The UN secretary general has called on all countries to declare a climate emergency.
António Guterres was speaking at a virtual summit on the fifth anniversary of the Paris climate agreement,
He criticised rich countries for spending 50% more of their pandemic recovery cash on fossil fuels compared to low-carbon energy.
Over 70 world leaders are due to speak at the meeting organised by the UK, UN and France.
Mr Guterres said that 38 countries had already declared a climate emergency and he called on leaders worldwide to now do the same.
He said the emergency would only end when carbon neutrality was reached.
On the Covid recovery spending, he said that this is money being borrowed from future generations.
"We cannot use these resources to lock in policies that burden future generations with a mountain of debt on a broken planet," he said.
The secretary general praised those countries who have come to today's meeting with new targets and plans.
media captionClimate change: How 1.5C could change the world
A number of big emitters, including Australia, Saudi Arabia, Russia and Mexico, are not taking part, as their climate actions were not deemed ambitious enough.
The UK has announced an end to support for overseas fossil fuel projects, and has today deposited a new climate plan with the UN.
It's the first time that Britain has had to do this, as it was previously covered by the European Union's climate commitments.
Today's virtual gathering is taking place after the pandemic caused the postponement of the annual Conference of the Parties (COP) meeting, which had been due to take place in Glasgow this year.
"Climate change is one of the great global challenges of our age, and it is already costing lives and livelihoods the world over, our actions as leaders must be driven not by timidity or caution, but by ambition on a truly grand scale," Boris Johnson said.
"That is why the UK recently led the way with a bold new commitment to reduce emissions by at least 68% by 2030, and why I'm pleased to say today that the UK will end taxpayer support for fossil fuel projects overseas as soon as possible."
The UK says that today's short, action-oriented summit will put a premium on new commitments from countries.
Around 70 leaders from all over the world will take part, including the Secretary General of the United Nations, and President Macron of France. Pope Francis will also address the meeting.
The UK will point to its new commitment on overseas fossil fuel projects as well as a new carbon cutting target of 68% by 2030, announced last week by the prime minister.
The EU will also present a new 2030 target of a 55% cut in emissions, agreed after all-night negotiations this week.
China and India will also be taking part, though the extent of their new commitments is not clear.
Australia had held out the promise of not using old carbon credits to meet future cuts in emissions.
But the UK felt that this didn't go far enough and the Australian Prime Minister Scott Morrison won't be taking part.
Russia, South Africa and Saudi Arabia won't be involved either.
Some observers believe this hard line is justified.
"From a kind of symbolic procedural point of view, it's good to have everybody on board," said Prof Heike Schroeder from the University of East Anglia.
"But from a proactive, creating some kind of sense of urgency approach it also makes sense to say we only get to hear from you if you have something new to say."
image captionHurricane Iota was one of a record number of storms to wreak havoc on the Americas this year
The UK wants the focus to be on the countries who are set to make new net-zero announcements, or present new plans for 2030.
The five years since the Paris agreement was adopted have been the warmest on record, according to the World Meteorological Organization (WMO), and emissions have continued to accrue in the atmosphere.
Over that period, many countries and businesses have started the process of decarbonisation.
The progress they've made now needs to be acknowledged and encouraged, says former UN climate chief Christiana Figueres.
"For this Saturday, they are focusing on the reduction of emissions, and that is a good thing because that progress that's been seen in the real economy has to be reflected and incentivised further by those additional commitments."
One area that's unlikely to yield any progress at this meeting is the question of finance. Rich countries have promised to mobilise $100bn a year from 2020 under the Paris agreement - but the commitments on cash are just not forthcoming.
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