A number of key net zero recommendations were only based on a single year of climate data.
Sir Chris Llewellyn Smith who led the UK Government's Climate Change Committee (CCC) said they only "looked at a single year" of data showing the number of windy days in a year.
The data research was related to the extent to which the UK could rely on wind and solar farms to meet net zero targets.
In a presentation given in a personal capacity in October Llewellyn Smith said: "They have conceded privately that that was a mistake." In contrast, the Royal Society review examined 37 years worth of weather data.
Sir Chris Llewellyn Smith (r)
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Last week Llewellyn Smith said that the remarks to which he was referring were made by the Climate Change Committee’s chief executive Chris Stark.
He said: "Might be best to say that Chris Stark conceded that my comment that the CCC relied on modelling that only uses a single year of weather data ... is ‘an entirely valid criticism’."
The body has admitted that its original recommendations in 2019 about the feasibility of meeting the 2050 net zero target were also based on just one year’s worth of weather data.
The 2050 target was made law by Theresa May in 2019. The then Prime Minister relied on the original recommendations when forming the legislation.
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Sir Chris Llewellyn Smith who led the Climate Change Committee (CCC) said they only "looked at a single year" of data showing the number of windy days in a year.
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In a presentation delivered on October 31 2023, Llewellyn Smith said: "By looking at one year you underestimate storage and you grossly overestimate the need for everything else.
"That’s exactly what the Committee on Climate Change have done."
He added: “The Committee on Climate Change, as I already said, looked at a single year and they have conceded privately that that was a mistake. But they are still saying they don’t differ that much from us. Well that’s not quite true."
The Royal Society report found that up to 100 Terawatt-hours (TWh) of storage will be needed by 2050, to mitigate variations in wind and sunshine. This was based on 37 years of weather data rather than the single year relied on by the CCC.
A CCC spokesman said: "Our recent report modelled the 12-month operation of Britain’s power system in 2035 using hourly energy demand and real weather data from a low-wind year, stress-tested to simulate a 30-day wind drought.
"We welcome Sir Chris’ work, which considers other aspects of the energy challenge in 2050, under different assumptions about the future energy mix."
When asked if the CCC disputed Sir Chris’s account, the spokesman said: "We’ve got nothing further to add."