For the first time, Hillary Clinton has openly criticised President Obama in her bid to win the Democrat nomination. In a tweet last night , she condemned the president’s decision to licence Arctic drilling that was he signed off earlier this week. “The Arctic is a unique treasure,” she said. “Given what we know, it’s not worth the risk of drilling.” Her criticism has been echoed around the world. Lobbyists globally are trying to work out why the man who has so clearly identified climate change as a legacy issue, and who in the past year has done so much to promote the green agenda, has taken such a controversial, counterintuitive step.

Back in May, when his decision was first indicated, the president justified it by arguing that it would be impossible to abandon fossil fuels until the transition to clean energy sources was finally accomplished. He said it would be better if the US, with its high safety standards, exploited all its available reserves, rather than importing oil from what he implied was any old foreigner. A few days later, he too took to Twitter to justify his actions, pointing out that Shell, the Anglo-Dutch oil giant which has already spent more than $6bn on the project of winning permission to drill, had been sent back to the drawing board and forced to redesign safety aspects of its plan. Supporters pointed out that the permissions had originally been granted under President George W Bush, and halting them, for example by a moratorium on the entire coastal shelf, as another Democrat candidate Bernie Sanders demanded, or by buying out the permissions, would require formidable amounts of compensation and would be likely to face a legal challenge. And, they say, it is only a permit to explore, not yet a contract for commercial exploitation. If the anticipated reserves are present, those contracts would be a matter for the next administration.

But his green critics, so recently heralding him as one of their own, are mystified and unimpressed. Only last February he vetoed a mischief-making Republican bill to allow oil from Canadian tar sands to be imported down the Keystone XL oil pipeline on the grounds that the US did not need more oil; now he is justifying giving the go-ahead to Shell – a European multinational, the greens point out – in the name of energy self-sufficiency.

This all feels familiar to observers of the British scene. Earlier this week, in the name of the search for energy security, more than 1,000 square miles of countryside was opened up for fracking applications. The decision comes hot on the heels of proposed changes to planning laws that will force councils to decide all relevant applications within 16 weeks, with the government waiting to step in and impose a decision, and after environmental campaigners complained that in the government’s first three months 10 different environmental policies had been watered down or scrapped altogether. Among them were support for renewable energy technology and the privatisation of one of the few flagship successes of the last parliament, the Green Investment Bank.

This is depressing evidence of a reluctance to tackle the biggest challenge of keeping the increase in global temperatures below two degrees. Since March, the Guardian has been campaigning to persuade investors that ultimately the majority of known fossil fuel reserves will have to be left in the ground. That requires a revolution not just in thinking now, but in the way we understand economic development. It can no longer be a question of exploitation of resources. Now sustainable economies must be built on a deliberate exercise in forgoing them. Every time a new area of fossil fuel exploitation is opened up, even if it is shale gas which appears less polluting than coal or oil, the risk is that the transition drifts a little further downstream. The Obama decision on Arctic drilling on the face of it is wrong because it jeopardises a unique environment. But it is even worse than that. It is another barrier to a change on which a sustainable future depends.