoto group of industrialised countries – could scupper the treaty. Canada now threatens the wellbeing of the world.

Why? There's a simple answer: Canada is developing the world's second largest reserve of oil. Did I say oil? It's actually a filthy mixture of bitumen, sand, heavy metals and toxic organic chemicals. The tar sands, most of which occur in Alberta, are being extracted by the biggest opencast mining operation on earth. An area the size of England, comprising pristine forests and marshes, will be be dug up – unless the Canadians can stop this madness. Already it looks like a scene from the end of the world: the strip-miners are creating a churned black hell on an unimaginable scale.

To extract oil from this mess, it needs to be heated and washed. Three barrels of water are used to process one barrel of oil. The contaminated water is held in vast tailings ponds, some so toxic that the tar companies employ people to scoop dead birds off the surface. Most are unlined. They leak organic poisons, arsenic and mercury into the rivers. The First Nations people living downstream have developed a range of exotic cancers and auto-immune diseases.

Refining tar sands requires two to three times as much energy as refining crude oil. The companies exploiting them burn enough natural gas to heat six million homes. Alberta's tar sands operation is the world's biggest single industrial source of carbon emissions. By 2020, if the current growth continues, it will produce more greenhouse gases than Ireland or Denmark. Already, thanks in part to the tar mining, Canadians have almost the highest per capita emissions on earth, and the stripping of Alberta has scarcely begun.

Canada hasn't acted alone. The biggest leaseholder in the tar sands is Shell, a company that has spent millions persuading the public that it respects the environment. The other great greenwasher, BP, initially decided to stay out of tar. Now it has invested in plants built to process it. The British bank RBS, 70% of which belongs to you and me (the government's share will soon rise to 84%), has lent or underwritten £8bn for mining the tar sands.

The purpose of Canada's assault on the international talks is to protect this industry. This is not a poor nation. It does not depend for its economic survival on exploiting this resource. But the tar barons of Alberta have been able to hold the whole country to ransom. They have captured Canada's politics and are turning this lovely country into a cruel and thuggish place.

Canada is a cultured, peaceful nation, which every so often allows a band of Neanderthals to trample over it. Timber firms were licensed to log the old-growth forest in Clayaquot Sound; fishing companies were permitted to destroy the Grand Banks: in both cases these get-rich-quick schemes impoverished Canada and its reputation. But this is much worse, as it affects the whole world. The government's scheming at the climate talks is doing for its national image what whaling has done for Japan.

I will not pretend that this country is the only obstacle to an agreement at Copenhagen. But it is the major one. It feels odd to be writing this. The immediate threat to the global effort to sustain a peaceful and stable world comes not from Saudi Arabia or Iran or China. It comes from Canada. How could that be true?