Tuesday, 21 June 2011

A Question of Scale

In considering physical systems I find it essential to have a concept of the relative size of things and quantities. For example, the carbon dioxide (CO2) concentration I referred to in my last (and first) post is usually quoted in parts per million (ppm) - actually measured by volume, so more accurately ppmv. Strictly, ppm refers to concentration by weight. The currently accepted concentration is 390 ppmv, which sounds a lot, until it's expressed as a percentage; 0.039% or just under four-hundredths of one percent. As a fraction it's 1/2564.

However, the atmosphere is very large, and that 0.039% represents about 2,500 gigatonnes (billion tonnes) of CO2. That seems pretty big, until it's compared with CO2 dissolved in the world's oceans - about 50 times as much, carbon in the biosphere (all living things, including bacteria and plants) - several hundred times as much, and in carbonate rocks in the earth's crust - several thousand times as much. For comparison, the amount of CO2 emitted annually as a result of mankind's activities is estimated to be about 32 gigatonnes.

So is that 0.039% important, or at all significant? Of course it is - the question though, is how significant. Despite contrary assertions, there is strong disagreement about the relative importance of that (increasing) amount on Earth's climate and ecology. Someone once said "There's what we know about the climate, there's what we don't know about the climate, and there's what we don't know we don't know about the climate. Of these three, the last is by far the largest". I would suggest that applies to every area of scientific theory and research, though perhaps in different proportions.

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