Sir - Your correspondent Michael Tyce (Letters, November 30) has demonstrated his doubts about the ability of human activity to change the atmosphere.

It seems so vast and endless from our perspective down here on the Earth's surface. Here are some facts which have informed my own perspective.

Ninety per cent of the atmosphere is below the altitude of 15km. If you scale the Earth down to the size of a "space-hopper" (1970s spherical toy about two feet in diameter), the atmosphere would be one millimetre thick!

99.9 per cent of the atmosphere is accounted for by nitrogen (78 per cent), oxygen (21 per cent), and argon (0.9 per cent). These gases have no greenhouse effect at all. If they were the only gases in the atmosphere, the average temperature at the surface would be about -15C (your freezer is -18C) instead of the average ten degrees we experience.

When I was at school in the 1960s I learned that carbon dioxide represented 0.03 per cent of the atmosphere. Today it is closer to 0.04 per cent. In fact it was 0.032 per cent in those days and 0.038 per cent today. Carbon dioxide is the major greenhouse gas, yet it is only a tiny component of the atmosphere, and the extra we put into it doesn't go away! It has increased by about a third in the last 300 years.

You will have heard that water is also a major greenhouse gas. This is true. However, if you put more water into the atmosphere it just rains or snows out again after a few days, and does not accumulate like carbon dioxide. The only way to get a gas to hold more water vapour is to warm it up. (Hmmm) This knowledge makes it much easier to understand what is happening. Want more detail? Try www.newscientist.com/climatemyths.

Steve Gerrish, Kidlington