Global leaders were due to announce more ambitious plans to combat global warming on Saturday, on the fifth anniversary of the signing of the landmark Paris Agreement.

The Climate Ambition Summit, being held online, comes as the United Nations warns current commitments to tackle rises in global temperatures are inadequate.

Britain, the UN and France are co-hosting the summit, which Prime Minister Boris Johnson will open at 14:00 GMT and which will be live-streamed at climateambitionsummit2020.org.

China's President Xi Jinping and France's Emmanuel Macron are among the heads of state taking part, with speaking slots handed to leaders of countries that submitted the most ambitious plans.

These include Honduras, and Guatemala, which were both recently hit by hurricanes, as well as India, which is battling increasingly erratic weather patterns and air pollution.

Business figures set to speak reportedly include Tim Cook, the chief executive of Apple, which has committed to making its whole supply chain carbon neutral by 2030.

But major economies including Australia, Brazil and South Africa are absent.

Australia has not committed to net-zero emissions by 2050 and has been accused of setting targets that are too weak.

Speakers will deliver short video messages, with organisers saying they will announce "new and ambitious climate change commitments" and there will be "no space for general statements".

The 2015 Paris climate accord saw signatories commit to take action to limit temperature rises to "well below" 2.0 Celsius above pre-industrial levels and try to limit them to 1.5C.

But the UN warned this week that under current commitments, the Earth is still on course for a "catastrophic temperature rise" of more than 3.0C this century.

It warned this will create a crisis that will "dwarf the impacts of Covid-19" and has said current pledges to cut emissions to meet the Paris accord were "woefully inadequate".

Record concentrations of greenhouse gases in 2019. Photo: AFP

'Moment of accountability'

Greenpeace called the summit -- seen as a warm-up for the UN's climate change conference COP26 in Glasgow, Scotland, next November -- a "moment of accountability for leaders".

Under the Paris deal's "ratchet" mechanism, countries are required to submit renewed emissions cutting plans — termed Nationally Determined Contributions or NDCs — every five years.

The deadline for this is December 31.

Countries are set to announce efforts to reduce national emissions, long-term strategies and financial commitments to support the most vulnerable.

More than 110 countries have committed to becoming carbon neutral by 2050.

China, the world's biggest polluter, announced in September it plans to achieve net-zero emissions by 2060.

The summit comes as EU leaders on Friday committed to the goal of reducing emissions by 55 percent by 2030 compared to 1990 levels.

Britain -- out of the EU since January -- this month announced it would seek to reduce emissions by 68 percent over the same period.

Johnson has presented plans for a "green industrial revolution" creating up to 250,000 jobs.

And before the summit opened, he committed to ending all direct government support for the fossil fuel energy sector overseas.

The last five years have been the warmest on record, according to the World Meteorological Organization, a UN agency, with concern at rising numbers of wildfires, storms and flooding.

The UN has said that the drop in emissions due to the global coronavirus pandemic is too small to halt the rising temperatures.

The United States, the world's second-largest polluter after China, left the Paris Agreement under President Donald Trump who questioned the accepted science behind climate change.

Incoming US climate envoy John Kerry plans immediately to re-enter the accord and President-elect Joe Biden has set a goal of carbon neutrality by 2050.