Former US President Barack Obama said Monday he understood why young people were "frustrated" with climate inaction from leaders and that "most countries have failed" to live up to promises they made in the Paris Agreement.

Obama, who was US leader in 2015 when the landmark accord was struck, said the world needed to "step up" its emissions-cutting pledges and work together to limit global temperature rises.

"We have not done nearly enough to address this crisis," he told delegates in Glasgow. "We are going to have to do more and whether that happens or not to a large degree is going to depend on you."

In the six years since the Paris deal -- which seeks to limit global heating to between 1.5 and 2 degrees Celsius -- planet-warming greenhouse gas emissions have continued to mount, and an assessment last week said that carbon pollution will rebound this year to pre-pandemic levels.

"By some measures the agreement has been a success," Obama said. "(But) we are nowhere near where we need to be yet."

He admitted that "some of our progress stalled" when his successor Donald Trump chose to unilaterally withdraw the US from the Paris deal.

US President Joe Biden re-joined the accord when he took office.

Obama said that China and Russia -- whose leaders skipped a high-level segment in Glasgow last week attended by more than 120 heads of state and government -- have shown a "dangerous lack of urgency" on climate commitments.

"Most countries have failed to be as ambitious as they need to be," he added.

"We need advanced economies like the US and Europe leading on this issue but you know the facts. We also need China leading on this issue and India leading on this issue," said the former president.

Addressing young activists who cheered and filmed his speech on their smartphones he said it was "not easy being young today."

"Most of your lives you've been bombarded with warnings about what the future will look like if you don't address climate change," he said.

"Meanwhile, you're watching many of the adults who are in positions to do something about either act like the problem doesn't exist or refuse to make the hard decisions necessary to address it."

"Get busy and vote"

He said that his generation's lack of urgency was "a real source of anxiety and real anger".

"You are right to be frustrated. My generation has not done enough to deal with a potentially cataclysmic problem that you now stand to inherit," said Obama.

Echoing advice he said his mother gave him whenever he was sad or frustrated, he advised young people: "Don't sulk. Get busy."

"Get to work and change what needs to be changed. Luckily that's exactly what young people around the world are doing right now," he said.

He spoke of his admiration for the next generation of leaders, including Swedish campaigner Greta Thunberg who inspired a global school strike for climate.

He joked how he was "unlike Greta. I was not on the cover of Time magazine when I was 16 years old. And if I was skipping school, it had nothing to do with climate change."

Obama called on young people who were eligible to turn out and vote "like your life depends on it".

"I recognise that a lot of young people might be cynical of politics. But the cold hard fact is we will not have more ambitious climate plans unless governments feel some pressure from voters," he said.