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# Climate change – a very difficult, very simple idea

Planet Earth doesn’t have ‘a temperature’, one figure that says it all. There are oceans, landmasses, ice, the atmosphere, day and night, and seasons. Also, the temperature of Earth never gets to equilibrium: just as it’s starting to warm up on the sunny-side, the sun gets ‘turned off’; and just as it’s starting to cool down on the night-side, the sun gets ‘turned on’. The ‘temperature of Earth’ is therefore as much of a contrived statistic as the GDP of a country. (If the Earth was in equilibrium, that is, if it absorbed and re-emitted the Sun’s radiation perfectly, as a ‘blackbody’, then its rotation would be irrelevant, and the temperature would be a constant 6 ⁰C. Mocking up the effects of Earth’s albedo brings the ‘blackbody’ temperature down to -18 ⁰C, and including greenhouse warming brings it back up to around 15 ⁰C.)

‘The climate’ is difficult to define: is it a trend over one decade, century, or millennium? What sized region is it defined for? Weather is very variable – how can we go from weather to climate? Furthermore, climate change on human timescales is a very small effect, and the empirical data needed for climate models have large ‘error’ bars. The models themselves have to make many assumptions, and sometimes lead to solutions that are unstable (small changes in the input data lead to large changes in the predictions).

However, despite the above, climate change, more specifically human-induced climate change, is a surprisingly simple idea.

The mathematician and philosopher, Bertrand Russell, summed up mankind’s activities as the rearrangement of matter on or near the Earth’s crust. Russell’s words need refining. The rearranging of matter is done in two different ways: by mechanical devices (such as levers), or by ‘heat-engines’ (such as muscle-power, or combustion engines). (Loosely speaking, anything that ‘farts’ is some sort of heat-engine.) A heat-engine consumes fuel and, by the Second Law of Thermodynamics, ‘always puts out some waste heat.’ Therefore, Russell’s summary should more accurately state: the activities of mankind can be summed up as the rearrangement of matter and the generation of heat. (By the way, ‘waste heat’ refers to direct heating and all other energy losses, not just the production of exhaust gases.)

The only way to reverse global warming is to change the interactions between the Earth and its surroundings…

There is still the question: is this generation of heat enough to cause climate change? Sceptics claim that the activities of people are completely swamped by the immense power of volcanoes, the variability of the Sun’s output, and other ‘natural’ effects. These effects are indeed immense, but note that life forms have affected Earth’s climate in the past (cf. cyanobacteria). Also, while the consequences of the behaviour of any one person are tiny, cumulatively there can be an influence. (In fact, monetarist economic models demand this – people are encouraged to buy things precisely because their individual behaviour affects the economy as a whole.) Finally, there is much evidence of mankind’s influence on the planet in ways not directly related to climate. For example, a ladle-full of seawater from any sea or ocean now always contains some detectable trace of plastic. Even where a material is introduced in miniscule amounts (on the planetary scale), there can be big effects. Witness the use and subsequent control of use of aerosols; this has changed the ozone layer, twice.

Sceptics also argue that natural ‘buffering’ mechanisms keep the Earth’s climate stable – yes, but not if pushed too far. How else has change happened in the past (Ice Ages, etc.)? (An analogy comes from dieting. Go on a weight-loss diet and your body thinks, “Aha, you’re on a fast, switch on the fasting survival-metabolism.” But if you go too far, you die.) If it wasn’t for the finality of it, the present era would be a moment for awe: science textbooks have talked of ‘the surroundings’ or ‘the environment’; now, for the first time, mankind is affecting ‘the surroundings.’

It is agreed that mankind does affect the microclimate (for example, cities are hotter than the surrounding areas) – but is not a whole climate made from microclimates knitted together?

In summary, it would be very surprising if humans were not affecting the climate. Affecting it how? By increasing the amount of global warming – this is what the Second Law of Thermodynamics tells us. According to this Law, so long as our activities concern the planet in isolation, there is nothing whatsoever that we can do to stop global warming (we can merely reduce its rate). The only way to reverse global warming is to change the interactions between the Earth and its surroundings; and this can only be done by reducing the net flux of Solar energy received, by reducing greenhouse gases and/or increasing the Earth’s albedo.

Headline image credit: Earth Atmosphere by NASA/ GSFC. Public Domain via Wikimedia Commons.

1. […] Oxford University Press talks about climate change, and more specifically, on how human-induced climate change is a surprisingly simple idea. Following the Second Law of Thermodynamics tells us that humans are affecting the climate by increasing the amount of global warming. […]

2. PETER CODNER

Do you agree with
Prof. Bjarne Andresen that the concept of a global temperature is thermodynamically as well as mathematically an impossibility?

If so, why and why is it a thermodynamic and mathematical impossibility?

From a philosophical and practical point of view it strikes me that there are conceptual difficulties with a global temperature, to say nothing of maths and thermodynamics-not my field, which is law.

My concern is with those that believe or disbelieve what they *want* to believe or disbelieve, which strikes me as a reasonable definition of religion. or the exact opposite of impartiality.
I take belief to be the exact opposite or inconsistent with Knowledge-as distinct from information.

Can it be said either that the earth is, or is
not, in thermodynamic equilibrium, and to what is that relevant?
As a lawyer/philosopher I need to be able to argue ‘both’ cases, and wonder if belief is helpful.