Campaigners and opposition politicians protested that the chancellor’s warning – revealed in a leaked letter – ignored the massive cost of failing to act on runaway climate change.
The move will raise hopes of meeting the call by the independent Committee on Climate Change (CCC) to quickly enact a legally-binding duty to hit the 2050 target – now threatened by Mr Hammond’s intervention.
Dr Doug Parr, chief scientist for Greenpeace UK, said: “The Treasury is putting their ideology before our wellbeing and trying to shape the public debate for political ends.”
And Ed Davey, a Liberal Democrat leadership candidate, warned: “Hammond might be trying to reclaim his crown as a fiscal hawk in the dying embers of May’s premiership, but this intervention is wrong headed and threatens our children’s future.”
The CCC called for the zero-emissions target for 2050 to be made law immediately – ahead of a UN summit in September – insisting “the quicker you do it the cheaper it is”.
Its blueprint would mean the end of petrol and diesel cars and gas boilers, a drastic cut in people's meat consumption and the planting of at least 1.5 billion trees.
Emissions from some activities, including air travel and farming, are viewed as unavoidable by 2050, but would be balanced by taking carbon out of the air by growing trees or burying carbon dioxide.
Now Mr Hammond has told the outgoing prime minister the commitment will shrink the money available for schools, the NHS, the police and public spending priorities.
His letter also warned that the target would render some industries “economically uncompetitive” without huge government subsidies.
The CCC estimated that reaching net zero will cost £50bn a year, but the department for business, energy and industrial strategy (BEIS) puts the figure at £70bn, according to the chancellor’s letter.
“On the basis of these estimates, the total cost of transitioning to a zero-carbon economy is likely to be well in excess of a trillion pounds,” he wrote.”
Mr Hammond said it was right for the UK to show leadership on climate change, but warned the prime minister that the implications of the policy needed to be “better understood”.
He urged Downing Street to support a Treasury review to minimise the cost of the policy for taxpayers and consumers to prevent “potentially damaging impacts”.
The letter also suggested that the government give itself an “explicit review point”, or a get-out clause to reconsider the target if other countries did not follow suit.
Dr Parr added: “If you want to know whether a policy is a good idea, you include the benefits as well as the costs – and, in this case, the benefits include an economy fit for the twenty-first century, cleaner air, warmer homes and maximising the chances of civilisation surviving.”
And Mr Davey said: “The cost of tackling the climate emergency is massively outweighed by the long-term cost of not acting. The chancellor has got his sums wrong.”
The prime minister’s spokeswoman would not say whether she intended to formally commit the government to the 2050 target before leaving office, at the end of July.
“In the week of the CCC report, we strongly welcomed the recommendations but set out that we would be formally responding as soon as possible. That remains our intention.”
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