Re: Paul Grimmer's letter ‘The Climate has always changed!' (The Weekend Sun Letters page, April 14, 2023).

Mr Grimmer quotes Dr Geoff Duffy, who often lectures to the public but to my knowledge has never published a single peer-reviewed paper on climate science probably because he is a chemical engineer!

Dr Duffy once said: 'doubling or trebling CO2 levels will have little effect on the atmosphere”. This is completely opposite to what all respected scientific heavyweight organisations like NASA say about CO2.

Mr Grimmer states: '99.8% of all greenhouse gases in NZ are from natural sources”. This completely ignores the fact that different greenhouse gases all have different warming effects – for exampl of the atmospheric methane (a potent greenhouse gas) 50% to 65% of this is from man-made sources and like CO2 increasing in concentration.

Mr Grimmer talks about the Medieval Warming Period being 'warmer than today” – this is an over simplification as this period was not like today's warming global event – some areas in Northern Europe were slightly warmer but many areas in for example South America were cooler. And importantly, the average temperature for the whole earth was cooler.

David Maxfield, Ohauiti.