Geopolitical tensions that have seen Russian President Vladimir Putin and Chinese leader Xi Jinping skip the talks mean the group is unlikely to even achieve the traditional final communique, let alone robust climate pledges.

That sets up a "potentially catastrophic" failure by nations that account for 80 percent of global power sector emissions, Amnesty International warned Thursday.

And it could lower expectations ahead of crucial COP28 climate talks that begin in November.

Three key climate issues will be on the table in New Delhi: a push to triple global renewable energy capacity by 2030; weaning economies off fossil fuels -- particularly coal; and finance for the green transition in developing countries.

The build-up to the summit suggests a difficult path to progress on any of those.

In July, G20 energy ministers failed to even mention coal in their final statement, let alone agree a phase down roadmap, and there was no progress on the renewables goal.

"The communiques that have come out are woefully inadequate," UN climate change chief Simon Stiell told AFP this week.

The backdrop to the talks could hardly be starker: the EU's climate monitor this week said this year is likely to be the hottest in human history, with UN chief Antonio Guterres declaring: "climate breakdown has begun."

"Our climate is imploding faster than we can cope," he warned.

Evidence of that has been abundant, with devastating flooding, record heat and ferocious wildfires across much of the world in recent months.

G20 countries account for 85 percent of global GDP and a similar amount of global climate warming emissions, making action in the forum crucial to real progress.

But per capita coal emissions have risen across the G20 since 2015, research showed this week, despite transition efforts by some members.

The nine percent hike was largely driven by increases in countries including host India, Indonesia and China.

That coal dependence, along with geopolitical fault lines on Russia's Ukraine invasion and disputes with Beijing, will make agreement on a phase down tricky.

"At the minimum, I hope they will include lines from Bali on coal phase down," said Madhura Joshi, a senior associate at climate think E3G, referring to last year's summit, where members committed to "accelerating efforts" on a "phasedown of unabated coal power, in line with national circumstances."

Another key sticking point may be financing for the energy transition.

Prime Minister Narendra Modi, who has tried to position himself as the voice of the "Global South", insisted ahead of the meeting that climate ambitions "must be matched with actions on climate finance and transfer of technology."

Wealthy nations have already failed to deliver on a pledge to provide $100 billion a year in climate finance to poorer nations by 2020.

Amnesty International has urged the developed world to "deliver, and substantially increase," that funding, warning of the "potentially catastrophic" consequences of inaction at the G20.

And African nations this week called for $600 billion of investments in renewable energy, paid for in part by a global carbon tax.

They also want debt relief and restructuring, and the swift implementation of a "loss and damage" fund for climate-vulnerable countries.

One bright spot in the talks could be the push for renewables, with draft documents reportedly containing a pledge to push for a global tripling of capacity by 2030.

"For the first time ever, a global renewables target... is being discussed by the world leaders," Joshi told AFP.

While the specifics of how to reach that goal remain to be hashed out, just agreeing the commitment "would be an important development."

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