I CAN'T decide whether Dave Christian accepts that temperatures are going up or not.

In his most recent letter (September 22) he does not question that the CO2 level is going up and he offers the nugget of wisdom that “CO2 lags temperature by nine months in the modern data record” from which it seems that he does not dispute that temperature is also going up.

However in other contexts he mocks the notion of a rising temperature trend.

It could be that he is saying that there is a rising trend but it is nothing to do with CO2.

That is a very useful argument for the fossil fuel industry which contributes far and away the largest slice of the CO2 that is appearing in the atmosphere.

So what's it to be Dave, are global temperatures going up or not? Or is all this just obfuscation?

And what about that extraordinary claim about the modern temperature record.

Every year in spring atmospheric CO2 drops slightly as plants draw it down into living biomass only for it go up again in winter.

That cycle happens in both hemispheres but six months out of phase while the best estimate of the rate of global temperature increase is about 0.017C per year between 1980 and 2010 (IPCC 5th Assessment Report 2013).

Good luck trying to match that rate of increase to the CO2 record on a time scale of nine months!

That totally ridiculous claim is just plain straightforward deception. Only the long-term record can illuminate climate trends and as David MacKay points out in his book “Sustainable Energy – Without the Hot Air” (an online version of which is referenced at gridwatch.templar.co.uk which Dave Christian lists as one of his sources) that record is clear.

The coincidence of timing seen in retrospect between the commencement of warming and the industrial revolution is highly suggestive.

Climate is the long-term average of weather patterns.

With any reasonably imaginable amount of data it is impossible to model the chaotic evolution of the atmosphere beyond a few days and occasional exceptional weather proves nothing.

(Incidentally the Met Office forecast the “Beast from the East” well in advance.)

However, if exceptional weather happens with greater frequency you have to admit that something is happening to the climate. Climate is much easier to model than weather and modelling, the extrapolation from what is known to infer something unknown, is what scientists have always done.

It is a key driver of scientific advance and the only difference that computers have made is that scientists now model much more complicated systems.

Of course the inferences of the model are speculative until they are tested against observation, experiment or both.

Climate system models are soundly based on settled physics and chemistry and climate scientists continually look for opportunities to test their predictions.

I see no problem with a model that correctly predicts the real world. The predicted temperature trends in the early assessment reports of the IPCC were vindicated in latter reports.

The global cooling following the Mount Pinatubo eruption in 1991 was correctly predicted and if CO2 climate forcing is omitted from the model the modern temperature trajectory cannot be reproduced.

This leaves those who want to absolve CO2 from responsibility with a problem and just saying the models don't work is nowhere near good enough.

I'll come to what Dave Christian says climate scientists have never addressed in a minute but what about what climate change deniers have never addressed.

They have never given a reasoned and consistent critique of the greenhouse effect to explain why it is inapplicable, they have never proposed an alternative to explain the warming that stands up to scrutiny and they have no data to back up their claims.

The only thing they can offer are obfuscation and increasingly farcical conspiracy theories.

So, the ice core record. Research on the Vostok ice core from Antarctica shows that warming after the last glacial maximum preceded CO2 rise

Climate change deniers want us to accept that this proves that CO2 rise is caused by rising temperature, a simple cause and effect relationship.

In reality CO2 and temperature are locked in a feed back loop. At that time climate forcing was from cyclic shifts in Earth's orbit caused by gravitational interaction with the major planets and that initiated a release of CO2 into the short-term carbon cycle from the deep ocean.

The greenhouse effect then came into play to exaggerated the warming and the loop continued to operate until a new stable state was reached.

That particular feedback loop is one source of Dave's “scary predictions”.

If it starts operating again because of what humanity is doing we could totally lose control of the climate.

Richard Russell

Southampton