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.
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