IN HIS letter (July 11, 'Beware of extremists'), Trevor Farrer refers to “global warming obsessives” and “green extremists”, and suggests that changes in the global climate are not occurring.

He offers no scientific facts to justify this, simply claiming that variable weather features “have existed forever”.

This is to confuse local, short-term, weather with long-term climate.

I invite readers of the Gazette to take a look at the global temperature trend on the NASA website. It shows an increase in global average land temperature of 1 degree centigrade since 1970. This is clear direct evidence that a major change is occurring.

And the climate models manage to simulate this trend pretty well, demonstrating that the knowledge on which the models are based is sound and reliable. Therefore Mr Farrer is wrong in his assertion that we are being misled by extremists – we are actually being very well informed by responsible scientists.

In the same edition of the Gazette, Clive Carroll makes a number of unsubstantiated claims about climate science. He states incorrectly that in “the past 18 years there has been no net warming”.

Again I draw readers’ attention to the NASA website, which shows that there has actually been quite a sharp increase in land temperature of 0.5 degrees centigrade during this period.

He also states that “changes in Earth’s surface temperatures are largely driven by variations in solar activity”. While it is true to say that solar radiation is a major factor in controlling the surface temperature (and this is taken into account in climate modelling), it is also the case that the greenhouse effect caused by atmospheric carbon dioxide plays a role.

By increasing the carbon dioxide content of the atmosphere, mainly through the burning of fossil fuels, humans have caused the warming that we are clearly witnessing. In fact, solar activity has declined over the last few decades, which means that the greenhouse effect has been dominant.

There is now no doubt that human activities have raised the temperature of the planet, and that consequently we face serious difficulties.

Ed Tipping