SIR - I learn something every day. I had no idea of the pet food industry influence on our resource use and the resulting amount of CO2. It seems that a quarter of the impact of meat production, involving the land, water, fossil fuels, phosphates and pesticides, results from feeding it to cats and dogs. That’s some pawprint.

I must own up as I have two cats, from a Buttershaw rescue centre, but they are both neutered, and it would certainly help if we had an effective system, perhaps related to licensing, that ensured that the breeding of all pet animals was controlled.

The problem stems from the sheer number of cats and dogs in the established industrial countries, and they are the same problem as our excessive numbers – we are just bigger, hungrier animals, and more numerous, but they still produce CO2 with our help.

So while the UK currently has around eight million cats and a similar number of dogs, with less than half the households owning one, the numbers have varied depending on the economy with a reduction of a couple of million of each since 2007.

It’s not just the poorer countries that have fewer pets, as Japan with double our population has only marginally more cats and dogs than us, and there are generally fewer per head of the population in the rest of Europe.

This trend is continued world wide with far fewer Indian and Chinese cats and dogs, with only a fifth of families owning one. However there’s one glaring exception, and as expected it’s the USA.

It’s the only country where two thirds of the households have a cat or dog, and there are at least 75 million of each animal.

However we can’t have both pets and a reduction in CO2, so at least half of them must go.

There’s a challenge for the government.