US climate envoy John Kerry has told the BBC a UN climate summit in the UK this November is "the last best chance" to avert the worst environmental consequences for the world.
He said years were lost on the climate issue under President Donald Trump, "who didn't believe in any of it".
Dozens of world leaders will attend the COP26 conference in Glasgow.
Mr Kerry spoke as President Joe Biden signed a flurry of executive orders designed to address climate change.
His latest edicts include a freeze on new oil and gas leases on public lands and set out to double offshore wind-produced energy by 2030.
Mr Biden - who re-entered the US into the 2016 Paris climate accord in one of his first acts as president - said America must lead the response to the crisis.
What did Kerry say?
The newly appointed US climate tsar said: "Glasgow will be extremely important.
"In fact, I would say that in my judgment, it is the last best chance the world has to come together in order to do the things we need to do to avoid the worst consequences of the climate crisis.
"Three years ago, we were told we have 12 years to avoid those consequences. Three of those years were lost because we had Donald Trump, who didn't believe in any of it. And now we have nine years left to try to do what science is telling us we need to do."
But at a White House news conference on Wednesday, Mr Kerry acknowledged the crisis would not be fixed even if the US reduced its emissions to zero.
He said: "He [Mr Biden] knows Paris alone is not enough. Not when almost 90% of all of the planet's global emissions come from outside of US borders.
"We could go to zero tomorrow and the problem isn't solved."
During his four years in office, Mr Trump rolled back environmental regulations as he sought to maximise the nation's fossil fuel production.
The Republican president also claimed credit for the US achieving a level of energy independence, becoming a net petroleum exporter, midway through his term.
Mr Trump - who once called climate change a hoax - withdrew the US from the Paris pact, arguing that it unfairly left the world's other two top polluters - India and China - free to use fossil fuels.
Nearly 200 countries signed up to the accord, which aims to keep the global temperature rise this century well below 2C above pre-industrial levels.
America's moral authority problem
By making John Kerry his Climate Envoy, Joe Biden isn't just appointing one of America's most experienced and senior politicians, he is also getting a man who is very deeply invested in making the process of international climate diplomacy work.
John Kerry was President Obama's Secretary of State and was instrumental in negotiating the landmark Paris climate agreement, the first time virtually all the nations of the world agreed they needed to act to tackle climate change.
No surprise then that he sees the Glasgow COP26 conference in November as the crucial forum for the world to come together to avoid really damaging climate change.
His tone is emollient, however. He knows the US's virtual absence from climate talks during the Trump administration means he has to approach other world leaders with a sense of humility.
America hasn't yet earned the moral authority to demand other nations take action.
It is clear, however, that this veteran and wily political operator has every intention of using his formidable negotiating skills to try to ensure the world makes the most ambitious deal possible in Glasgow in November.
What else did Biden say?
The new US president indicated he would not wait until November's summit in Scotland to engage in multilateral talks on the climate issue.
The series of executive orders that Mr Biden signed on Wednesday announced a US summit of leaders to be held in April on Earth Day.
"Just like we need a unified national response to Covid-19, we desperately need a unified national response to the climate crisis because there is a climate crisis," he said, establishing a White House office of domestic climate policy.
Mr Biden directed the Department of the Interior to pause oil and gas drilling leases on federal lands and water and to launch a review of existing energy leases.
Mr Biden aims to conserve at least 30% of federal lands and oceans by 2030.
The new president - whose fellow Democrats control Congress - has signed over three dozen executive orders in his first week in office, more than any of his predecessors.
Critics note he told ABC News while campaigning last October that only a "dictator" would use executive orders excessively. "We're a democracy," said Mr Biden. "We need consensus."
Mr Biden endured a storm of criticism for last week's executive order halting construction of the Keystone XL pipeline, that would have transported oil from Canada through the US.
His White House is trying to get ahead of more criticism by addressing job creation.
Mr Biden argued that "millions" of Americans would be able to find employment "modernising our water systems, transportation, our energy infrastructure - to withstand the impacts of extreme climate".