The main greenhouse gas emissions driving climate change have all reached record levels, a UN report said on Thursday.
Carbon dioxide (CO2), methane and nitrous oxide were now far above pre-industrial levels, with no sign of a reversal of the upward trend, the Guardian quoted the World Meteorological Organisation (WMO) report as saying.
"The last time the Earth experienced a comparable concentration of CO2 was 3-5m years ago, when the temperature was 2-3C warmer and sea level was 10-20 metres higher than now," WMO Secretary-General Petteri Taalas SAID.
"The science is clear. Without rapid cuts in CO2 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."
Levels of CO2 rose to a global average of 405.5 parts per million in the atmosphere in 2017 - two-and-a-half times higher than before the industrial revolution.
Levels of methane, a potent greenhouse gas responsible for about 17 per cent of global warming are now 3.5 times higher than pre-industrial times owing to emissions from cattle, rice paddies and leaks from oil and gas wells.
Nitrous oxide, which also warms the planet and destroys the Earth's protective ozone layer, was now at more than double pre-industrial levels, according to the WMO report.
About 40 per cent of N2O comes from human activities including soil degradation, fertiliser use and industry.
Efforts to cut emissions were increasing and on Wednesday the UN's climate change body published a report on the commitments made in 2018.
It found 9,000 cities in 128 countries were taking action, along with 240 states and regions in 40 countries and more than 6,000 businesses in 120 countries.