How a few physics lessons, over 50 years ago, made a lifelong climate sceptic of me.

I left school in 1969, and in the two weeks between the end of our final exams and the end of term, my physics master took the opportunity to introduce us to two subjects he thought would be significant in our lives – computers and climate change. At that time, computers were the size of wardrobes, and their ownership was the preserve of large companies and government departments. The term ‘personal computer’ had yet to be coined, and the idea that one might have one in one’s home was the stuff of science fiction. But he introduced us to Moore’s Law, which roughly states that every two years the power of computers will double, and their price and size will halve. That’s been more or less true throughout my life.

Next, he introduced us to ‘the greenhouse effect’. He started by showing us how an actual greenhouse worked, its glass having the intriguing property of converting short-wave radiation in the ultra-violet spectrum into longer-wave radiation in the infra-red spectrum, in which form it warmed the soil and air in the greenhouse, the warmth being trapped by the physical barrier of the glazing.

He then introduced us to the ‘Tyndall gases’ which, like glass, absorb high frequency radiation (visible and ultra-violet light) and re-emit it at a lower frequency (infra-red). He explained why the presence in the atmosphere of these gases, and in particular water vapour, was the reason our planet maintained a habitable temperature. He drew attention to Svante Arrhenius’ observation that industrial emanations of CO2 might, by adding to the prevalence of that gas in the atmosphere, have the effect of slightly warming the world.

He was at pains to point out, however, that while the greenhouse gases in the atmosphere inhibited the rejection of heat to spacer by radiation, the atmosphere did not, unlike a real greenhouse, have a roof to prevent heat being transported to the upper atmosphere by convection, and then rejected to space by radiation. He then showed us an example of a very simple climate-modelling equation, which I regret to say I have entirely forgotten. However, it was clear that a very large number of iterations of the equation were necessary to produce any meaningful results.

And at that point, he reintroduced the computer, observing that with its arrival, it was for the first time possible to perform in seconds a large series of recursive iterations of these equations that would take a researcher months to perform, very possibly at the cost of his sanity. He cautioned us, however against concluding that it was now possible to predict the future course of the climate in any meaningful way.

In the first place, the Earth’s climate is a nonlinear, chaotic system, so that even to achieve an approximately accurate prediction of its future course, it would have to be modelled, in its entirety, in tiny volumes, each interacting with its neighbours – a computational task far beyond the abilities of the computers of the day, and still not achievable, despite the vast development of computer technology that has ensued.

Secondly, while the primary thermal consequence, all other factors being equal, of a rise in CO2 concentration is relatively uncontroversial, all other factors would not remain equal, giving rise to secondary effects, or feedbacks, which could either enhance or mitigate the primary effect. To take but one example, the atmosphere is subject, in any case, to great fluctuations in humidity – that is, in the concentration of water vapour. Water vapour is a far more effective greenhouse gas than CO2. Any thermal consequences of CO2 increase would tend to induce changes in the atmosphere’s water content, whose effect would have to be determined, and then included in the model.

Each greenhouse gas has a unique absorption spectrum, that is, a range of frequency bands to which it is opaque. However, some of these bands are shared by two or more gases, and this is true of water vapour and CO2.

As the above chart shows, there is a considerable overlap between the absorption spectra of CO2 and water vapour. This means that on a humid day, the frequency bands that are shared by water and CO2 are saturated by the water vapour, rendering the greenhouse effect of the CO2 nugatory for those frequencies. We might also reflect that our atmosphere has experienced wildly fluctuating concentrations of water vapour throughout its existence, without these leading to ‘runaway warming’, the catastrophic outcome with which the comparatively puny greenhouse effect of CO2 is said to be threatening us.

This is far from a comprehensive account of the obstacles to any attempt to model the earth’s climate, and I was taught it over half a century ago, so I hope I may be forgiven the odd solecism.

Our teacher concluded his disquisition by telling us, with a prescience that still impresses me, that some day, armed with the outputs of computer models, scientists would try to persuade us that man-made emissions of CO2 were dangerously altering Earth’s climate. He urged us to remain sceptical of such claims. I have.