Speaking after two weeks of technical talks in Bonn, Stiell closed the annual climate diplomacy event saying: "We need to go further, faster, and fairer."

Bonn is home to the UN Climate Change Secretariat, which coordinates international climate policy and hosts preparatory talks each year ahead of climate summits.

"I'm not going to sugar coat... we have a lot more to do before we meet again in Belem," he said.

COP30 is due to be held on November 10-21 in the Amazonian city which is the capital of Para state.

At last year's UN COP29 summit in Azerbaijan, rich nations agreed to increase climate finance to $300 billion a year by 2035, an amount decried as woefully inadequate.

Azerbaijan and Brazil, which is hosting this year's COP30 conference, have launched an initiative to reduce the shortfall, with the expectation of "significant" contributions from international lenders.

This year's COP comes as average global temperatures in the past two years have exceeded the 1.5 degrees Celsius benchmark set under the Paris climate accord a decade ago.

"There is so much more work to do to keep 1.5 alive, as science demands. We must find a way to get to the hard decisions sooner," Stiell said.

Under the Paris Agreement, wealthy developed countries -- those most responsible for global warming to date -- are obliged to pay climate finance to poorer nations.

Other countries, most notably China, make voluntary contributions.