Changes in the Radiative Effects of Clouds

Clouds exert two radiative effects on the Earth. They reflect the solar insolation and reduce the warming of the Earth (albedo effect). In addition, they create a greenhouse effect by utilizing the thermal radiation emitted from the Earth, which reduces the radiative cooling of the Earth. Lower clouds have a smaller greenhouse effect, because the emission from the clouds is closer to the emission from the surface temperature. Higher clouds exert a larger greenhouse effect.

Figure 20.6 demonstrates the changes in the radiative effect of clouds on the Earth (albedo effect and greenhouse effect) in NICAM and MIROC. A positive or negative sign indicates that the change warms or cools the Earth,

Figure 20.6 Difference in cloud radiative effect (cloud radiative forcing) (W m 2) between the control experiment and the +2K experiment in NICAM (solid line) and in MIROC (solid line with filled circle). Red line indicates greenhouse effect (longwave forcing) and blue line indicates albedo effect (shortwave forcing).

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Figure 20.6 Difference in cloud radiative effect (cloud radiative forcing) (W m 2) between the control experiment and the +2K experiment in NICAM (solid line) and in MIROC (solid line with filled circle). Red line indicates greenhouse effect (longwave forcing) and blue line indicates albedo effect (shortwave forcing).

respectively. Although net change is positive in both models, the magnitudes of the changes in both radiative effects in NICAM are much larger than in MIROC. The signs of the changes in the two cloud radiative effects vary between the two models. In MIROC, both the albedo effect and greenhouse effect weaken, with the weakening larger in the albedo effect. This corresponds to the result that the decrease in low cloud is dominant in MIROC. In NICAM, both the albedo effect and greenhouse effect intensify, and the intensification of greenhouse effect is larger than that of albedo effect. This corresponds to the significant increase in high clouds in NICAM.

Net radiative effect of high clouds depends on optical thicknesses. If the optical thickness is thin enough, the greenhouse effect exceeds the albedo effect (Inoue and Ackerman 2002), and an increase in these high thin clouds intensifies the warming.

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