Sorry Mr Wroe (Letters, December 12, 'Is the climate so different?'), but the fact there were floods and downpours in the eleventh century doesn’t prove anything about climate change – not even when you put 'QED' at the end. Weather is unpredictable, and it is impossible to tell with complete certainty what causes any single event.

The evidence for man-made climate change comes from a whole series of careful measurement made by scientists around the world. Carbon dioxide levels in the atmosphere, static for centuries, started to rise at the start of the industrial revolution, when we first started burning coal in substantial quantities. They’ve now passed 400 parts per million, more than 40 per cent higher than before the industrial revolution.

The mechanism by which CO2 causes global warming was worked out at the beginning of the 20th century by the Nobel prize-winning Swedish chemist Svante Arrhenius. CO2 preferentially absorbs low-energy heat radiation from the Earth, reducing the amount leaking out into space and hence warming it up – a phenomenon familiar to anyone who owns a greenhouse.

True to Arrhenius’ predictions, average global temperatures have shown a gradually rising trend over the last 150 years. There have, of course, been considerable fluctuations from year to year. Climate change deniers are fond of quoting these fluctuations as 'proof' that global warming doesn’t exist.

But the overall trend – especially since 1980 – is up, and it’s no coincidence that 2015 is on course to be the hottest year globally since records began. Higher temperatures mean more energy in the atmosphere, which makes extreme weather events such as hurricanes or floods more likely.

So are the recent floods are result of climate change? No-one can tell for sure. But the fact is that Carlisle has now suffered three floods, of the sort that were supposed to happen only once a century, in the last 10 years. Coincidence – or a wake-up call? I know which I think more plausible.

John Eakins

Flookburgh