"There is no high-carbon growth story in anything like the medium or long term," he told business and government officials at an innovation forum held as part of London Climate Week events.

The growing climate threat "creates an environment so hostile that growth will be reversed," said Stern, who chairs the Grantham Research Institute on Climate Change and the Environment in London.

Unexpectedly rapid change is already underway in some parts of the global economy, with car manufacturers now discussing the end of the internal combustion engine, he said, and renewable energy cheaper than fossil fuels in many places.

But while efforts to shift economies are accelerating, they are still "not nearly fast enough", he said.

Around the world, "you have to go zero right across the board, country by country, sector by sector. Basically you need to go to zero everywhere" in terms of emissions, he said.

Ovais Sarmad, deputy head of the U.N. climate secretariat, said recognition of the threat presented by climate extremes was growing quickly, including among business and financial leaders.

"But recognizing and doing something about it are different things," he said.

To bring about the level of changes needed, "everybody has to make this a priority in their lives and in their business," he said. "That's the only way we'll drive the deep and systemic change that's needed."

Shifting existing ways of doing business, however, is challenging, not least because government policy in many places has yet to change to enable the needed shifts, experts said.

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Britain last week passed a legally binding commitment to reach net zero greenhouse gas emissions by 2050, making it the first of the major G7 countries to set such a goal.

The decision followed a surge of civil disobedience - including traffic disruptions - across the country by protest organisations including Extinction Rebellion and a growing student school strike movement.

But making the zero-carbon commitment a reality will require not just investment but wide-ranging changes to legal structures and policy on everything from infrastructure to town planning, Stern said.

Needed economic shifts are gaining momentum with "mediocre" policies in place, he said, but "just imagine how fast we could go if those policies are good".

Still, the scale of what needs to be done is daunting, government officials said.

Getting more efficient heating boilers into 25 million homes and 6 million businesses in Britain took more than 30 years, said Julian Critchlow, head of energy transformation and clean growth for Britain's Department of Business, Energy and Industrial Strategy.

Finding ways to now get zero-carbon heating into homes, in even less time, will be "a significant challenge", he said.

From getting fossil-fuel cars off the road to planting billions of trees and making millions of old houses low-carbon, "the shift that is required to reach net zero... is incredible," said Chris Stark, head of Britain's Committee on Climate Change, an independent advisory group.

It will also require not just new technology and policies, but shifts in behavior by a wide range of people, not all of them eager yet to make the changes, he admitted.

To do what's needed, "we have to engage real people, not people who come to conferences like this, but people going about their daily lives," he said.

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