The Green party manifesto, launched on Tuesday, outlined ambitious climate change targets that would see the UK running a zero carbon economy by 2050. But energy analysts have told the Guardian that while laudable, elements of the Greens’ climate agenda may be impossible to implement even if the funding floodgates are opened.

The Green party’s platform improves on the UK’s current carbon reductions plan, which would see cuts of 80% by the middle of the century. Total decarbonisation would be achieved by doing the heavy lifting in the next 15 years, via a 90% reduction in emissions by 2030 based on 1990 levels. This policy requires the UK to make double the cuts required by EU legislation in the next 15 years.

The UK’s current carbon targets are not enough, the manifesto said, to keep the world within the agreed safe level of warming of 2C. The rich, carbon-intensive UK economy must take on a larger share of the burden of emissions reductions than developing nations, it said.

To achieve the targets, the Greens would spend £80bn to foster an energy transformation of unprecedented scope. Solar capacity would increase from just over 5GW to 25GW by 2020. To do this it would need to grow faster than in Germany in 2014 during the massive boom inspired by the country’s energiewende transition to renewable power. Offshore wind energy would grow enormously from 8GW to 42GW by 2020 and 60GW by 2030.

The Greens said the parameters for this expansion were set in consultation with renewable energy industry professionals. But Rob Gross, the director of Imperial College’s Centre for Energy Policy and Technology, said the Greens’ timeframes may be too ambitious.

“I wouldn’t say they are impossible, but you’d really have to be able to put things on a war footing to get to those levels of roll-out in such a short space of time. It would be unprecedented, for offshore wind in particular,” he said.

Gross said current pipeline projects have the UK reaching around 10GW of offshore wind by 2020. Jim Watson, the director of the UK Energy Research Centre said increasing this to more than 40GW was likely to be unattainable.

“It seems extremely ambitious, just because of the lead times of getting those project up and running. Even were you to throw a lot of money at it now you’d struggle to meet those targets,” said Watson.

Solar energy has proven more scaleable, said Gross, but with any such large scale, disruptive push for a certain energy type there is the risk it will increase prices and drive the industry offshore.

“Trying to go too far too fast can drive up costs. There’s also a danger that if you want to do this to build UK skills, you can’t actually get those skills quickly enough, so the only way you can hit those targets is by bringing in people from other countries. I’m not against that, but if what you are trying to do is create jobs for people in this country then you need to give those people enough time to get trained,” he said.

There was also a question, said Gross, about whether distribution networks could cope with such high levels of decentralised renewable energy - as opposed to the highly-centralised power from coal, gas and nuclear. Gross said the grid in Germany was already experiencing some problems coping with the increasingly fragmented supply driven by the energiewende.

“In some respects one would welcome that and in the longer term that might be exactly what we would like to happen. But there’s an adjustment period that’s needed. Building energy infrastructure does tend to take time,” said Gross.

The UK’s Committee on Climate Change (CCC) is responsible for drafting legally binding advice on how the UK can meet its carbon targets. John Gummer, a Tory member of the House of Lords and the chair of the CCC, said the Greens’ manifesto raised important questions about the UK’s climate ambitions, but their timeframes for implementation were fundamentally flawed.

“This has to be an evolutionary process, building stage upon stage,” he said. “The Green party is helpful in stretching our ambition but its dogmatic and doom-laden approach will simply not gain enough public support till it’s too late and disaster is upon us. They also carry a good deal of political baggage which distracts from their core environmental message. I admire their tenacity, share their concerns, but doubt their tactics and question their solutions.”

Along with their expansion of renewables, the Greens said they would implement a policy of stopping any new nuclear renewable developments (including scrapping the already approved plans for the Hinkley Point C station) and ban fracking for shale gas. Both of these are seen by their advocates as energy sources required for the transition away from coal and eventual decarbonisation.

Gross said the Greens position on these energy sources was defensible. “We could do it without nuclear, I don’t see nuclear as being essential to decarbonisation and I think fracking is an irrelevance,” he said.

But if their emissions reduction targets were to remain feasible, Gross said the £24bn billpayers are set to pay for Hinkley would have to be redirected towards carbon capture and storage technology (CCS) or developing ways to store energy from the fluctuating supply from renewable sources. Both technologies are currently in their infancy.

Watson agreed: “In principle, if you look at modelling and analysis that’s been done you could meet the climate targets that we have now without nuclear. This does place more emphasis on other options like CCS, or a breakthrough in energy storage, but there are uncertainties about these.”

Note: the statistics for onshore wind and solar capacity were changed to reflect the latest statistics. The article originally said solar was at 1GW and wind at 1.6GW.