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Concentrations of key gases in the atmosphere that are driving up global temperatures reached a new high in 2017.

In their annual greenhouse gas bulletin, the World Meteorological Organization (WMO) says there is no sign of reversal in this rising trend.

Carbon dioxide levels reached 405 parts per million (ppm) in 2017, a level not seen in 3-5 million years.

Researchers also note the resurgence of a banned gas called CFC-11.

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What are concentrations?

Concentrations differ from emissions in that they represent what remains in the atmosphere after some of the gases are absorbed by the…