The middle curve is the transient calculation with increasing greenhouse gas and aerosol forcing, the first 100 years of which was shown in Figure 1. The uppermost curve is the result of an experiment in which the green house gas forcing evolves as in the previous case, but the aerosol amount is fixed at its 1900 level. Modelled global, annual average surface air temperature from 1900 to 2100. Lower curve is from a control simulation with constant greenhouse gas and aerosol forcing. Middle curve is from a simulation with increasing greenhouse gas and aerosol forcing.
Upper curve is from a simulation with greenhouse gas forcing only.
See text for further details. One can contrast these results with the equilibrium calculation of Boer et al. They obtained a global average warming of 3.5 o C upon doubling CO 2 concentration. The difference between the transient warming at the time of CO 2 doubling, 2.7 o C, and the equilibrium value of 3.5 o C illustrates the ocean's role in sequestering heat. A notable difference is the hemispheric asymmetry in warming whereby the warming is larger in the northern hemisphere than in the southern hemisphere, in contrast to the equilibrium calculation where the warming is roughly equal in the northern and southern polar regions.