Greenhouse gases in the atmosphere hit a new record this year, according to a new report by the United Nations. 

It's the latest warning from meteorologists concerned time is running out to stop irreversible climate change. But as Owen Fairclough reports, there's also concern that the collective will to do anything about it is diminishing.

Sydney engulfed in a bushfire haze on Friday-catapulting Australia's largest city into the world's top ten most-polluted cities. 

And on Monday, the United Nations announced greenhouse gases that trap heat in Earth's atmosphere hit a record this year. 

"We have again broken records in carbon dioxide concentrations, and we have already exceeded 400ppm level, which is regarded as a critical level that happened already two years ago and this growth of carbon dioxide concentration continues," said Petteri Taalas. He is the secretary-general of the World Meteorological Organization. 

Bushfires may be common in Australia. But meteorologists say record heat caused by climate change is partly to blame-and its impact across the world is accelerating from California's wildfires to the hottest summer ever in France and Arctic ice melt, changing the planet's entire water system. 

And if advanced economies were once responsible for nearly all pollution, that's changing. 

"There has also been fairly strong growth in the emissions of non-OECD countries, and this is demonstrating that if we want to solve this problem, we have to have global efforts. The European Union nor the USA cannot solve it alone, nor China, we have to have all the countries onboard," added by Petteri Taalas. 

But the U.S. isn't on board. Under President Trump, environmental regulations are being rolled back to favor fossil fuels such as coal. 

And earlier this month the Trump administration formally served notice that it would withdraw from the Paris accord designed to limit harmful emissions. The U.N. says unless temperatures start to fall next year, irreversible climate change can't be prevented.