The study of the uncertainties in future climate projections requires large ensembles of simulations with different values of model characteristics that define its response to external forcing. These characteristics include climate sensitivity, strength of aerosol forcing and the rate of ocean heat uptake. The latter can be easily varied over a wide range in an anomaly diffusing ocean model (ADOM). The rate of heat uptake in a three-dimensional ocean general circulation model (OGCM) is, however, defined by a large number of factors and is far more difficult to vary. The range of the rate of the oceanic heat uptake produced by existing Atmosphere-Ocean General Circulation Models (AOGCMs) is narrower than the range suggested by available observations. As a result, simpler models, like an ADOM, are useful in probabilistic climate forecast type studies as they can take into account the full uncertainty in ocean heat uptake.
        To evaluate the performance of the ADOM on different time scales we compare results of simulations with two versions of the MIT Integrated Global System Model (IGSM): one with an ADOM and the second with a full three dimensional OCGM. Our results show that in spite of its inability to depict feedbacks associated with the changes in the ocean circulation and a very simple parameterization of the ocean carbon cycle, the version of the IGSM with ADOM is able to reproduce important aspects of the climate response simulated by the version with the OCGM through the 20th and 21st century and can be used to obtain probability distributions of changes in many of the important climate variables, such as surface air temperature and sea level, through the end of 21st century. On the other hand, the ADOM is not able to reproduce results for longer term climate change and specifically those concerned with details of the feedbacks on the heat and carbon storage. Such studies will require the use of the OGCM and uncertainties in those results will be limited.