US climate envoy John Kerry has been ridiculed for saying that 50% of the required carbon reductions to reach 'net zero' will come from technologies that have yet to be invented.
Kerry suggested that this could mean Americans would not have to change aspects of their lifestyles to achieve net zero emissions.
He told the BBC's Andrew Marr: "You don’t have to give up a quality of life to achieve some of the things that we know we have to achieve."
Climate campaigner Greta Thunberg was scathing on Twitter, saying: "Great news! I spoke to Harry Potter and he said he will team up with Gandalf, Sherlock Holmes & The Avengers and get started right away!"
Dr Robert Rohde, lead scientist at Berkeley Earth, warned: "This sort of techno-optimism can be dangerous.
Watch: Top tips for helping the environment on a tight budget
"We can achieve most of the reductions we need with technology that we already have... so we should get started. Time is short. If better options come along, then great, but we shouldn't delay starting now."
Kerry suggested in the interview that Americans might not have to reduce the amount of meat they eat.
He told Marr: "That’s the brilliance of some of the things that we know how to do.
"I am told by scientists that 50% of the reductions we have to make to get to net zero are going to come from technologies that we don’t yet have. That’s just a reality.
"And people who are realistic about this understand that’s part of the challenge. So we have to get there sooner rather than later."
Kerry is visiting London to meet UK government representatives ahead of the COP26 climate conference, scheduled to be held in Glasgow in November.
The US is the second-largest emitter of greenhouse gases after China.
Kerry said: "We're determined to turn that around. We are going to be moving very rapidly to a new economy, building out a new grid, moving towards alternative renewable energy, and pushing the curve on the discovery of new technologies.
"There are a lot of possibilities out there."
Watch: Britain to treble tree planting