The G20 major economies committed on Sunday (Oct 31) to the key goal of limiting global warming to 1.5 degrees Celsius, but some disappointed leaders warned more was needed to make a success of UN climate talks beginning in Glasgow.

British Prime Minister Boris Johnson, the host of the COP26 summit that opened on Sunday, said the pledge from world leaders after two days of talks in Rome was "not enough", and warned of the dire consequences for the planet.

"If Glasgow fails, the whole thing fails," he told reporters, saying the G20 commitments were "drops in a rapidly warming ocean".

UN Secretary General Antonio Guterres said he left Rome "with my hopes unfulfilled - but at least they are not buried".

The G20 nations between them emit nearly 80 per cent of carbon emissions, and a firm commitment on action was viewed as vital for the success of the UN's COP26.

US President Joe Biden said the summit made "tangible" progress on many issues but said he found it "disappointing" that Russia and China, whose leaders attended only via videolink, did not offer stronger climate pledges.

He vowed to "continue to focus on what China is not doing, what Russia is not doing, and what Saudi Arabia is not doing."