Here's the essential fact to bear in mind. The tar sands of northern Alberta are the second-largest pool of carbon on earth, second only to Saudi Arabia. It's burning Saudi Arabia, more than any other single thing, that has raised the temperature of the planet by a degree so far. But when oil was discovered in the Middle East, we knew nothing about climate change – it's not surprising that we started pumping. In the case of Canada, however, we've taken 3% of the oil from the sands. We're still at the start. If, knowing what we now know about climate change, we just keep going, then we're idiots.

That realisation explains why Americans rose up in remarkable numbers to fight the proposed Keystone XL pipeline. In August 1,253 people were arrested outside the White House during the largest civil disobedience action in a generation. Citizens ringed the president's mansion in a line a mile long and five people deep. A couple of weeks ago, the president announced that he would delay the pipeline for a new environmental review, which would cover not only the route across the country but also climate change, public health, and other issues.

That announcement caught industry off guard. Transcanada Pipeline had already mowed the strip they planned to put the pipe on, and had carried vast quantities of steel across the border. They're fighting back with every tool they can find, but for the moment they're delayed and in trouble. It's a win, though like all environmental wins a temporary one. And it's a tribute not only to an organising effort that brought everyone from Nebraska ranchers to Occupy Wall Street protesters together, but also to the slowly dawning realisation that this was a big deal. As the leading climatologist James Hansen puts it, tap heavily into tar sands oil and it's "essentially game over for the climate".

Which is where Europe comes in. Canada wants to sell some of this oil on the continent, and as today's revelations in the Guardian reveal they've dispatched endless teams of diplomats and oil barons to make the case. They have a difficult row to hoe – because the oil is embedded in sand, it takes lots of energy to get it out of the ground and hence it's even more carbon-intensive than regular oil. The EU has provisionally imposed penalties on that extra carbon severe enough to make it difficult for Canada to sell Europeans its filthy oil.

But now, for reasons not entirely clear, the UK seems to have emerged as Canada's partner in crime, leaning on Brussels to let this crud across the borders. No one seems to know exactly why. Lingering colonial attachment? Kinship among Tory governments? The effect, however, is clear. Any good that Britain's government does with new efficiency standards, runway halts, windmills, you name it; all that will be outweighed if it manages to broker a deal to bring this oil into Europe.

Just as it was for Obama, it will be among the biggest single environmental decisions the Cameron government makes. So far it's been hidden behind some obscure jargon in Brussels, but history will expose this as one of those fateful choices humans sometimes get to make. Faced with a huge new pool of carbon, will we simply make the easy choices for short-term profit? Or will we actually figure out that it's time to think anew? Odd that in this day and age choices so important to the future of an oilfield a hemisphere away, and to the entire atmosphere, would be made in Whitehall, but that's the case here. Around the world environmentalists are watching, and hoping Britain strikes a serious blow for the future.

• Bill McKibben is the author of Eaarth: Making a Life on a Tough New Planet, and an organizer at 350.org