Simulated climate change in the 20th and 21st centuries

Two global coupled models, the DOE Parallel Climate Model (PCM) and the NCAR Climate System Model (CSM) are run in a series of climate experiments with identical forcings to produce 20th and 21st century climate simulations. Both models use the same atmospheric model with different sea ice and ocean components. Comparison of these experiments provides insight into the role of ocean and sea ice in climate variability and climate change. The CSM and PCM exhibit similar globally averaged responses to changes in forcing in terms of surface air temperature, suggesting that the major feedbacks in the climate system are managed by the atmosphere. Regional and subsurface ocean responses differ between the two, consistent mainly with the different ocean model characteristics. There are greater differences in the model realizations at high latitudes associated with interannual and decadal variability that is largest at those latitudes. An El Nino-like response (SSTs warm proportionately more in the central and eastern equatorial Pacific than in the western Pacific) appears in the CSM 20th century experiment, but not in the PCM partly due to different cloud liquid water formulations. This regional response is modulated by changes in monsoon strength in the 21st century experiments in the CSM. The monsoon effects are related to changes in the land-sea heating distribution in the south Asian region associated with the time-varying forcing. Analysis of several different forcing scenario experiments for 21st century climate shows that the details of such scenarios do not make much difference for large-scale patterns of surface temperature response in most regions in the models.