4.5 Climate feedbacks involved in global warming

Figure 17: Comparison of climate feedbacks for a $CO_{2}$ doubling simulated by SimClimat and by climate models participating to CMIP. (a) Global warming and its contributions simulated by climate models in average. (b) Standard deviation of the different contributions to the warming simulated across the different climate models. (c) Evolution of the temperature simulated by SimClimat, with and without the different feedbacks. The red curve is a pre-industrial simulation, the blue curve is a simulation with double $CO_{2}$ (560 ppm), the purple curve is a simulation with double $CO_{2}$ and constant albedo, and the green curve is a simulation with double $CO_{2}$ and constant water vapor concentration. Panels (a) and (b) are from [Dufresne and Bony, 2008].

The increase in the global temperature in response to a doubling of the atmospheric $CO_{2}$ concentration can be decomposed into the effect of several processes:

1. the greenhouse effect directly related to $CO_{2}$;
2. water vapor feedback: the warmer the atmosphere, the moister the atmosphere. Since water vapor is a a greenhouse gas, this leads to increase the temperature;
3. ice albedo feedback: as temperature increases, ice melts more easily, so the Earth's albedo decreases, so the Earth absorbs more solar radiation and therefore the temperature increases even more.
4. Cloud feedbacks: These are very diverse and are not represented by SimClimat.

In climate models participating to CMIP, more than one-third of the simulated warming is caused by the direct effect of $CO_{2}$. A small third is caused by the water vapor feedback. The albedo feedback accounts for only 5% to 10% of the warming (Figure 17a). These proportions are reproduced by SimClimat (figure 17c). However, SimClimat does not represent cloud feedbacks, which account for nearly a quarter of global warming, but is subject to high uncertainty ( figure 17b).

Figure 18: (a) Evolution of the global-mean temperature since 1900 for observations (black), for models participating in CMIP (yellow) and for the average between all CMIP models (red), when the greenhouse gas concentrations increase in the same way as in the observations. (b) Change in global temperature since 1900 for observations (black), for models participating in CMIP (light blue) and for the average between all CMIP models (dark blue), when the greenhouse gas concentration remain constant. Figure from the 5th IPCC Report ([IPCC, 2013]).