In its latest assessment of the climate crisis, the UN Environment Programme (UNEP) says delaying action by even one year will mean emissions need to be cut even faster and at greater cost - with a dwindling chance of success.
Inger Andersen, UNEP's executive director, said it is an urgent problem as "we need to catch up on the years in which we procrastinated".
Latest statistics show the equivalent of 55.3 billion tonnes of carbon dioxide were released into the atmosphere last year as a result of burning fossil fuels and other human impacts on the planet, such as deforestation.
That is up from 53.5 billion tonnes the previous year, despite increasing pressure on world leaders to act.
The UN report calculates that emissions need to be more than halved to just 25 billion tonnes by 2030 to stand a 66% chance of keeping the rise in average global temperature to 1.5C.
It would mean cutting emissions by 7.6% every year for the next decade.
"The summary findings are bleak," the report says. "Countries collectively failed to stop the growth in global greenhouse gas emissions, meaning that deeper and faster cuts are now required."
Ms Andersen said: "Countries simply cannot wait until the end of 2020, when new climate commitments are due, to step up action.
"They - and every city, region, business and individual - need to act now."
The report says that if serious climate action had begun in 2010 the year-on-year cuts to emissions needed to reach 1.5C would only have been 3.3%.
The Intergovernmental Panel on Climate Change has warned that warming beyond 1.5C will increase the frequency and intensity of heatwaves and storms, accelerate sea-level rises and significantly reduce the yields of food crops.
The UN report urges the countries with the biggest economies to do more.
The G20 nations account for 78% of all emissions, but only five, including the UK, have so far committed to reducing them to net zero by 2050.
Source: Sky News