Greenhouse gas levels have reached new record highs, prompting experts to warn “the window of opportunity for action is almost closed” to tackle climate change.

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, the World Meteorological Organisation (WMO) said.

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.

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.

WMO secretary general Petteri Taalas: “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.

“The window of opportunity for action is almost closed.

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

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 1.5C above pre-industrial levels and reduce the risks of climate change.

IPCC chairman Hoesung Lee said: “The new IPCC special report on global warming of 1.5C 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.

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

Prof Corinne Le Quere, director of the Tyndall Centre for Climate Change Research at the University of East Anglia, said she was “not surprised but I am very concerned” that the major greenhouse gases are rising unabated.

“It seems the urgency and extent of the actions needed to address climate change have not sunk in.

“Low-carbon technologies like wind, solar, and electric transport need to become mainstream, with old-fashion polluting fossils pushed out rapidly,” she said.–PA