Don Graham is right that natural processes emit greenhouse gasses (Letters, June 13, 'A natural phenomenon?'). However, the numbers he has given are incorrect.

Compared to current human yearly emissions, all volcanoes together contribute 1-2 per cent, bushfires five-10 per cent, and, at its peak, Eyjafjallajökull 0.3 per cent. Mount Pinatubo, finally, spewed out 0.01 per cent of total human-caused emissions.

This is why reducing greenhouse gas emissions matters: human-caused emissions vastly outpace natural processes and unsettle a balance that has lasted millions of years. He, I, and everyone alive are now responsible to change that.

Henk Muller