Environmentalists and those on the front line of climate change have told Sky News of their relief at seeing Joe Biden sworn in as America’s new president.

The inauguration of a new world leader who describes climate change as an “existential threat” is a lifeline for countries whose people and land are being devastated by global warming.

Analysis: Biden wastes no time in undoing Trump’s work

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Tina Stege is climate envoy for the Marshall Islands, a group of atolls lying just two metres above sea level in the northwestern Pacific Ocean.

She said: “For a country like mine which is really on the front lines of climate change, we now have optimism. It’s got to be cautious optimism when the challenges are this big.

“But with a partner like the US and with all the resources that the US can bring to bear, with this president we are at the start of a process that provides some hope.”

Immediately after taking office, Mr Biden signed executive orders to rejoin the landmark Paris climate agreement (which Donald Trump pulled out of), when countries came together in 2015 to pledge to reduce devastating levels of global emissions.

Mr Biden has also rolled back a host of executive orders put in place by Mr Trump which weakened efforts to tackle climate change.

They include revoking the presidential permit granted to the controversial Keystone XL pipeline delivering hundreds of thousands of barrels of crude oil each day from Canada to be refined in the US.

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Joye Braun has been fighting the pipeline for a decade and was – as she calls it – “boots on the ground” from day one “until we were evicted”.

A member of the Cheyenne River Sioux Tribe and Indigenous Environmental Network, she said: “To have Keystone Pipeline XL go through is a climate changer – we’ve always said that. It’s an absolute necessity that the Keystone XL pipeline be stopped.

“Watching the inauguration, I felt a huge sigh of relief. For 10 years I’ve been working on this. We’ve gone cold, we’ve gone hungry. Thank you President Biden.”

Throughout his campaign Mr Biden had promised rejoin the Paris agreement on “day one” of his presidency.

America will be back in the club in 30 days after notifying the United Nations.

Remy Rioux, the head of the French development agency, was a lead negotiator for the Paris agreement.

He said: “I remember in 2015 it was an executive order by President Obama which had the US joining the Paris agreement so there’s no need for Congress approval to join or to come back within the agreement. President Trump withdrew by a single executive order as well so it can be very fast.”

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While Mr Biden wants to be a global leader on the climate, his credibility rests on transforming decarbonising the US.

He’s promised a $2tn plan to create millions of jobs in clean energy and energy retrofits.

His climate strategy is underpinned by the belief that the climate is inextricably linked to America’s health, wealth and national security.

He subscribes to a global recovery from coronavirus being green and will likely announce net zero goals.

Joe Biden has become the 46th president of the United States, after taking his oath of office in a heavily scaled back inauguration ceremony in Washington DC.

He swore to preserve, protect and defend America to the sound of cheers and applause from former presidents both Democrat and Republican – though Donald Trump decided to break precedent by skipping the event.

It came minutes after new Vice President Kamala Harris took her oath, too.

Biden’s inauguration – follow live updates

Mr Biden stressed the fairness of last November’s election result in the opening of his inaugural address by declaring: “This is democracy’s day. The will of the people has been heard and the will of the people has been heeded.

“We’ve learned again that democracy is precious, democracy is fragile and at this hour my friends, democracy has prevailed.”

Mr Biden promised to “press forward with speed and urgency” during a “winter of peril” to tackle the “once-in-a-century virus that silently stalks the country”, also vowing to confront white supremacy and terrorism.

He stressed his prevailing focus after a divisive election campaign will be on “uniting our nation”, adding: “With unity, we can do great things, important things – we can right wrongs.”

And he said he wanted to “make America once again a leading force for good in the world”, seemingly in a snub to Mr Trump commenting: “Let’s start afresh… all of us.”

Mr Biden urged people to “join forces, stop the shouting and lower the temperature”, for, he explained, without unity there will be “no nation, only a state of chaos”.

Speaking as he looked out on to the National Mall lit by a bright sunshine, Mr Biden continued: “Politics doesn’t have to be a raging fire destroying everything in its path.

“Every disagreement doesn’t have to be a cause for total war and we must reject the culture in which facts themselves are manipulated and even manufactured.”

Repeating a motif from his victory speeches in the days after winning the Electoral College vote, Mr Biden promised to be “a president for all Americans”.

Winding up his address, he struck an optimistic tone, saying: “Together we shall write an American story of hope not fear, of unity not division, of light not darkness.”

He ended with: “May God bless America and may God protect our troops, thank you America.”

Lady Gaga, wearing a large dove broach on her top and clasping a golden microphone, had just performed a rousing rendition of the national anthem – and Jennifer Lopez followed with an “American musical selection”.

Former presidents Barack Obama, George W Bush and Bill Clinton attended the event at the Capitol – and Mr Biden was greeted by cheers and applause as he walked up to the stage.

Mr Biden shared a fist-bump with Barack Obama before the pair took their seats, and then a series of speeches got underway – all sharing a theme of unity.

But as the new president prepared to take the oath of office, Donald Trump was landing in Florida.

Mr Trump is the first outgoing president since 1869 to skip an inauguration ceremony, but departing Vice President Mike Pence was in the audience.

As the inauguration ceremony took place in a chilly Washington DC, where it was trying to snow, the White House was getting a deep clean that was set to cost $500,000 (£366,000).

Shortly before the ceremony began, Mr Biden declared on Twitter: “It’s a new day in America.”

Mr Trump gave a parting message before boarding Air Force One, telling a small group of supporters and family members gathered on the tarmac of Joint Base Andrews that “we will be back in some form”.

“I wish the new administration great luck and great success,” he added, before boarding the plane, which took off to the booming soundtrack of Frank Sinatra’s My Way.

Mr Biden is only the second Catholic to hold the office of president.

His team have already announced he will sign a series of executive orders reversing several of Mr Trump’s policies, including on COVID-19, climate change and racial inequality.

Tennis star Novak Djokovic has insisted he was not being “selfish, difficult and ungrateful” after making a list of requests for players in quarantine ahead of the Australian Open.

The men’s world number one reportedly sent a letter to Australian officials asking for a reduction in the time players spend in isolation, permission to see coaches and for athletes to be moved to private houses.

His suggestions were firmly rebuffed by Victoria’s premier Daniel Andrews, who said: “People are free to provide lists of demands, but the answer is no… There’s no special treatment here.”

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A total of 72 players are in quarantine after 10 people who flew to Melbourne for the first Grand Slam of the year tested positive for coronavirus – leaving many forced to train in their hotel rooms.

Djokovic has since defended speaking out about the quarantine conditions, writing in a lengthy social media post: “My good intentions for my fellow competitors in Melbourne have been misconstrued as being selfish, difficult and ungrateful.

“This couldn’t be farther from the truth.”

He said his email exchange regarding suggestions for the quarantine conditions was an “opportunity to brainstorm” and he was “aware that the chances were low that any of our suggestions would be accepted”.

“There were a few suggestions and ideas that I gathered from other players from our chat group and there was no harm intended to try and help,” he said.

While many players are under the strictest quarantine conditions and unable to leave their rooms, others who were not on the affected flights – including Djokovic – are able to train outside for five hours a day under COVID-secure protocols.

The star player said he wanted to use his “position of privilege” to help others.

“I’ve earned my privileges the hard way and for that reason it is very difficult for me to be a mere onlooker knowing how much every help, gesture and good word mattered to me when I was small and insignificant in the world pecking order,” he said.

He added: “Things in the media escalated and there was a general impression that the players (including myself) are ungrateful, weak and selfish because of their unpleasant feelings in quarantine.

“I am very sorry that it has come that because I do know how grateful many are.”

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Going ahead with the tournament amid the global pandemic and harsh restrictions in Melbourne has caused some controversy, particularly as many Australians remain stuck overseas.

Three new coronavirus cases related to the tournament were reported on Wednesday, including a player who has been in hard lockdown since they arrived.

The second case related to another player and the third is a support person with the player.