What are the relative sizes of these heat inputs and outputs? In looking at very large quantities of heat, it is common to measure heat in Q-units (1 Q is equal to 1018 BTUs, or about 1021 joules).
Differing estimates of the Earth's total stocks of fossil fuels range between about 40 and 200 Q. We have been using these for well over a century, and although current rates are higher than ever, we are probably still not injecting more than 2-3 Q of heat into the biosphere each year. The potential reserves of fissionable nuclear fuels are much higher than for fossil fuels, but even so we are not actually using as much each year. A figure of 5 Q/yr for Man's budget-negative injection of heat into the biosphere is probably well on the high side.
Such a figure pales into complete insignificance when placed against the heat the Earth receives from the Sun, which is about 5000 Q/yr. Hold up one square metre of surface at right angles to the Sun, out in Earth orbit, and it receives the equivalent of 1.8 horsepower or 1.3kW of radiant energy -- enough to drive an average air-conditioner.
Not all the energy sent by the Sun is absorbed by the Earth, about 30% is reflected back into space. This isolates one of the crucial factors. If the amount of heat reflected increased by only 1% (say with the average reflectance rising from 30.3 to 30.6%), this would slough off another 50 Q/yr, some ten times the most Man is adding. Our fuel-use activities are irrelevant in the face of this factor.
Proposition 17H
Man's influence on the amount of heat added to the biosphere is insignificant compared to the effects of small variations in reflected solar radiation
With all this heat coming from the Sun, why don't we just heat up and vaporize? This is the other side of the energy balance -- the Earth is itself radiating heat off into space. We move on now to look at the effect of small changes in this balance.
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