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The pledge comes alongside a warning from the prime minister that it is “one minute to midnight” in the fight against the climate disaster and an appeal for the world “to act now”.
“If we don’t get serious about climate change today, it will be too late for our children to do so tomorrow,” Mr Johnson is expected to tell 120 world leaders at the Cop26 opening ceremony in Glasgow.
But the United Nations summit gets underway with some of those leaders being accused of having already “fluffed their lines” after the G20 summit in Rome failed to beef up commitments to cut carbon emissions fast enough.
A gloomy prime minister has downgraded his hopes for Glasgow – calling it only a stopping point towards halting climate change, with “no chance” of a deal to keep global temperature warming to 1.5C.
There was anger when wealthy nations announced last week that they would not achieve a long-promised $100bn (£73bn) annual target for the fund for developing countries until 2023 – three years late.
The UK is currently contributing around £2.3bn a year, but had refused to increase its share in the run-up to Cop26, even as other countries did so.
It also stands accused of breaking the rules of the initiative because, as The Independent revealed, the cash will be swiped from the overseas aid budget – despite a requirement that it be “additional”.
Think tank Overseas Development Institute also suggested the UK was short-changing poor countries by around £1.9bn a year, based on its population size and historic carbon emissions.
Now Mr Johnson has pledged the extra £1bn – but only by 2025 and if the UK economy grows fast enough to revert the aid budget back from 0.5 per cent of national income to 0.7 per cent.
The cash would fund programmes for developing nations to cope with the devastating impact of climate change, helping to protect nature and supporting a transition to clean and green energy.
In Glasgow, the prime minister will also say: “Humanity has long since run down the clock on climate change. It’s one minute to midnight and we need to act now.”
Mr Johnson will add: “We have to move from talk and debate and discussion to concerted, real-world action on coal, cars, cash and trees.
“Not more hopes and targets and aspirations, valuable though they are, but clear commitments and concrete timetables for change.”