Biden calls climate change the 'existential issue humanity faces' as Kremlin accuses him of pushing world to brink of WWIII

President Joe Biden ignored growing concerns that he's starting 'World War III' with Russia as he focused Tuesday on warning that climate change is an 'existential issue.' 

Biden was attending a second day of meetings as part of the G20 in Rio de Janeiro, Brazil as Russia is threatening to move closer toward using its nuclear arsenal.

The president hasn't taken questions from the press for the entirety of the trip - and since before Election Day - so hasn't gone on-the-record about giving Ukraine permission to use long-range weapons inside Russia's borders. 

But Russian President Vladimir Putin responded Tuesday morning by signing off on an updated version of the Kremlin's nuclear doctrine.

It says that Moscow can use its weapons if Russia is threatened by a non-nuclear nation supported by a nuclear power - which perfectly fits the Ukraine-U.S. relationship. 

The move has stoked World War III fears after Ukraine attacked Russia's Bryansk region using U.S.-supplied ATACMS missiles. 

But the growing international tensions didn't come up during Biden's remarks at the G20 on Tuesday, where he focused on environmental issues.

'When it comes to the adoption of clean energy we've made remarkable progress together, but we all know we have so much more to do. We all know what it takes,' Biden said. 'History is watching us, history is watching.'  

'I urge us to keep faith and keep going,' Biden said. 'This is the single greatest existential threat to humanity if we do not deal with climate change.'

He thanked the G20 delegations for focusing on this issue.

'I really do think it's the existential issue humanity faces,' the president added. 

Russian Foreign Minister Sergei Lavrov, who is attending the G20 in Rio alongside Biden, said in a press conference Tuesday morning that U.S.-supplied missile attack was a clear sign the West wanted to escalate the war in Ukraine. 

'The fact that ATACMS were used repeatedly in the Bryansk region overnight is, of course, a signal that they want escalation,' Lavrov said. 'Without the Americans, it is impossible to use thes

e high-tech missiles, as Putin has repeatedly said,' he added. 

He also told reporters that he hoped Russia's new nuclear doctrine would be read. 

'I hope that they will read this doctrine... in its entirety,' he said. 

President-elect Donald Trump - who will take over in two months and one day - told supporters throughout his 2024 bid that he would prevent World War III from happening. 

He has also floated that he would stop the robust funding going toward Ukraine's war effort and work on a peace plan instead - which likely means territorial giveaways. 

A number of Trump allies have publicly taken on Biden for his decision to greenlight long-range weapons being used in Russia. 

'The Military Industrial Complex seems to want to make sure they get World War 3 going before my father has a chance to create peace and save lives,' Donald Trump Jr. posted on X. 

Republican Rep. Marjorie Taylor Greene also chimed in: 'Biden is committing an Act of War, without approval from Congress, by authorizing Ukraine use of long range missiles into Russia,' she posted to X on Monday. 

Ric Grenell, Trump's former acting director of national intelligence wrote: 'No one anticipated that Biden would ESCALATE the war in Ukraine during the transition period.' 

The 81-year-old president heads back to Washington, D.C. later Tuesday after spending six days in South America, avoiding questions from the press, in what's expected to be his final appearance at global summits. 

He first attended APEC in Lima, Peru, before highlighting his green agenda with a visit to the Amazon rainforest - becoming the first sitting U.S. president to do so. 

Biden then arrived in Rio Sunday night for two days of meetings at the G20.