Ms Stratton said that the science was clear that faster action is needed to bring down greenhouse gas emissions in order to stop global temperature increases by 2030.
Her comments are likely to alarm backbench Conservative MPs who fear that voters are being asked to shoulder too great a financial burden to give the UK a world-leading role in the fight against climate change.
The UK was the first major industrialised country in the world to sign the 2050 target into law in 2019, and is aiming to persuade other nations to follow suit at the COP26 summit which Mr Johnson is chairing in November.
But Ms Stratton stressed that the plan envisaged immediate action, with interim targets of 68 per cent reductions by 2030 and 78 per cent by 2035 to put the UK on a “glide path” to net zero.
Speaking on BBC Radio 4’s World This Weekend, Ms Stratton said that the technology needed to reduce emissions was becoming cheaper all the time, making the net zero target no more expensive than the previous 80 per cent target, at about 1 per cent of GDP.
And she made clear that urgency was needed to meet the COP26 target of keeping global warming to a maximum of 1.5C above pre-industrial levels.
“We have to feel the fierce urgency of now,” she said. “I feel the fierce urgency of now. We have to bring countries to COP26 in November in Glasgow with real substantial plans.”
Ms Stratton, who speaks on COP26 on the prime minister’s behalf, pointed to the government’s 10-point plan for a “green industrial revolution” and to pledges from FTSE100 companies to go net-zero as signs of progress towards the 2050 goal.
But she added: “Every bit of society is moving in tandem towards this net zero in 2050, but let’s be honest that’s too far away.
“Net zero is the glide path. What we have to be doing more quickly – the science is clear – [is] we have to be changing our carbon emissions output right now, so that we can stop temperature increase by 2030.”
Amid growing concerns that the UK is lagging behind in its drive to secure international agreement on ambitious targets, Ms Stratton acknowledged that progress had been delayed by the Covid-19 pandemic.
But she said that plans will be unveiled when parliament returns in September for projects like the replacement of gas boilers with more climate-friendly alternatives over the coming decade.
Ms Stratton admitted that ministers had to overcome distrust from voters in light of fast-changing advice on issues like diesel cars – which were promoted as environmentally friendly for many years before the harmful effects of their exhaust became known.
“My granny says to me ‘Why would we believe government when look what they encouraged us to buy and how quickly they changed their opinion?’” she said.
“But that only underscores the reason why this has to be done properly so we take people with us because it would be so damaging to have another version of that.”
She said that the government had a “balancing act” between moving swiftly enough to respond to the scale of the challenge and taking enough time to ensure that new technology like electric cars works as it should.
“This is a long-term journey we are all on,” she said. “This is a journey to 2050.
“This is not going to happen overnight. This is going to be a conversation we have with the British people about what is fair, protecting vulnerable families from some of the more difficult decisions they will have to make.”
Ms Stratton declined to discuss reports that chancellor Rishi Sunak is holding out against green taxes to pay for action on climate change.
“What worries me and what worries members of the government is the extreme climate change and weather events that we are seeing in this country now,” she said.
“The climate has warmed by 1.2 degrees. We are trying to limit that increase to 1.5. Everyone can see that the margin we have right now is not big. We are headed for 3. If we think about the weather we’re seeing right now at 1.2, what the weather be like at 3 doesn’t bear thinking about.”
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