7 Models for Predicting How Greenhouse Gases Affect the Global Climate

General circulation models (GCMs) are the sophisticated computer models of the global climate system that are used to predict the effect of changes in greenhouse gas concentration on climate in the future. They incorporate fluid dynamics, thermodynamics, chemistry, and to some extent biology into systems of equations that model changes of state for a three-dimensional system of grid points over multiple time periods (a typical time increment in the model is 1 day). They expand on the simple concepts laid out using the single-layer model explained above by incorporating the following additional elements:

1. Dynamic modeling: Whereas the single-layer model is continued to be in static equilibrium with no change to the solar constant or Ts and Ta, GCMs are dynamic models where quantities of thermal or kinetic energy are retained from one time period to the next, and temperatures at specific locations in the models vary accordingly in time.

2. Multiple vertical layers: The atmosphere is modeled as having on the order of 20 layers, in order to track temperature gradients from the top to the bottom of the atmosphere. Also, for portions of the globe covered by oceans, these areas are modeled with 15–20 layers, so as to capture the effect of temperature gradients in the ocean on CO2 absorption and release, and other factors.

3. Horizontal matrix of grid points: The globe is covered in a grid of points, so that the model can capture the effect of uneven heating of the globe due to the movement of the sun, change in reflectance based on changing ice cover in different parts of the globe, and other factors. The result is a three-dimensional matrix of grid points covering the globe.

4. Convection: Heat transfer between grid points due to convection is explicitly included in the model.

5. More sophisticated temperature forcing: In the single-layer model, the forcing function is the solar input, which is assumed to be constant. GCMs are able to incorporate historic and anticipated future variations in solar input due to variations in solar activity, as well as volcanic activity and other deviations.

6. Biological processes: The models capture the effect of future climate on the amount of forest cover, which in turn affects CO2 uptake and the amount of sunlight reflected from the Earth’s surface.

7. Changes in ice cover: Similar to biological processes, the models capture the effect of changing temperature on the amount of ice cover, which in turn affects the amount of sunlight reflected or absorbed in polar or mountain regions where ice exists.

GCMs have been developed by a number of scientific programs around the world. Some of the best-known models include those of the Center for Climate System Research (CCSR), the Hadley Centre, the Max-Planck Institute, and the National Center for Atmospheric Research (NCAR). Rather than choose a single model that is thought to best represent the future path of the climate, the IPCC, as the international body convened to respond to climate change, uses the collective results from a range of GCMs to observe the predicted average values for significant factors such as average temperature, as well as the “envelope” created by the range of high and low values predicted by the models.

This post is excerpted from Energy Systems Engineering, Third Edition, coauthored by Francis M. Vanek, Ph.D. You can download Dr. Vanek’s free white paper, An Orderly Transition to Renewable Energy.

Tags: , , ,