RESPONDING to Tony Kelly, who mentions our “greed and stupidity” may cause another “wholesale mass extinction” (HAS, Feb 23), I would refer readers to Cosmos Magazine issue of July 6, 2015, where the last five big events are summarised.

We may deduce the following from the article:

End Ordovician Period, 444 million years ago (MYA) – probable Ice Age caused by mountain-building and carbon dioxide (CO2) drawdown.

Late Devonian, 375 MYA – probable algal blooms triggered by newly-evolved land plants.

End Permian, 251 MYA – mass eruptions in Siberia triggering CO2 spike (warming) then methane spike (more warming), ocean acidification, then poisoning by hydrogen sulphide.

End Triassic, 200 MYA – the cause is an enigma.

End Cretaceous, 66 MYA – volcanic activity, climate change and asteroid impact.

But two years after the Cosmos article, in 2017, Swiss teams analysing rock strata in the Nanpanjiang Basin in China found strong evidence that the Permian extinction coincided with a short, 80,000-year Ice Age that preceded an age of global warming and desertification by 500,000 years.

In conclusion, the Permian extinction mentioned by Tony Kelly was caused by sudden extreme cold, and the victim species were already fossils when CO2 spiked due to volcanism.

If our CO2 addition is guilty of anything it is to help feed plants and fortify their root systems, especially in arid conditions, fixing nutrients and fibre into the soil in fragile regions like the Sahel.

Michael Watson, Darlington.