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Greenhouse gas levels have reached new record highs, prompting experts to warn that without rapid cuts climate change will have increasingly destructive and irreversible impacts.

Average concentrations of carbon dioxide hit new highs of 405.5 parts per million (ppm) in 2017, up from 403.3 ppm in 2016 and 400.1 ppm in 2015, levels not seen for millions of years.

Levels of other key greenhouse gases methane and nitrous oxide in the atmosphere also rose, says the UN’s World Meteorological Organisation (WMO).

There is no sign of a reversal in the trend in increasing greenhouse gas levels, which is driving climate change, sea level rises and more extreme weather and making oceans more acidic, the UN experts warned.

In its annual bulletin on greenhouse gas levels, the WMO also warned of a resurgence in a potent greenhouse gas and ozone-depleting substance known as CFC-11, which has been linked to illegal refrigerator factories in China.

“The science is clear. Without rapid cuts in carbon dioxide and other greenhouse gases, climate change will have increasingly destructive and irreversible impacts on life on Earth,” said WMO secretary-general Petteri Taalas.

“The last time the Earth experienced a comparable concentration of carbon dioxide was 3-5 million years ago, when the temperature was 2-3°C warmer and sea level was 10-20 metres higher than now,” said Taalas.

The latest findings come after a report from the UN’s Intergovernmental Panel on Climate Change (IPCC) found net emissions of carbon dioxide must reach zero by around 2050 to keep temperature rises to 1.5°C above pre-industrial levels and reduce the risks of climate change.

The new IPCC special report on global warming of 1.5°C shows that deep and rapid reductions of emissions of carbon dioxide and other greenhouse gases will be needed in all sectors of society and the economy,” said IPCC chairman Hoesung Lee.

“The WMO greenhouse gas bulletin, showing a continuing rising trend in concentrations of greenhouse gases, underlines just how urgent these emissions reductions are,” said Lee.