Nick Bridge, Britain’s senior climate envoy, said his country want s to position itself as “a global green hub.”

Bold promise, meet the mess known as Brexit.

Britain’s economy is slowing precisely at a time when the country will need to invest the equivalent of at least 1 percent of its gross domestic product to meet its net-zero target, according to the Committee on Climate Change, a government advisory body. Moreover, at a time of peak political dysfunction, the government has not implemented the policies needed to get to net zero , nor mapped out how it will pay for the transition.

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“Brexit is a horrible distraction,” Ms. Worthington said. “Politically there’s not enough oxygen to even have a conversation.”

Meanwhile, the clock is ticking. Glasgow is scheduled to host a crucial round of United Nations climate negotiations next year. With Britain having adopted such a lofty law, it remains t o be seen whether the country will have anything to show for it.

At the moment, the country is not on track to meet its earlier target for an 80 percent reduction in emissions from 1990 levels.

That’s not for a lack of knowing what to do.

Alex Kazaglis, an economist who served on the Committee on Climate Change and now heads the energy division at Vivid Economics, a consulting firm, offered to show me some of the challenges on a walk through Central London.

To get to net zero, he said, Britain needs to generate much more electricity than ever before — all of it from non-fossil fuel sources.