Globally, averaged concentrations of CO2 reached 405.5 parts per million (ppm) in 2017, up from 403.3 ppm in 2016 and 400.1 ppm in 2015.
Concentrations of methane and nitrous oxide also rose, whilst there was a resurgence of a potent greenhouse gas and ozone depleting substance called CFC-11, which is regulated under an international agreement to protect the ozone layer.
Since 1990, there has been a 41 per cent increase in total radiative forcing – the warming effect on the climate – by long-lived greenhouse gases. CO2 accounts for about 82 per cent of the increase in radiative forcing over the past decade, according to figures from the US National Oceanic and Atmospheric Administration.
The last time the Earth experienced a comparable concentration of CO2 was 3-5 million years ago, when the temperature was 2-3°C warmer and sea level was 10-20 meters higher.