The goal of reducing greenhouse gas emission to net-zero by 2050 is already “extraordinarily ambitious”, according to the head of the government’s independent climate adviser.

Chris Stark, chief executive of the Committee on Climate Change (CCC), said there are “huge risks” in bringing forward the target to an earlier date as demanded by environmental activists.

Stark was responding to criticism from Extinction Rebellion, which accused the CCC of betraying future generations when it recommended the government set a legally binding target of reaching net-zero emissions by 2050. The campaign group has instead called for the UK economy to be completely decarbonised by 2025.

Speaking at an event hosted by Aurora Energy Research, Stark recalled having “sleepless nights” over its advice to government: “I was very concerned we were going to say something that the government wasn’t ready to accept.

“And what happened over the seven months or so that we worked on this was something that I just hadn’t predicted…

“It was an astonishing thing… The Overton window shifted in a way that no one expected and the opportunity to have a target like this moved with it.”

Since publishing its report, Stark said the majority of debate has focused on whether the CCC’s recommendations are too conservative.

“In one sense that’s a good debate,” he remarked. “That’s not something I expected would happen a year ago. But my main response to Extinction Rebellion and others who look for a tougher target is great, but there are some very big barriers to achieving an earlier net zero date.”

He continued: “It’s possible we might get there sooner, but there are huge risks in naming an earlier date when you have no certainty that policies can do that kind of heavy lifting.”

Stark said progress is likely to be held back by the decarbonisation of heating, which remains the “biggest single challenge” facing both the government and industry: “The overall cost we assess of net zero is between 1 and 2 per cent of GDP – you might think of that as annual set of costs between now and 2050.

“The consensus now is about 1.3 per cent. About 1 per cent – or more than half of that – is heat in buildings. That is a big, big, big challenge. And never mind the costs of this; it’s the challenge of intervention in the home and putting in place a proper policy to do this over 30 years.”

He said heating systems will need to be replaced in roughly 30 million homes – the equivalent of one million per year between now and 2050.

“We do not have yet the right skills in the economy to deploy the solutions we need to get to net zero at the appropriate scale, I think, sooner than 2050,” he added. “Until we see those kind of policies, 2050 is absolutely at the end of what I think is feasible.”

To maintain pressure on the government to adopt the CCC’s recommendations, the chair of the Business, Energy and Industrial Strategy committee, Rachel Reeves, is planning to present a bill to parliament shortly enshrining the 2050 target in law.