"If you don't put more ambition into your pledges, the NDCs [nationally determined contributions]... then you can forget about the 1.5 degrees or even the two degrees because you won't be able to make it," John Christensen, lead author of the UN Environment's annual Emissions Gap report, told RFI on Tuesday.
The report comes just days before the next world climate conference in Bonn, Germany.
Pledges made so far cover only one-third of the cuts needed by 2030 to keep temperature levels at below 2.0°C, the review warns.
The implications for the 2015 Paris climate deal are huge.
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With less than three years to go before countries must submit their revised pledges for keeping within the Paris deal, the report has underscored their inability to stick to even the first pledge.
"There's a sense in wealthier countries that this is something we have the capacity to adapt to," explains Brandon Wu, Campaign Director for ActionAid USA in Washington.
But, he warns, "for vulnerable countries, this is really an existential crisis right now."
Developing countries committed to doing much more than rich countries did in the Paris climate pledges, he said, adding that that is "the opposite of what it should be, of course."
Paris deal flawed
For Wu, the Paris deal was inherently flawed from the start.
"The fact that this was non-binding, the fact that the pledges are voluntary and really don't amount to very much, puts us on the road to disaster;" he comments.
The evidence is already there, as witnessed this year alone in the flash floods that pounded the Caribbean, Florida and Texas.
"Hurricane activity has been made worse by climate change," says Bob Ward of the Grantham Research Institute on Climate and the Environment.
"Sea level rises there have made the storms there just more devastating," he points out. "So I think people can see that keeping with status quo is not going to be a viable option," he told RFI.
Government action needed
But government and non-state actors are still dragging their feet, in keeping within the temperature limits agreed in the Paris climate pact, the UN report highlights.
Not everyone agrees.
"If we're talking about cities, I would not say there is a lack of climate action," Simon Hansen, Regional Director for the Cities for the C40 Cities Climate Leadership Group, told RFI. "On the contrary, I would say that in many cities there is a real commitment to turn the Paris agreement into real action on the ground."
He cites last week's meeting of representatives of several cities, who pledged to only purchase zero-emission buses from 2025.
"We're not saying there's no action happening, we're simply saying there's not enough action happening," replies John Christensen of UN Environment.
While he commends non-state actors like cities for taking the lead in many areas, he says there is still confusion about whether this action is part of countries' pledges to stick to the Paris deal.
Closing the gap
Going forward, the UN report does outline measures for closing the gap and reaching the 1.5°C target, notably by adopting new technologies and increasing national goals.
"The issue is we know how to do this," says Ward, "But there is a question now of getting the investment pointing in the right direction."
That direction should be away from investing in the coal industry, which the United States under President Donald Trump has revived, he reckons.
"The US has been overtaken by corporate interests," reckons Brandon Wu, before adding "politics change, we can't base what the world does on what the US does or doesn't do."
"We know there are going to be a few glitches," acknowledges Bob Ward. "The United States for instance if it remains in the Paris climate deal looks likely to submit a weaker pledge."
However, he insists that the momentum galvanised by the Paris accord is still intact.
"Most countries recognise that this is no case for going slow on this, all the pressure is for stronger action sooner," Ward insists.