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OCR for page 21
Carbon Dioxide and Climate: A Scientific Assessment Appendix: Comparison of Snow-Ice Effects in the Models Examined Major differences between and within the two series of model predictions appear in high latitudes. For example, in M2, which does not have a seasonal change, the maximum surface ΔT is about 8°C at approximately 83° N, whereas in H2, which likewise does not have a seasonal change, the maximum surface ΔT is about 10°C at approximately 60° N and also at 70° S. In such predictions, the latitude of maximum ΔT should be near the latitude where the maximum decrease of albedo occurs, and this seems to be the case for both M2 and H2. In these cases, the latitude of maximum ΔT should be poleward of the mean snow-ice boundary of the control run with the present CO2 concentration, and equatorward of the mean snow-ice boundary in the prediction with the increased CO2 concentration. Judging from the albedo changes, we infer that the mean snow-ice boundary is too far equatorward in H2 and too far poleward in M2. The reason for these discrepancies is not clear because so many factors, such as horizontal resolution, land-sea distribution, and snow and ice albedos are different in the two model predictions. Both H1 and M3 show large seasonal fluctuations in ΔT.* This is to be expected because the snow-ice albedo feedback differs considerably from one season to another. The feedback will not be relevant in the polar region of the winter hemisphere, where there is no solar radiation, and over the regions of melting snow and ice in the summer hemisphere, where the surface * A large seasonal fluctuation is also predicted by Wetherald in calculations with a sector model that is similar to M2 with quadrupled CO2 but includes both hemispheres and neglects interactive clouds. In this model, the global mean surface ΔT is about 4°C. On the assumption that ΔT is linear in the CO2 concentration, we obtain 2°C for this model for doubled CO2.
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Carbon Dioxide and Climate: A Scientific Assessment temperature must be near freezing. The maximum changes due to the feedback are to be expected in subpolar latitudes in winter and in polar or subpolar latitudes in spring when both snow and sea-ice changes are important. In H1, the snow-ice albedo feedback mechanism is significant even in winter because the maximum ΔT in that prediction is in subpolar regions between 45° N and 70° N. In M3, on the other hand, the snow-ice albedo feedback seems to be most significant in spring when a maximum in ΔT occurs around 65° N. In M3 there is another, even stronger, maximum in winter near the north pole. This cannot be interpreted as the result of a snow-ice albedo feedback because there is no solar radiation. It has been suggested that it is a result of a sea-ice thickness feedback: When the sea ice in the model becomes sufficiently thin, the surface air becomes strongly coupled by conduction to the ocean immediately below the sea ice, which must be near freezing. This gives a warming effect and therefore a positive feedback. The warming is further enhanced by the circumstance that the polar ice in this model (for quadrupled CO2) is completely melted so that the polar seas beneath the ice in winter will be warmer. In H1, the sea-ice thickness feedback cannot be clearly seen in winter. Instead, H1 shows a maximum ΔT near the north pole in spring when the sea ice is thin enough and the leads wide enough to permit effective atmospheric communication with the ocean. In the annual average, H1 shows a large ΔT poleward of about 45° N, with a flat maximum of about 7°C near 60° N.