The UN climate conference in Glasgow, Scotland, is the former American president’s first since he helped deliver the triumph of the 2015 Paris climate accord, when nations committed to cutting fossil fuel and agricultural emissions fast enough to keep the Earth’s warming below catastrophic levels.

Climate summits since then have been less conclusive, especially as the US under President Donald Trump dropped out of the Paris accord. President Joe Biden has since rejoined.

Obama’s appearance on the sidelines of the talks is meant to remind governments of the elation that surrounded the striking of the Paris accord, and urge them to more immediate, concrete steps to put the 2015 deal into action.

In prepared remarks obtained by The Associated Press ahead of a speech to activists, Obama noted efforts by the United States — the world’s second-worst climate polluter now after China — stalled when Trump pulled out of the climate accord.

“I wasn’t real happy about that,” he admitted.

Despite opposition within Biden’s own Democratic party that has blocked the climate-fighting legislation, Obama said he was confident that some version of Biden’s ambitious climate bill will pass in Congress in the weeks to come.

“It will set the United States on course to meet its new climate targets,” he said.

And while in 2015, rapport between Obama administration negotiators and their Chinese counterparts was seen as paving the way to the global Paris accord, Obama on Monday criticised Chinese President Xi Jinping and Russian President Vladimir Putin for not joining other global leaders at the climate talks in Glasgow.

“It was particularly discouraging to see the leaders of two of the world’s largest emitters, China and Russia, decline to even attend the proceedings, and their national plans reflect what appears to be a dangerous absence of urgency,” Obama said in the prepared remarks.

Obama spoke earlier Monday to a session on Pacific Island nations, including ones whose existence is threatened by rising oceans under climate change.

“All of us have a part to play. All of us have work to do. All of us have sacrifices to make” on climate, he said.

“But those of us who live in wealthy nations, those of us who helped to precipitate the problem” of global warming, “we have an added burden,” Obama said.

The two-week climate talks are at their midpoint, after Biden and scores of other global leaders launched the summit last week with pledges of action and calls for more.

Scientists say the urgency of global warming is as great as the dire speeches at Glasgow have conveyed, with the planet only a few years away from the point where meeting the goals set in the Paris accord becomes impossible, due to mounting damage from coal, petroleum, agriculture and other pollution sources.