What Are Global Climate Models

The scientific assessment that the global climate will continue to become warmer due to increasing greenhouse gases is based on studies using global climate models, also called GCMs. What are global climate models, and how are they used in this assessment of greenhouse gas-induced warming? GCMs are large computer programs that produce a mathematical representation of the physical climate system, including:

a. Equations of motion of the atmosphere, i.e., winds, pressure, temperature, and air density.

b. Equations of heat and energy transfer in the atmosphere, including sunlight (solar radiation), infrared (thermal) radiation, and their effects on air, land, and water temperatures.

c. Equations for water in the atmosphere - water vapor, cloud formation (condensation), precipitation, and evaporation from the surface.

Many global climate models also include equations for the currents, temperature, and salinity of the world's oceans, as well as sea ice in the Polar Regions, hydrology, and other terrestrial processes. Since all of these components of the system interact, these models are also called coupled atmosphere-ocean-ice GCMs.

The temperatures, winds, and other variables in the equations are computed at every point on a spherical grid of the Earth, and at multiple heights (and depths) in the atmosphere and the ocean. The resolution of this grid, i.e., the distance between grid points, defines the spatial detail that the climate model can represent, just as a high-resolution photo or image can capture small-scale features that cannot be seen at lower resolution. At their present resolution of about 300 km between grid points, GCMs can capture large-scale weather patterns on Earth, as well as some of the day-to-day variations in weather. They cannot resolve smaller-scale weather features, such as hurricanes (though still large), severe storms, flash floods, or the details of narrow mountain ranges like the Andes or Cascades. Scientists have been improving the resolution of GCMs as the size and speed of supercomputers have increased. General circulation models are now able to simulate important phenomena, such as El Nifio in the Pacific Ocean and its global-scale effects.

The equilibrium temperature of the Earth's climate - the temperature at which the incoming energy absorbed from the Sun balances the outgoing energy from the Earth in the form of infrared radiation - depends on the concentrations of greenhouse gases in the atmosphere. Increasing concentrations of the greenhouse gases can absorb more of the outgoing infrared radiation from the surface and increase air temperatures. Greenhouse gases that are included in recent GCMs include water vapor (the most abundant greenhouse gas), carbon dioxide, methane, ozone, nitrogen oxides, and chloroflu-orocarbons (CFCs). The sensitivity of the Earth's climate to changes in greenhouse gases has been studied by comparing climate model simulations using different gas concentrations. These simulations show, in general, that doubling the present-day values of CO2 concentration would produce a global average warming between 3 (1.5 °C) and 9°F (5 °C). The range of warming reflects the results of climate models that feature different assessments of the atmosphere, ocean, and sea ice, and different model grid resolutions.

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