It’s unrealistic to expect the UK to reach net-zero emissions sooner than 2050, said the head of the British government’s official climate policy advisory body in response to calls by Extinction Rebellion and other activists to speed up the process.

Chris Stark, chief executive of the Committee on Climate Change (CCC) – which is recommending the ambitious goal to UK ministers – said there “are very big barriers” to achieving an even earlier date, as demanded by the Extinction Rebellion movement which in April brought parts of London and other cities to a standstill by blockading roads.

Stark said before the CCC announced its recommendations he’d expected much of the backlash to claim 2050 was too ambitious. In fact “the majority of the debate that’s been had has been with the lobby that would like us to go faster and sees our recommendations as being overly conservative”, he told a conference organised by Aurora Energy Research on Wednesday.

“That’s not something I expected a year ago. My main response to Extinction Rebellion and others looking for a tougher target is: 'great, but there are some very big barriers to achieving an earlier net-zero date'.”

They include the need for a massive number of trees to grow and above all tackling the decarbonisation of heat – “by far the biggest policy challenge” – in UK buildings.

“We do not have, yet, the right skills in this economy to deploy the solutions we need to get to net-zero, at the appropriate scale to do something, I think sooner than 2050,” said Stark, claiming that to set an earlier target would be a “huge risk” without the policies in place to deliver them.

“The target means nothing unless there are a set of policies in place to meet it,” said Stark, who warned the UK’s political parties that they risk losing younger voters unless they swing behind ambitious climate action.

Stark’s comments came as latest Aurora research said complete decarbonisation of the economy as recommended by the CCC will require 5-10GW of new wind and solar to be added every year in the UK, representing an investment of £4-9bn ($5-11.4bn).

The “radical transformation” of the energy system necessary will bring big challenges to renewables investors and the government.

Under its high-renewables secenario for 2050, Aurora said the huge amount of clean generation coming online would leave ever-growing levels of power unsold during windy and sunny periods, and depress wind prices to less than half the current £50/MWh.

“These issues mean that it becomes difficult for renewables to recover their costs in the wholesale and balancing markets. In other words, the more wind and solar on the system, the less revenue they make from the energy market per unit of electricity and the more revenue they will need from other sources to justify investment,” said the study.

Ben Collie, principal at Aurora, said the UK government faces a “stark choice about how to reach net zero: a market-driven approach which enables renewables investment and operational decisions through market forces; or a stateled approach characterised by more subsidies and central planning”.