A curious thing happened on the way back from the UN's Climate Change Summit.

In his speech to Tuesday's one-day gathering of world leaders in New York, David Cameron stressed Britain's commitment to push for a legally binding international agreement on tackling climate change in 2015.

Activists responded that the Prime Minister should live up to his rhetoric and commit to phasing out highly polluting coal power within a decade.

And then, rather unexpectedly, he appeared to come close to doing just that. A post on Twitter from the UK's team at the UN attributed to Mr Cameron a firm commitment to phase out existing coal power stations over the next 10 to 15 years.

The low-key nature of the announcement instantly made environmental groups suspicious and they are now pressing for more details. But it shows why many campaigners feel confused and frustrated by the UK Government when it comes to the matter of tackling climate change. How committed are ministers to leading internationally on this issue?

Prior to 2010, there was some cautious hope among environmental lobbyists about the Conservatives' belated enthusiasm for tackling climate change. Remember that image makeover David Cameron gave the party logo, which briefly became a green oak tree? (It is now a Union flag in the shape of a tree.) Remember the huskies in the North Pole? Remember the cycling to work (with the brief case following behind in a car)?

Since then, the UK Government has fallen short of many people's hopes. Mr Cameron appointed a Chancellor who does not want Britain to lead the world on tackling climate change, and for two years an environment minister, Owen Paterson, who regarded the environmental lobby as the "green blob" and provoked suspicions in office that he was a climate change sceptic (now out of office, he is to give the keynote address next month at Lord Lawson's climate change-doubting Global Warming Policy Foundation). Mr Cameron's government opposes an EU-wide renewables target for 2030 and the Prime Minister provoked conflict with the LibDems last year with hints he would scrap green levies on fuel bills (in the end, a £50 cut in levies was met from general taxation). On the other hand, solar power and renewable electricity capacity has increased in England and Wales and the government has established the Green Investment Bank.

Mr Cameron insisted this week that economic growth and tackling global warming could go hand in hand, and indeed many campaigners also see opportunities in Britain's transformation into a low-carbon economy. But the Prime Minister's claim that efforts to rein in global warming could be assisted by developing shale gas (which produces fewer carbon emissions than coal but more than renewables), met with frustration from campaigners.

David Cameron says he believes he has lived up to his famous promise to head "the greenest government ever", but he will have to achieve a good deal more before campaigners endorse that view.