SNP and Green ministers have been accused of adding “relentless” pressure onto Scotland's over-stretched bus operators after a funding black hole totalling hundreds of millions of pounds has been revealed in a vision to decarbonise public transport.

Analysis has showed that up to £640 million of missing cash will need to be found to upgrade polluting diesel buses – amid concerns that operators will need to make up the shortfall, despite facing financial strain due to the pandemic.

The revelation is a dent to the SNP-Greens' climate credentials ahead of Scotland hosting the COP26 climate summit in Glasgow which starts on October 31.

The shortfall has emerged after a report commissioned by the transport arm of the Scottish Government warned it will be “impossible to deliver a viable alternative to cars” if “insufficient government investment” is put forward to decarbonise Scotland’s bus fleet.

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Transport accounts for around 11Mt of carbon emissions each year in Scotland – but ministers hope to cut this to 6.5Mt by 2030.

In its Programme for Government last month, the Scottish Government announced it would remove all remaining diesel buses currently on the road in just two years’ time.

Transport Minister Graeme Dey has confirmed that “there are approximately 4,400 public service buses” in Scotland, adding “the majority of which will have to transition to zero emission buses in order to meet the 2023 ambition”.

The policy will contribute to a commitment to cut car kilometres in Scotland by 20 per cent by 2030 and cut emissions by 75 per cent of 1990 levels in nine years’ time.

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Through the Scottish ultra-low emission bus scheme, 272 green buses were commissioned from £36.9 million of funding that was made available in 2020/21.

The first phase of the Scottish zero emission bus challenge fund is now underway with £50 million being available for buses and infrastructure.

But the Scottish Conservatives have warned that the funding will only stretch far enough to decarbonise 343 buses, falling well short of the Scottish Government's target.

Analysis from the Scottish Parliament Information Centre (Spice) shows that as of March 2020, “93% of buses used to provide local services in Scotland were diesel powered”, with just 1% “fully zero emissions”.

The study adds: “There are 4,400 buses used to provide local services in Scotland, meaning that there were 4,092 diesel powered buses”.

At least 2,046 diesel buses will need to be replaced by December 2023 under the SNP-Greens commitment, which would be “roughly 76 buses a month for the next 27 months”, according to the Spice research.

It adds: “By comparison, the Scottish Government’s Scottish green bus fund supported the purchase of 475 low-emission buses between 2011 and 2018 – many of which were diesel powered.”

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The Scottish Conservatives have claimed that ministers will need to provide around £640 million of funding for bus operators to make the switch.

Scottish Conservative transport spokesperson, Graham Simpson, said: “This is yet another prime example of a SNP-Green policy bound by unachievable timescales and overbearing costs. It won’t be the Government paying for it, but rather already overstretched bus operators.

“The pressures that the nationalist coalition are putting on our bus industry are relentless.

“Bus operators have already made huge strides in greening fleets, infrastructure and their overall operations.

“If the SNP-Green Government aren’t willing to significantly increase the funding they have made available, then they must rethink the timescales they have set to remove these diesel vehicles.

“Right now they are setting the industry up for failure. It is clear that this SNP-Green Government are not matching their rhetoric on climate change with the funding needed to hit crucial targets.”

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Paul White, Scotland director of Confederation of Passenger Transport (CPT) UK, has issued a warning over the short timescale to decarbonise bus fleets as operators face pandemic pressures.

He said: “The Scottish bus sector is committed to decarbonisation and has invested over £70 million in zero emissions buses over the last 18 months. The Scottish Government target for greening half the Scottish fleet by 2023 is extremely ambitious and has been set despite the impact of Covid-19 leaving patronage levels suppressed, thereby restraining the sector’s ability to ramp up investment.

“The majority of the cost of every zero emission bus is met by the operator. There are limited levers for operators to pull to reduce costs or increase revenue to divert resource towards fleet investment.

"These could include reviewing fares or services levels – neither of which would help build back bus patronage, nor encourage more people to use bus, thereby reducing congestion and carbon emissions associated with car use. The transition to zero emission must be just for bus passengers and bus operators.

“CPT Scotland continues to contribute to the government’s bus decarbonisation taskforce to identify an evidence-based pathway to net zero that is challenging but achievable. We would urge the government to provide continuing support for decarbonisation based on the findings and timetable identified by the taskforce.”

A coalition of organisations representing bus operators, local authorities, manufacturers and workers has called on the UK Government to speed up its zero emission bus rollout or risk missing key targets.

In a letter sent to Transport Secretary Grant Shapps and the Chief Secretary to the Treasury, Simon Clarke, a plea has been made for UK ministers to use the forthcoming spending review to set out a clear roadmap to deliver significantly more buses on an accelerated timetable and ensure funding reflects the full costs of zero emission vehicles and and infrastructure.

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Last month, a report published by Element Energy, commissioned by Transport Scotland, starkly warned that “insufficient government investment in delivering a zero-emission world class public transport network will make it impossible to deliver a viable alternative to cars, limiting both cars and bus decarbonisation efforts”.

The document adds that the Scottish Government's timeline for decarbonising the bus fleet was “very ambitious requiring strong financial support”.

It said: “As with cars, the main barriers to the introduction of zero-emission buses and coaches are cost, refuelling infrastructure, and supply of vehicles.”

The report highlights that Alexander Dennis offers “both electric and hydrogen bus models".

It adds: “The Scottish Government should continue to work with original equipment manufacturers across Europe to introduce zero-emission buses into the Scottish market as quickly as possible, taking advantage of the more advanced position of zero-emission bus supply and domestic production in the bus sector that Scotland cannot rely on in other road vehicle sectors.”

A Scottish Government spokesperson said: “We’ve been clear that our ambition to remove the majority of fossil fuel buses from public transport by the end of 2023 is not something government can achieve alone.

“That is why within our first 100 days we convened the bus decarbonisation taskforce to agree with leaders from the energy, finance, bus manufacturing and bus operating sectors all the actions needed to transition to zero emission vehicles.

“We have made £50 million available now, through the first phase of the new Scottish zero emission bus challenge fund, to support the actions needed. This builds on the £50 million we made available last year, which unlocked over £71 million of private investment, resulting in 272 new zero emission bus orders, of which 207 are currently being built here in Scotland.

“This new funding round, which is part of the £120 million we are making available to the sector to invest in zero emission technologies, is designed to encourage the market to bring forward new and innovative ways to finance zero emission buses into the future.”