DOUG Mills (Letters 4 October) tells us what would happen if Earth magically shifted 30 million miles and adopted the orbit of Venus around the sun. I think his point was somehow to suggest global warming is not a problem.

In truth, it would only require a shift of three million miles in orbit to boil the Earth’s oceans off into the atmosphere. But three million miles is an unthinkable distance for an orbit shift.

Our orbit round the sun has ensured the oceans don’t boil but the oceans have frozen over a couple of times in the past. It is CO2 that is the big regulator of climate, keeping global temperatures roughly at today’s level for millions of years.

The impact of a drop of 6ºC is commonly appreciated. This causes an ice age with glaciers reaching down to North London.

The impact of a rise of 6ºC is less commonly understood. Such temperatures haven’t occurred for 40 million years. They would render much of the tropics unliveable without air conditioning. And a lot more bad stuff as well.

This situation is a problem because a 6ºC rise in global temperature is not unthinkable. Unlike Doug Mills’ shift in orbit, if mankind continues pumping CO2 into the atmosphere, that 6ºC rise is very likely to occur.

DR MARTIN RODGER

Bloxworth Road

Poole