It seems a deal has finally been brokered on the UK's long-term climate change targets, with an announcement expected this afternoon from the energy and climate change secretary committing the UK to a 50% cut in greenhouse gases – compared with 1990 levels – by 2025.

The bitter divisions in the cabinet over whether to accept the advice of Lord Turner's independent Committee on Climate Change and commit to a fourth carbon budget are incredibly revealing. The Treasury and Department for Business, Innovation and Skills (BIS) reportedly mounted the greatest resistance to this pillar of existing climate policy, making a mockery of George Osborne's pre-general election claim that, under his chancellorship, the Treasury would be "a green ally, not a foe".

A positive decision on climate targets is certainly welcome, and it's a good news story for a coalition which has so far failed to impress with its shambolic environmental policy – so naturally the prime minister is keen to take credit. The idea, though, that a principled intervention from David Cameron saved the day rings a little hollow.

Since pledging that the coalition would be "the greenest government ever" and posturing as the "fourth minister" at Decc, Cameron has shown little interest in green policy and the sustainability agenda. Even his new environmental adviser, a former policy director for BP's now defunct Alternative Energy – and commended for the No 10 post by Lord Browne, former BP chief executive, no less – comes with a health warning.

And the truth is that signing up to the fourth carbon budget is the very least this Conservative-led administration should be doing. The previous government accepted all of the three previous carbon budgets set out by the CCC and the coalition is bound by law under the Climate Change Act to set this fourth by the end of June. That this legally binding element was ever in any danger hints at the level of scepticism amongst the government's biggest players.

Furthermore, the coalition is set to ignore CCC advice to toughen up the existing targets for 2013-2023. And campaigners are rightly concerned that a concessionary review clause in the fourth budget, set for 2014, could allow the government to backtrack on its promises.

Perhaps worst of all, it appears that the government is also disregarding the committee's advice that the budget should be met through domestic action alone. Allowing the use of trading mechanisms such as offsetting essentially means outsourcing our emission reduction responsibilities to other countries – and will weaken the drive to achieve more green technologies and industries, with all the jobs those would bring, here in the UK.

The dramatic politicking over the fourth budget – which ironically will cost Cameron nothing in this parliamentary term – comes precisely a year after the formation of the coalition. Ultimately, actions speak louder than targets, so what of the Lib Dem-Conservative environmental record so far?

To be the greenest government ever isn't a particularly ambitious goal, given Labour's record on the environment. While the Climate Act was undoubtedly a milestone achievement, road building under Labour increased, aviation grew, Britain languished at the bottom of the EU's record on renewable energy, and fossil fuel use actually rose.

Perhaps the coalition was mindful of the fact that Labour didn't set the bar too high when it decided, just a few weeks into its new term, to scrap the very body designed to measure its environmental performance – the Sustainable Development Commission.

A strong and independent voice providing well-informed scrutiny of government policy, the SDC worked towards mainstreaming sustainability throughout Whitehall.

Ministers insist that the independent monitoring and scrutiny role can now be fulfilled by the Environmental Audit Committee, on which I sit. But its chair and its members strongly disagree, since the capacity and the resources are simply not there.

The government has also ignored advice, including from the EAC, to make the cabinet responsible for mainstreaming and embedding sustainable development across all departments – instead giving that role to Defra, ensuring that it continues to be perceived as a marginal environmental issue.

The "green investment bank" is next on the list of coalition disappointments, as perhaps the most inappropriately named government initiative to date – given that it is neither very green nor a fully fledged bank, as I've noted before. This squandered opportunity to lever sufficient capital to help everything else in the low-carbon economy get going says much about the government's lack of vision on green policy.

Not only is the government actively considering using the bank to subsidise nuclear power, the bank's wings are being clipped from the outset by insufficient capitalisation and no initial borrowing capacity. So rather than unlocking the £450bn in finance for renewable energy and the low carbon infrastructure needed in the next 15 years, the bank's impact will, initially at least, be limited to the original £3bn funding.

The government's seeming determination to back nuclear power and channel taxpayers' money into the industry without having to publicly renege on the "no subsidy for nuclear" coalition pledge manifests itself in imaginative ways.

The proposed electricity market reform, for example, has the potential to help update our ageing energy infrastructure and facilitate urgently needed investment in renewables.

But, as the committee pointed out in a report yesterday, ministers have failed to be upfront about the fact that nuclear is set to benefit massively – especially from the carbon floor price, which at £30 a tonne, would give nuclear power companies a windfall subsidy of anything from £1.3bn to £3bn over 13 years.

And while the government remains in thrall to nuclear, uncertainty over renewables seems inevitable.

The extraordinary U-turn on feed-in tariffs, for instance, left the solar industry reeling in shock. The government's decision to indicate it will slash financial support to solar over 50 kW has effectively pulled the rug out from under the industry, creating significant investment uncertainty in one of the few industries to have created thousands of new jobs.

Also falling short of expectations was the energy bill, presented to parliament for its second reading last week by the energy and climate change secretary.

Despite some significant statements of ambition on improving energy efficiency in housing and positive moves to address standards in the private rented sector through the green deal, the proposals lacked detail, joined-up thinking and realistic strategies to deliver on the scale really needed to address fuel poverty and reduce climate emissions.

Meanwhile, coalition pledges to enhance and protect our natural environment have been completely undermined by a considerably weakened Defra – most clearly illustrated by the debacle over the reckless, ideologically motivated plans to sell off the nation's woodland and forests.

It's clear that some in the coalition are more committed to a greener economy than others. But the overwhelming impression of government policy is unambitious and incoherent.

If it's not actively challenging the principles of green politics – lumping essential legislation such as the Climate Change Act into its "red tape challenge", for example – the government is refocusing the political debate in favour of deregulation in order to return the economy to an unsustainable "business as usual".

Despite all this, could this still be the greenest government ever?

In the words of George Eliot: "It is never too late to become what you might have been". But the prime minister and his colleagues must start showing real political leadership on the environment and commit to ambitious policy reforms – otherwise this government's legacy will be a failure to realise the UK's significant potential as a leading green economy.