Former US secretary of state John Kerry will act as “climate tsar” when US President-elect Joe Biden takes office.
Mr Kerry was one of several people named for top positions by the Biden transition team on Monday.
Other key picks include Avril Haines as the first woman to lead intelligence, and long-time Biden aide Antony Blinken as secretary of state – the most important foreign policy position.
It comes as calls are growing for Donald Trump to concede the election.
He has made unsubstantiated claims of widespread electoral fraud and is continuing to pursue legal challenges over the result.
Mr Biden is projected to beat President Trump by 306 votes to 232 when the US electoral college meets to formally confirm the winner on 14 December. This is far above the 270 votes he needs.
In a statement following the announcement on Monday, Mr Biden said: “I need a team ready on day one to help me reclaim America’s seat at the head of the table, rally the world to meet the biggest challenges we face, and advance our security, prosperity, and values. This is the crux of that team.”
Some of the positions require confirmation in the US Senate.
What will John Kerry do?
Mr Kerry was chosen for the role of special presidential envoy for climate.
The Biden transition team said the position would see him “fight climate change full-time”. He is also set to be the first official dedicated to climate change to sit on the National Security Council.
Mr Kerry signed the Paris climate agreement on behalf of the US in 2016. The deal commits countries to working to limit the rise in global temperature.
Under Mr Trump, the US this month became the first country to formally withdraw from the agreement. But Mr Biden has said he plans to re-join the accord as soon as possible.
Under the rules, all that is required is a month’s notice and the US should be back in the fold.
In a tweet following the announcement on Monday, Mr Kerry wrote: “America will soon have a government that treats the climate crisis as the urgent national security threat it is. I’m proud to partner with the President-elect, our allies, and the young leaders of the climate movement to take on this crisis as the President’s Climate Envoy.”
Mr Kerry previously served as secretary of state during Barack Obama’s second term as president. A veteran Democratic politician, he lost to incumbent Republican George W Bush in the 2004 presidential election.
He was a senator for 28 years and chairman of the foreign relations committee.
He endorsed Mr Biden to be the Democratic Party’s candidate in the 2020 race and joined him on the campaign trail.
What about the other roles?
Mr Blinken was nominated as secretary of state. The 58-year-old is a long-time adviser to the president-elect. He was deputy secretary of state and deputy national security adviser during the Obama administration, in which Mr Biden served as vice-president.
He is expected to manage a Biden foreign policy agenda that will emphasise re-engaging with Western allies.
Avril Haines was nominated director of national intelligence. Ms Haines is a former deputy director of the CIA and deputy national security adviser.
The Biden transition team said Alejandro Mayorkas was the first Latino and immigrant nominated to serve as secretary of homeland security. He previously served as deputy secretary of homeland security under President Obama.
Jake Sullivan was named White House national security adviser. Mr Sullivan served as Mr Biden’s national security adviser during Mr Obama’s second term.
In a tweet following the announcement, he said Mr Biden “taught me what it takes to safeguard our national security at the highest levels of our government”.
Long-time diplomat Linda Thomas-Greenfield was nominated US ambassador to the UN. She also served under President Obama, including as assistant secretary of state for African affairs between 2013 and 2017.
US media reports say economist and former Federal Reserve chair Janet Yellen is Mr Biden’s pick for Treasury secretary. Mr Biden’s team has not confirmed this.
The president-elect previously said he had chosen someone for the role who would “be accepted by all elements of the Democratic Party”.
What about the calls for Trump to concede?
President Trump is continuing to refuse to concede and facilitate a smooth presidential transition. He has been pursuing so-far fruitless legal challenges in several states to try to overturn his loss, but calls are growing for him to accept defeat.
Former New Jersey Governor Chris Christie, a prominent Trump ally, called the president’s legal team a “national embarrassment”.
“I have been a supporter of the president’s. I voted for him twice. But elections have consequences, and we cannot continue to act as if something happened here that didn’t happen,” he told ABC’s This Week programme on Sunday.
How is President Trump challenging the result?
‘Overvoting’ and other US fraud claims fact-checked
High-profile Trump supporter Stephen Schwarzman, the CEO of investment company Blackstone, also said it was time for Mr Trump to accept he lost.
“Like many in the business community, I am ready to help President-elect Biden and his team as they confront the significant challenges of rebuilding our post-Covid economy,” he said in a statement reported by US media.
Maryland’s Republican Governor Larry Hogan told CNN that the Trump camp’s continued efforts to overturn the election results were “beginning to look like we’re a banana republic”.
Some Republican lawmakers have also moved to acknowledge Mr Trump’s defeat in the election.
What’s the latest with the challenges?
The Trump campaign has lost a slew of lawsuits contesting results from the election, and its latest efforts focus on stopping the swing states that handed Mr Biden his win certifying the results – an essential step for the Democrat to be formally declared victor.
The president’s latest legal setback came on Saturday when a judge dismissed his attempt to have millions of postal votes in Pennsylvania invalidated.
In a scathing ruling, Judge Matthew Brann said his court had been presented with “strained legal arguments without merit and speculative accusations”.
The move paves the way for Pennsylvania to certify Mr Biden’s win on Monday. However, the Trump campaign is appealing against the ruling.