This will be the first opportunity, however, for the 19 to voice a collective condemnation.

“At this summit what the Germans and the rest of us want is basically to isolate the US so that the US doesn’t contaminate any other countries,” said Mohamed Adow of Christian Aid, an expert observer of the two-decade negotiation that culminated in the Paris Agreement in 2015.

In May, before Trump made his announcement, the other six in the G7 group of rich nations reaffirmed support for the hard-fought global pact.

The G20 includes the G7 as well as developing nations such as China, Brazil, Mexico, South Africa, South Korea, and Indonesia.

It also counts as members the world’s biggest oil producers - Russia and Saudi Arabia - and major emitters of planet-warming greenhouse gases, like India, which required much arm-twisting to greenlight the agreement in the first place.

“Trump has potential allies there: Saudi Arabia, Turkey and Russia,” said Celia Gautier of the Climate Action Network, an NGO grouping.

Russia and Turkey have signed the agreement but not ratified it. Saudi Arabia, which was one of the last countries to hold out, has both signed and ratified.

Climate champions among the bloc’s developing country members need to be vocal supporters of Germany’s bid to emerge from the summit on Saturday with strong language on climate, observers say.

If not, “you could easily end up with Trump feeling that he can weaken the G20 outcome and... build connections with these other countries,” said Adow.

According to Alden Meyer of the Union of Concerned Scientists, there are “rumblings that Saudi Arabia has been pushing back on some elements of the text”, possibly in “co-ordination” with Washington.

But the geopolitical turmoil surrounding the meeting may turn out to be a blessing in disguise.

Saudi Arabia and Russia, respectively involved in the crises in Qatar and Syria, may not wish to pick a new fight - on climate - at this point, observers say.

The summit is meant to come up with a closing “communique” that includes elements on climate change.

Countries have also been negotiating the text of an “action plan” on climate and energy.

The question is what the final text will look like.

Will 19 countries speak with one voice on climate? Or will they splinter, with some aligning themselves with the US?

Will accommodations be made to America that there is a joint 20-member statement but with watered-down climate language?

Whatever the outcome, observers say G20 summits are not the place for direction-changing decisions.

They are more a gauge of mood and a barometer of thinking on issues such as putting a price on carbon pollution, or ending fossil fuel subsidies.

Having said that, it “would be a concern if you saw some watering down of past language,” in the G20 statement, Meyer added.

“Certainly anything that gave a signal that ‘clean coal’ is acceptable would not be good.”

This is a term for technology that allows continued coal use by lessening its polluting effects. Critics say the focus should be on a wholesale move to non-polluting energy sources like the Sun and wind.

The G20 represents about 85 per cent of global GDP and 80 per cent of energy-related CO2 emissions.

Under the Paris pact, countries agreed to cut planet-warming greenhouse gas emissions from burning coal, oil and gas.

The goal is to limit average global warming to two degrees Celsius over pre-Industrial Revolution levels - the threshold for a liveable planet, according to scientists.

The only response to Trump’s undermining of the deal is for other countries to meet, and step up, their own commitments, analysts say.

But on this front, the week brought bad news.

A report by Climate Transparency, a network of research and advocacy groups, said G20 fossil fuel investment was not in line with the 2-degree C goal.

In 2014, G20 countries provided more than $230 billion in fossil fuel subsidies, it said.

“The overall growth of greenhouse gas emissions is slowing, but is not yet in decline,” said Niklas Hohne of the NewClimate Institute, which contributed to the study.

“Renewables are on the rise, but coal and other fossil fuels still dominate the G20’s energy mix.”