4. Conclusion

[49]  The INM coupled atmosphere-ocean general circulation model was used to conduct experiments on the simulation of climate changes under doubled and quadruple CO2 concentrations; the climate of the 20th century under prescribed variations in external forcing; and climate changes in the 21st-22nd centuries under scenarios B1, A1B, and A2, and the control experiment 330 years in duration for the conditions of the year 1871. In the INM model, the non-equilibrium sensitivity to doubled CO2 is somewhat lower than its mean value over all models and is equal to 1.57 K. However, the equilibrium sensitivity of the atmosphere model with a homogeneous 50-m ocean layer is the lowest among the models under consideration and is equal to 2.1 K. The model adequately reproduces observed variations in surface air temperature for 1871-2000. In the model, the warming over the 20th century is 0.7 K, which is close to observational data.

[50]  The features of the observed time variation in air temperature such as warming in 1940-1950 and its slowing down in 1960-1970 are reproduced by the model. According to model data, the increase in the mean air temperature at the surface at the end of the 22nd century as compared to the end of the 20th century is about 2, 3, and 5 K for scenarios B1, A1B, and A2, respectively. This is slightly lower than the mean value for all models used in such experiments [Solomon et al., 2007]. The maximum warming occurs in the Arctic. For all scenarios, in the model, the sea-ice area in the Arctic is significantly reduced at the end of summer and, for scenarios A1B and A2, in the 22nd century, the Arctic ice almost completely melts by the end of summer. In the model, the sea-level rise in 2100 as compared to 1990 is 11 cm, 16 cm, and 18 cm for scenarios B1, A1B, and A2, respectively. These are among the lowest values for the models whose results are given in [Solomon et al., 2007].

[51]  The response of the MHT to global warming is formed mainly owing to the changes occurring in the pattern of meridional circulation and heat store in the tropics of the Pacific. The response in the Atlantic suggests a strengthening of the northward heat flux. Conversely, the southward heat transfer is strengthened in the Pacific and Indian oceans. A significant peak is observed in the response of the meridional circulation in the Southern Ocean. This peak corresponds to an increased heat transfer to Antarctica and is caused mainly by the response in the sector of the Pacific and Indian oceans.

[52]  In conclusion, we note that the correct reproduction of the time variation in air temperature for the 20th century does not guarantee that the model predictions for the 21st-22nd centuries, even under the assumption that one of the aforementioned scenarios of variations in external forcing is exactly fulfilled, will also be correct. This result is due to the fact that there are great uncertainties in estimating variations in radiation forcing for the 20th century, which are associated primarily with an uncertainty in a sulfate-aerosol forcing, whose estimates vary from 0 to 2 W m-2 [Solomon et al., 2007]. Therefore, an incorrect sensitivity of the model to variations in the concentrations of greenhouse gases can be compensated by incorrect variations in the radiation forcing from sulfate aerosol and, as a result, the reproduction of warming in the 20th century may prove to be correct. According to the scenarios under consideration, in the 21st century, the concentration of greenhouse gases will be increased, while the concentration of sulfate aerosol will be increased only at the beginning of the 21st century and then will be decreased (Figure 1). Therefore, it is not improbable that the model correctly reproducing the warming in the 20th century can underestimate or overestimate the warming in the 21st-22nd centuries.


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