US climate change envoy John Kerry has urged the world's top 20 polluters which create 81% of emissions between them to reduce CO2 immediately.
He was speaking after meeting Prime Minister Boris Johnson and other senior UK figures in London to plan two upcoming international climate summits.
He praised the UK for phasing out coal, and for its "ambitious" climate goals.
But he told BBC Newsnight that the UK - along with other major nations - must deliver their proposed emissions cuts.
"China, the US, Russia, India, the EU, Korea, Japan and others all have to be part of this effort," he said. "Twenty countries. Eighty one percent of the emissions."
Asked during the interview whether the UK should be planning a controversial new coal mine in Cumbria, he replied: "The marketplace has made a decision that coal is not the future.
"All over the world people have made a decision to move to cleaner fuel than coal, which is the dirtiest fuel in the world. In America and elsewhere …most banks will tell you we're not going to fund a new coal plant."
Earlier after talks with Mr Johnson and other senior ministers, Mr Kerry hailed the UK as a "strong partner" in the fight to safeguard the planet.
Fantastic meeting today with COP26 President @AlokSharma_RDG, including a surprise visit from Prime Minister @BorisJohnson. Only 8 short months until Glasgow -- thrilled to have both these leaders as strong partners in the work ahead. pic.twitter.com/eF7q8b0Lb3— Special Presidential Envoy John Kerry (@ClimateEnvoy) March 8, 2021
And the prime minister said the two countries had an "exciting shared agenda" in driving down global emissions in the run-up to November's COP 26 UN summit in Glasgow.
Great to welcome @ClimateEnvoy John Kerry back to London today and discuss our joint efforts to protect our planet.— Boris Johnson (@BorisJohnson) March 8, 2021
The UK and US have an exciting shared agenda to drive down global emissions ahead of @COP26 in November.
Mr Kerry, a former US Secretary of State appointed to the role by Mr Biden in November, spent several hours in Downing Street with Alok Sharma, the cabinet minister who is chairing November's gathering.
Mr Kerry was also due to meet other senior UK figures, including Chancellor Rishi Sunak, Foreign Secretary Dominic Raab, and Business Secretary Kwasi Kwarteng.
On Tuesday, climate diplomacy sees him in Paris and Brussels for talks with European leaders, who have been praised for their recent target to cut emissions 55% on 1990 levels.
Leaders are wrestling with gloomy news from China, whose recent five-year plan takes tiny steps to decarbonisation.
But they will be heartened by President Biden's $1.9 trillion stimulus package agreed by the Senate, which will support “green” economic growth.
There is positive news too, from Brazil, which – under US pressure – says its previous stance blocking climate talks was misunderstood.
Monday's meetings may go some way to helping the UK focus its objectives for the November gathering.
Ministers were accused recently by MPs on the Business and Energy Select Committee of failing to set clear goals.
The committee said the key areas identified by the UK for action - adaptation and resilience; nature based solutions; energy transitions; clean transport and switching the finance system to low-carbon investments - were too broad and "without clear measures for success".
'Up for grabs'
It said more focus needed to be given to the "overriding necessity" of agreeing deliverable policies that keep global temperature rises to as close to 1.5C as possible.
Nick Mabey, from the think tank e3g, told the BBC there was the potential to achieve multiple goals – including banning new coal power plants, ending banks' fossil fuel investment and supporting poorer nations to adapt - and that these should be debated publicly.
“This debate is up for grabs” he said. “It should be a public debate because we’re talking out how to change whole economies. A lot of the outcomes from Glasgow will be decided in the court of public opinion.”
John Kerry's meetings with the UK politicians who will be running the COP26 summit may help bring some much-needed clarity to efforts on this side of the Atlantic.
But he will also be assessing just how much political capital Team Biden should invest in Team Boris.
So far, the messaging from the UK on what COP26 can realistically achieve has been muddled. If every country is going to put a new climate plan on the table before Glasgow, what's the actual point of the meeting?
Mr Kerry will also be assessing how much diplomatic heavy lifting he should undertake on behalf of the Brits.
To give Glasgow a chance, both India and China will have to come up with significant advances on their current actions on carbon.
Others, including the Saudis and the Brazilians, will have to show flexibility. Kerry's involvement may be critical in delivering these outcomes.
But his impression of the British effort will also influence events back home. President Biden is organising a climate summit of world leaders for 22 April.
That meeting may become the bigger focus for the US if Kerry takes a dim view of UK's efforts on the COP.
Follow Roger on Twitter @rharrabin