Nils Simon wrote:
> I have come across a small puzzle, but am unable to solve it myself. I
> was looking around a bit to find our present concentration of GHGs
> measured in CO2-equivalents. I took all following numbers from the 4AR
> or calculated them from these. Here's what I found, and I'm only
> looking at the three most important GHGs:
>
> CO2 - 380ppm
> CH4 - 1,774 ppb, or 1.774 ppm, with a global warming potential (GWP)
> of 25 equals 44.35 ppm CO2-eq
> N2O - 319 ppb, or 0.319 ppm, with a GWP of 298 equals 95.06 ppm CO2-eq
>
> Together, I get about 519 ppm CO2-eq, a surprisingly high figure.
> (GWP figures are from p. 33 in the Technical Summary)
>
> Now another table gives expected temperature increase for certain
> concentrations of GHGs, also measured in CO2-eq. You can find it on p.
> 66. It says that for the following concentrations, we could expect the
> following temperatur increase:
>
> CO2-eq Best guess temperature increase (+ likely range)
> 350 ppm 1.0 (0.6-1.4) °C
> 450 ppm 2.1 (1.4-3.1)
> 550 ppm 2.9 (1.9-4.4)
> 650 ppm 3.6 (2.4-5.5)
> 750 ppm 4.3 (2.8-6.4)
> and so on
>
> Now I was in the belief that we would already have crossed or nearly
> crossed the 2°C threshold (with 0.74 already realised and the rest in
> the pipeline), since the Stern Review says our present GHG
> concentration is about 430ppm CO2-eq, while George Monbiot gives a
> figure of 440-450 ppm CO2-eq (don't ask for his source). I don't have
> the faintest idea why my simple calculation gives me 519 ppm, which
> would mean that we'd soon be commited to about 3°C, and all that just
> for the three top gases. Anyone able to help me out on this? If
> anywhere, I would guess I've made a mistake with the nitrous oxide
> figure since it seems unlikely high, but I can't find it.
>
> Btw, what puzzles me even more is that the pre-industrial
> concentration appears to have been 280 ppm + 18.25 ppm + 80.46 ppm =
> 378.71 ppm CO2-eq. That's strange.
CO2-eq is used to mean the equivalent CO2 concentration, holding other
gases fixed at the pre-industrial level. So you have to subtract off the
pre-ind level off CH4 and N2O from your sums. I think what you have
effectively done is work out the CO2-equivalent if all other gases were
zero.
James
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