Geoscience Reference
In-Depth Information
Transient (70 years)
Long run or equilibrium
0
1
2
3
4
5
6
Temperature change from CO
2
doubling (°C)
Figure 7.
Temperature response of models in IPCC Fourth Assessment Report. The curves
show a smoothed distribution of the calculated transient temperature (darker line, at
left) and equilibrium temperature (lighter line, at right) increases from eighteen models.
all the models was a little above 3°C, which is not quite two times the
short-run or transient response.
How do these idealized experiments compare with the projected con-
centrations of CO
2
and other gases? Most integrated economic-climate
models suggest that CO
2
-e (the CO
2
equivalent of all GHGs) will be two
times preindustrial levels by around 2050. So the solid curve to the left in
Figure 7 would be a rough estimate of the temperature response around
2050 according to the best-guess emissions paths. If we look at the eco-
nomic models, they are close to the middle of the left-hand curve, show-
ing an estimated temperature increase of 1.8°C for 2050.
We might also compare this estimate with what has actually
occurred to date. The instrumental record indicates that the actual in-
crease of global temperatures is around 0.8°C over the last 100 years. So
the models suggest that temperature will warm another 1°C over the
next four decades (but with substantial disagreement among models).
Now look at the light curve to the right in Figure 7, which shows the
long-run or equilibrium temperature increase. The average long-run
