Contrary to countless reports, the debacle in Copenhagen was not everyone's fault. It did not happen because human beings are incapable of agreeing, or are inherently self-destructive. Nor was it all was China's fault, or the fault of the hapless UN.

There's plenty of blame to go around, but there was one country that possessed unique power to change the game. It didn't use it. If Barack Obama had come to Copenhagen with a transformative and inspiring commitment to getting the US economy off fossil fuels, all the other major emitters would have stepped up. The EU, Japan, China and India had all indicated that they were willing to increase their levels of commitment, but only if the US took the lead. Instead of leading, Obama arrived with embarrassingly low targets and the heavy emitters of the world took their cue from him.

(The "deal" that was ultimately rammed through was nothing more than a grubby pact between the world's biggest emitters: I'll pretend that you are doing something about climate change if you pretend that I am too. Deal? Deal.)

I understand all the arguments about not promising what he can't deliver, about the dysfunction of the US senate, about the art of the possible. But spare me the lecture about how little power poor Obama has. No president since FDR has been handed as many opportunities to transform the US into something that doesn't threaten the stability of life on this planet. He has refused to use each and every one of them. Let's look at the big three.

Blown Opportunity No 1: The Stimulus Package

When Obama came to office, he had a free hand and a blank cheque to design a spending package to stimulate the economy. He could have used that power to fashion what many were calling a Green New Deal – to build the best public transit systems and smart grids in the world. Instead, he experimented disastrously with reaching across the aisle to Republicans, low-balling the size of the stimulus and blowing much of it on tax cuts. Sure, he spent some money on weatherproofing, but public transport was inexplicably short-changed while highways that perpetuate car culture won big.

Blown Opportunity No 2: The Auto Bailouts

Speaking of the car culture, when Obama took office he also found himself in charge of two of the big three carmakers, and all of the emissions for which they are responsible. A visionary leader committed to the fight against climate chaos would obviously have used that power to dramatically re-engineer the failing industry so that its factories could build the infrastructure of the green economy the world desperately needs. Instead Obama saw his role as uninspiring downsizer-in-chief, leaving the fundamentals of the industry unchanged.

Blown Opportunity No 3: The Bank Bailouts

Obama, it's worth remembering, also came to office with the big banks on their knees – it took real effort not to nationalise them. Once again, if Obama had dared to use the power that was handed to him by history, he could have mandated the banks to provide the loans for factories to be retrofitted and new green infrastructure to be built. Instead he declared that the government shouldn't tell the failed banks how to run their businesses. Green businesses report that it's harder than ever to get a loan.

Imagine if these three huge economic engines – the banks, the car companies, the stimulus bill – had been harnessed to a common green vision. If that had happened, demand for a complementary energy bill would have been part of a coherent transformative agenda.

Whether the bill had passed or not, by the time Copenhagen had rolled around, the US would already have been well on its way to dramatically cutting emissions, poised to inspire, rather than disappoint, the rest of the world.

There are very few US presidents who have squandered as many once-in-a-generation opportunities as Obama. More than anyone else, the Copenhagen failure belongs to him.

• Read Naomi Klein's blog from Copenhagen at EnviroNation