New US climate envoy John Kerry said he regretted his country's absence from the fight against climate change under Donald Trump, at a online summit of world leaders on Monday.
President Joe Biden's pointman on the issue committed the United States to efforts to make communities more resilient to the effects of climate change, the focus of the meeting hosted by the Netherlands.
German Chancellor Angela Merkel, French President Macron and British Prime Minister Boris Johnson also pledged their support for the cause in video statements to the Climate Adaptation Summit.
"Three years ago scientists gave us a pretty stark warning. They said we have 12 years within which to avoid the worst consequences of climate change," Kerry said in his video statement.
"Now we have nine years left and I regret that my country has been absent for three of those years."
Under Trump the United States pulled out of the Paris climate agreement, and one of Biden's first moves in office after his inauguration on January 20 was to rejoin.
"President Biden has made fighting climate change a top priority of his administration. We have a president now, thank God, who leads, tells the truth and is seized by this issue," said former secretary of state Kerry.
"So we're proud to be back, we come back, I want you to know, with humility, for the absence of the last four years, and we'll do everything in our power to make up for it."
The aim of the summit -- the first of its kind to focus on dealing with the effects of climate change rather than its causes -- is to produce an "adaptation action agenda".
This will help communities, especially the poorest and most vulnerable, to cope better with effects such as rising sea levels, extreme weather and crop failure.
- 'Fundamentally unliveable' -
Boris Johnson announced the launch of an Adaptation Action Coalition, in partnership with Egypt, Bangladesh, Malawi, the Netherlands, Saint Lucia and the United Nations.
"It is undeniable that climate change is already upon us and is already devastating lives and economies," Johnson said. "We must adapt to our changing climate, and we must do so now."
Merkel and Macron both agreed on the need to promote adaptation to climate effects, and spoke of the sums the German and French governments had committed to the issue.
Kerry however warned that it was not enough to just deal with the effects, saying that the "best adaptation" was to do more to hold the earth's temperature rise to 1.5 degrees Celsius.
A rise of 3.7 to 4.5 degrees Celsius, the scenario if action is not taken on emissions, would create "fundamentally unliveable conditions" for all but the richest places on the planet, Kerry added.
Former UN secretary general Ban Ki-moon welcomed the return of the United States to the Paris pact.
"I'd like to take this opportunity to highly commend the visionary leadership of president Joe Biden of the United States for his decisive action to rejoin the Paris climate change agreement as the first act of his presidency," said Ban.
"This means the multilateralism which has been in disarray during the last four years is now back."
The summit is being held almost entirely online because of the coronavirus pandemic.
Hosting the summit, Dutch Prime Minister Mark Rutte said his country, a third of which lies below sea-level, had centuries of experience in keeping out the water and hoped to teach others.
Plans under discussion at the summit could include shoring up sea defences but also taking advantage of opportunities including longer growing seasons for crops, and new areas for cultivating, organisers said.