The clench-jawed, white-knuckled handshake between Donald Trump and Emmanuel Macron at the Nato summit in Brussels on Thursday was about more than proving who is more “virile” – Le Figaro’s word – or the stronger “alpha male”, as the Washington Post put it. It was also about saving the planet.

As president of France, Macron feels proprietorial about the Paris Agreement on climate change, which was concluded under his predecessor.

During his presidential campaign, Trump repeatedly contradicted the overwhelming body of scientific evidence on global warming, calling climate change “a hoax” and promising to withdraw from the agreement or “renegotiate” it once he was elected.

So far, Trump has done neither. After discussing climate change with the US president over lunch in Brussels, Macron said he did “not want the US to rush to decisions”. As long as Trump had not said no, a source at the Élysée explained, there was still hope.

Pope Francis pitched in on Wednesday, when he gave Trump Laudato si, his 200-page plea for the protection of the environment.

Macron and five other leaders at the G7 summit, which ends in Taormina, Sicily, on Saturday, can cite two reports issued this week in discussions with Trump.

A Nato report emphasised the “strategic imperative” of fighting climate change, while an OECD report predicted industrialised countries would gain an extra 2.8 per cent in GDP by 2050 if they adopt policies in line with the Paris Agreement.

For the past decade, climate change has been at the heart of every final communique issued by G7 summits (or G8, as it was until Russia’s departure over the annexation of Crimea in 2014).

Final decision

Trump said he was unlikely to take a final decision on the Paris Agreement before returning to Washington. The six other leaders at the summit debated whether to leave the issue out of the communique, or push the US leader for some kind of mention. An Élysée source predicted that negotiations on wording would continue all night on Friday.

Trump’s close adviser Steve Bannon and the climate-sceptical head of the Environmental Protection Agency, Scott Pruitt, reportedly want him to renounce the Paris Agreement altogether.

Trump’s daughter and adviser Ivanka, and the energy secretary Rick Perry, would prefer that the US remain in the agreement, but with a lower commitment to reducing greenhouse gas emissions.

Under President Barack Obama, the US promised to decrease emissions 26-28 per cent by 2025 compared to 2005 levels. Trump’s vow to “renegotiate” the Paris agreement is a misnomer, since the 147 countries who have ratified the accord set their own level of contributions.

Experts debate which would be more damaging, a total US withdrawal, or a downward revision of the US commitment. Either “would be very bad news”, the Élysée says, “because other actors would probably follow”.

Under article 28 of the agreement, the US could cancel its ratification of the accord. It’s a long procedure, which would not take effect until November 2020, when the US will again hold presidential elections.

But, cautions Todd Stern, the former US special envoy for climate change, in an interview with Libération newspaper, “Once a head of state has announced he’s pulling out, it’s over. The country no longer goes to meetings and doesn’t even send delegates to climate convention summits.”

Reduce emissions

The fact that Washington sent only 16 negotiators to this month’s climate meeting in Bonn – compared to 42 for France – was seen as a bad omen.

Paragraph 11 of article 4 of the agreement says members can “modify” their commitments to reduce emissions “so as to heighten the level of ambition.” US environmental groups say the text would not allow Trump to lower the US commitment. Stern said a downward revision would be “slightly less bad” than a US departure.

Célia Gautier of the Réseau Action Climat warns that the US should not be allowed to remain a party to the agreement “at any price . . . It’s hard to imagine an accord that is relevant in the battle on climate change without the US, who are the world’s second polluter after China. But that would be better than . . . allowing four or five climate sceptics in the White House to decide, alone, for the rest of the world.”