In 1984 I read a prediction by the biological scientist and Nobel Laureate Konrad Lorenz that heat produced by human activity would necessarily increase Earth's average atmospheric temperature. His prediction said nothing about changes in the composition of the atmosphere. Clearly this would involve a vast number of individual factors. For examples:

  • changes in ground cover such as replacing forests with grass fields, pavement, buildings, etc.,

  • capturing solar radiation which is then returned to the environment as the thermal byproduct of useful work,

  • release of chemical energy stored in fossil fuels

  • and etc.

How would, for example, paving and painting a field differ from planting a crop of the same color? Does a forest absorb and communicate heat to the ground in such a way that it doesn't significantly impact atmospheric temperature?

I am unfamiliar with any effort to quantify or even to qualify such impacts on global temperature. Have such studies been conducted?


Yes, such effects are quantified and in fact this is an area of intensive study.

Direct effects

Human energy production (via burning fossil fuels, nuclear power, solar power, anything) has an absolutely minute direct impact on temperature. It is easy to see this by considering a simple-minded black-body model of the planet. With fairly mild assumptions (the planet is far enough from the Sun and either is perfectly conductive or rotating fast enough) you get a surface temperature for such a black-body planet of

$$T_S = \left(\frac{F}{4\sigma}\right)^{\frac{1}{4}}$$

Where $F$ is the flux of energy from the Sun. For Earth $F \approx 1360\,\mathrm{Wm^{-2}}$ and this gives a surface temperature of about $278\,\mathrm{K}$ which is plausible (it's too cold, but only about ten degrees too cold, so as a sanity check it's fine).

You can then take account of the difference in temperature from human power generation, which is

$$ \begin{align} \Delta T &= \left(\frac{F}{4\sigma} + \frac{H}{4\pi R^2\sigma}\right)^\frac{1}{4} - \left(\frac{F}{4\sigma}\right)^\frac{1}{4}\\ &\approx 0.007\,\mathrm{K} \end{align} $$

Where here $R$ is the radius of the Earth & $H$ is human power generation: $H \approx 1.8\times 10^{13}\,\mathrm{W}$ in 2013.

So the direct effects are tiny and can safely be ignored (but see caveat below).

Indirect effects

Indirect effects include things like albedo changes due to land-use change, soot deposition, changes in the size of ice-sheets and the amount of sea-ice, changes in the composition of the atmosphere in the form of both greenhouse gasses (raise surface temperature), aerosols & dust (generally reduce surface surface temperature) and so on. These effects currently completely dwarf direct effects.

If we leave out atmospheric effects (which are probably the dominant ones) then we're left with albedo changes &c. These are fairly significant, and people modelling climate therefore need to take them into account. There have been two approaches to this.

  • They can be prescribed: whatever model you are trying to use is just given values for the various parameters as boundary conditions, and then computes the rest (typically it tries to model the atmosphere and probably the ocean only).
  • They can be included in the model to a greater or lesser extent. For instance it's possible to try and estimate things like soot deposition (soot on snow is a big deal), or changes in vegetation as a result of changes in temperature and other changes in the atmosphere. Some typically still have to be prescribed (I think things like area-covered-by-cities are in this category) because it's not really possible to model them plausibly.

The general term for models which try to take these things into account is 'Earth system models' and this is a very active area of research. Note that all such models need to take the atmospheric and ocean changes into account as well: it's not really possible to isolate things in any easy way as the system has a large number of dependencies.

A caveat

The underlying reason that indirect effects are so much more important than direct ones is is that the amount of power the Sun dumps into the system and that the system must then reflect or absorb and reradiate is enormously larger than human power usage: it's about $1.7\times 10^{17}\,\mathrm{W}$. But human power usage has been increasing approximately exponentially, at around 2% every year (rather more in fact) for a long time. This means that, in due course & unless something changes, direct effects will start to matter. This won't happen for quite a long time (a few hundred years) and it seems unlikely to be a problem in practice.


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