JOHN Howson wrote to the Gazette (Letters, March 5, 'Veganism and the Ice Age') on two interesting topics: the absorption of CO2 by grassland, and human diet. Some clarification is needed.

He has been told grassland absorbs four to six tonnes of CO2 per hectare per year.

This is not quite right but not far off.

In the absence of fertiliser the annual growth of grass in Britain is equivalent to about 15 tonnes of CO2 per hectare.

If fertiliser is applied the amount is about twice that.

To put this in context, human emissions of CO2 average about 15 tonnes per hectare across the whole of the UK.

However, whereas the human emissions continually add CO2 to the atmosphere, the plant growth only removes CO2 temporarily.

This is because when the grass dies it decomposes, releasing CO2 back to the atmosphere, so there is hardly any net change.

Therefore, just growing grass makes little difference to atmospheric CO2.

Planting trees might help since they will store CO2 in the accumulating wood, but in my opinion this option might not be as effective as its champions claim.

Perhaps we will see.

Turning to human diet, Mr Howson thinks the start of the Ice Age two million years ago prompted humans to turn to a meat-based diet because they could not find enough vegetation to eat.

Actually, humans only evolved about 300,000 years ago, in Africa during a period of dramatic climate change.

They spent most of the time between then and now as hunter-gatherers, with a diet of meat, fish, fruit and nuts.

Only about 12,000 years ago did humans invent farming, in what we now call the Middle East.

They were thereby relieved from endless journeying in search of food.

They increased their consumption of cereals and vegetables, and had time to start civilisation.

According to some, they have never been happy since!

Ed Tipping