The Goddard Institute for Space Studies General Circulation Model II, described fully in Hansen et al. (1983), is a three-dimensional global climate model that solves numerically the physical conservation equations for energy, mass, momentum and moisture as well as the equation of state. The standard version of this model has a horizontal resolution of 8˚ latitude by 10˚ longitude, nine layers in the atmosphere extending to 10mb, and two ground hydrology layers. The model accounts for both the seasonal and diurnal solar cycles in its temperature calculations. Cloud particles, aerosols, and radiatively important trace gases (carbon dioxide, methane, nitrous oxides, and chlorofluorcarbons) are explicitly incorporated into the radiation scheme. Large-scale and convective cloud cover are predicted, and precipitation is generated whenever supersaturated conditions occur. Snow depth is based on a balance between snowfall, melting and sublimation. The albedo of snow is a function of both depth and age. Fresh snow has an albedo of 0.85 and ages within 50 days to a lower limit of 0.5. The sea ice parameterization is thermodynamic with no relation to wind stress or ocean currents. Below –1.6°C ice of 0.5 m thickness forms over a fractional area of the grid box and henceforth grows horizontally as needed to maintain energy balance. Surface fluxes change the ocean water and sea ice temperature in proportion to the area of a grid cell they cover. Conductive cooling occurs at the ocean/ice interface, thickening the ice if the water temperature remains at –1.6°. Sea ice melts when the ocean warms to 0°C and the SST in a grid box remains at 0°C until all ice has melted in that cell. The albedo of sea ice (snow-free) is independent of thickness and is assigned a value of 0.55 in the visible and 0.3 in the near infrared, for a spectrally weighted value of 0.45.

Vegetation in the model plays a role in determining several surface and ground hydrology characteristics. Probably the most important of these is the surface albedo, which is divided into visible and near infrared components and is seasonally adjusted based on vegetation types. Furthermore, the assigned vegetation type determines the depth to which snow reflectivity can be masked. Hydrological characteristics of the soil are also based upon the prescribed vegetation types; the water holding capacity of the model's two ground layers is determined by the vegetation type as is the ability of those layers to transfer water back to the atmosphere via transpiration. Nine different vegetation classes, developed by Matthews (1984) for the GISS GCM to represent major vegetation categories and the ecological/hydrological parameters which are calculated from the vegetation. Since the GISS GCM is a fractional grid model, more than one vegetation type can be assigned to each grid box.

Sea surface temperatures (SST) are either specified from climatological input files or may be calculated using model-derived surface energy fluxes and specified ocean heat transports. The ocean heat transports vary both seasonally and regionally, but are otherwise fixed, and do not adjust to forcing changes. This mixed-layer ocean model was developed for use with the GISS GCM and is often referred to as the “Qflux” parameterization. Full details of the Qflux scheme are described in Russell et al. (1985), and in appendix A of Hansen et al. (1997). In brief, the convergence (divergence) at each grid cell is calculated based on the heat storage capacity of the surface oce