UK Prime Minister Boris Johnson says the promises made in the landmark Paris climate accord are starting to sound “frankly hollow” six years later, as he called leaders’ latest commitments “drops in a rapidly warming ocean”.

Mr Johnson struck a grim note on Monday morning (Australian time) at the end of a G20 summit in Rome where leaders agreed on a final statement that urges “meaningful and effective” action to limit global warming in a declaration which offers few concrete commitments.

• Read the G20 Rome Leaders’ Declaration, here

“If we don’t act now, the Paris agreement will be looked at in the future not as the moment humanity opened its eyes to the problem but the moment we flinched and looked away,” the UK leader said.

The 2015 Paris accords seek to keep the rise “well below” 2C and to “pursue efforts” to limit it to 1.5C.

The United Kingdom had hoped for a “G20 bounce” going into the UN climate change conference that started on Sunday in Glasgow, Scotland.

But Mr Johnson said the group of large economies needed to go much further.

The G20 bloc, which includes Brazil, China, India, Germany and the United States, accounts for an estimated 80 per cent of global greenhouse gas emissions.

The result of days of tough negotiation among diplomats leaves huge work to be done at a broader United Nations climate summit in Scotland, to where most of the G20 leaders will fly directly from Rome.

The final document says current national plans on how to curb emissions will have to be strengthened “if necessary” and makes no specific reference to 2050 as a date to achieve net zero carbon emissions.

“We recognise that the impacts of climate change at 1.5C are much lower than at 2C. Keeping 1.5C within reach will require meaningful and effective actions and commitment by all countries,” the communique said.

The 1.5C threshold is what UN experts say must be met to avoid a dramatic acceleration of extreme climate events like droughts, storms and floods, and to reach it they recommend that net zero emissions should be achieved by 2050.

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The leaders recognised “the key relevance” of achieving net zero carbon dioxide emissions by the middle of this century.

China, the world’s biggest CO2 emitter, has set a target date of 2060 and other large emitter such as India and Russia have also not committed to the 2050 target date.

UN experts say that even if current plans of countries are fully implemented, the world is headed for global warming of 2.7C with a catastrophic acceleration of events such as drought, storms and flooding.

The draft includes a pledge to halt financing of overseas coal-fired power generation by the end of this year but set no date for phasing out coal power, promising to do so “as soon as possible”.

They also set no date for phasing out fossil fuel subsidies, saying they will endeavour to do so “over the medium term”.

On methane, which has a more potent but less lasting impact than carbon dioxide on global warming, they watered down their wording from a previous draft.

-with AAP