Climate experts have warned the Scottish government its ambitious plans to cut greenhouse gas emissions lack credibility and risk stalling unless its strategies improve dramatically.

The Committee on Climate Change (CCC) said the devolved government had so far led the UK in its efforts to cut emissions, reducing its actual CO2 emissions by 38% by 2015 compared with 35% at UK level. Scotland is now on the brink of meeting its 2020 target to cut emissions by 42% several years early.

Nicola Sturgeon’s government is consulting on plans to set a fresh target of cutting emissions by 90% by 2050 but faces heavy criticism from anti-poverty charities and the Scottish Green party, who have urged the first minister to take faster, tougher action.

The CCC, the UK’s government-funded advisory committee, said the policies needed to hit that target were too weak and ill thought out, singling out proposals in a draft plan to heavily cut emissions by sharply increasing low carbon home heating to 80% by 2032. Those were “very unlikely to be feasible”, it warned.

The committee said ministers needed instead to put far greater emphasis on cutting the country’s rising transport emissions – an area largely neglected by the Scottish government until now.

So Sturgeon’s proposals to dramatically increase the use of electric and ultra low emission vehicles by helping phase out sales of new petrol and diesel-powers vans and cars by 2032, outlined in her new programme for government earlier this month, were a significant step forward, it added.

In an unusually critical progress report for the Scottish parliament, the CCC said: “Without firm new policies, reductions in Scottish emissions are unlikely to continue in the 2020s. The final version of Scotland’s plan should also build as fully as possible on the UK government’s clean growth plan, which will set out how UK emissions targets to 2032 will be met.

“Greater ambition will be required to reduce emissions in the transport sector, as announced in the recent programme for government, with less reliance on rapid deployment of low-carbon heating. The plan as it stands lacks credibility in meeting the emissions targets to 2032 and fails to prepare properly for deeper decarbonisation in the longer term.”

Roseanna Cunningham, the Scottish environment secretary, at the parliament building in Edinburgh. Photograph: Andrew MacColl/REX/Shutterstock

Roseanna Cunningham, the Scottish environment secretary, said her government was now putting action on climate at the heart of its policymaking. Its final plan, due to be published in 2018, would reflect the committee’s advice.

“We acknowledge that there are areas where more needs to be done in order to continue meeting our ambitious targets and to prepare for even greater future ambition under our proposed climate change bill,” she said.

Scotland’s emissions reduction had been driven largely by sharp increases in renewable electricity, funded by UK consumers, and deep cuts in coal-fired power, bolstered by forestry acting as a carbon sink.

Scotland’s last coal powered station, Longannet, closed down in 2016 and electrification of the country’s rail network is increasing, but the committee said Scottish ministers could no longer rely on renewable electricity investment to hit future targets.

That switch to renewables was driven by Alex Salmond, Sturgeon’s predecessor as first minister, but the Scottish government has been accused of pursuing contradictory policies on climate. Salmond championed North Sea oil exports to fund Scottish independence, while investing billions on new road projects, and Sturgeon plans to phase out air passenger duty to boost aviation.

While the Scottish government has no direct say over vehicle emissions standards and little influence over manufacturers, committee officials said it could boost electric vehicle use by greatly increasing charging infrastructure and having tougher planning and parking policies.

The committee said the 2032 policy would lead to the complete phasing out of all petrol and diesel powered cars by 2050. Ministers are also introducing new low emission zones in Scotland’s four largest cities, to control pollution and inefficient vehicles.

Patrick Harvie, the Scottish Greens’ co-convenor and parliamentary leader, accused Sturgeon of lacking ambition. He said that cutting emissions by 90% by 2050 represented a slowing down of action. If Scotland continued to cutting annual emissions at the present rate, it could reach zero net emissions by 2040.

“Other countries have already set net-zero targets before the year 2050, including Norway and Sweden”, Harvie said last Friday. “Scotland has a chance to continue showing leadership and adopt a net-zero target and I’m glad the first minister has at least committed to ‘proper consideration’ of consultation responses.”