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Climate Models and Global Climate Change
(Released December 2007)

  by Christopher Readinger  


Key Citations




Climate Models in Action


As early as the 1960s the question of what rising levels of carbon dioxide might do to climate began to be raised. The best method to determine an answer was to be found in climate models with the aid of various types of proxy data, and with data collected from meteorological networks around the world.

chart showing shift in weather variation
The effect on extreme temperatures when the mean temperature increases, for a normal temperature distribution
Intergovernmental Panel on Climate Change
As the leading climate change research group in the world, the Nobel Prize winning International Panel on Climate Change (IPCC), established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), places great emphasis on GCMs to forecast the possible changes caused by increased levels of carbon dioxide. The latest publication by the IPCC, the Fourth Assessment Report (AR4), outlines the improvements made to the more than 20 computer models since the Third Assessment Report (TAR) from 2001, and discusses in detail the results of the GCMs used in the experiments.

There were approximately 23 different models that were used in AR4 and all were evaluated extensively before inclusion in the report. In the evaluations, models are run for historical periods to assess their ability to simulate known conditions as well as climatic variability and extremes. In addition they are often compared against periods from the distant past to assess how they handle climates that are very different from today. To participate, models run specific experiments designed to assess how much influence CO2 has on global averaged temperatures, sea level, droughts, extreme temperatures, etc., for specific periods in the future. The results of all the models are then evaluated using several techniques, including looking at all of the results together and determining the "ensemble mean." That is, since it is difficult to determine which model performs best, the average of all of the models is taken for the most robust result.

charts of climate change in various countries
Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906 to 2005 (black line) plotted against the centre of the decade and relative to the corresponding average for 1901-1950
Intergovernmental Panel on Climate Change

To help determine the difference that increases in compounds like CO2 and CH4 can cause, experiments are conducted using varying levels of CO2, including pre-industrial levels. The lack of warming seen in these simulations compared to the meteorological observations and projections for the next few decades shows just how much humans have influenced the climate system. To date, the results of these model experiments have been much what one would expect on a global scale with warmer averaged temperatures, rising sea levels, and thinning sea ice; however, it is the details and regional changes that interest governments and policy makers the most. Those changes are far more difficult to quantify, in part because of the model resolution and the difference in scale between modeling the global climate system and determining how much smaller feedbacks and trends influence a much smaller regional area.

I have attempted to describe the history and make-up of climate models in a very basic way; much more information can be found in scientific literature and from reliable sources on the internet. While early computer models produced suspect results and lacked many physical parameters known to play a role in Earth's climate, today's computer models have reached a point where the best models can begin to be treated with confidence if evaluated properly. Although there is still plenty of work that can be done in the atmospheric sciences and numerical modeling to improve today's models significantly, it is time for policy makers and the public to begin listening to what the experts have to say.

chart of sea level over time
Averages of the global mean sea level based on reconstructed sea level fields since 1870 (red), tide gauge measurements since 1950 (blue) and satellite altimetry since 1992 (black). Units are in mm relative to the average for 1961 to 1990
Intergovernmental Panel on Climate Change

For readers who may be interested in getting involved with climate models without going back to school, a program that may be of great interest is (for a discussion of the principles behind this, see the Discovery Guide Grid Computation: The Fastest Supercomputer in the World). This is a climate model experiment similar to the SETI program in that it uses thousands of idle online personal computers to run a climate model, as opposed to an on-site supercomputer.

© 2007, ProQuest LLC. All rights reserved.

List of Visuals


Journals and Books

Delworth, T., 2006: Peface. J. Climate, 19, 641.

Doney, S.C., K. Lindsay, I. Fung, and J. John, 2006: Natural Variability in a Stable, 1000-Yr Global Coupled Climate-Carbon Cycle Simulation. J. Climate, 19, 3033-3054.

Gent, P.R., 2006: Preface to Special Issue on Community Climate System Model (CCSM). J. Climate, 19, 2121.

Giorgetta, M.A., G.P. Brasseur, E. Roeckner, and J. Marotzke, 2006: Preface to Special Section on Climate Models at the Max Planck Institute for Meteorology. J. Climate, 19, 3769-3770.

Johns, T.C., C.F. Durman, H.T. Banks, M.J. Roberts, A.J. McLaren, J.K. Ridley, C.A. Senior, K.D. Williams, A. Jones, G.J. Rickard, S. Cusack, W.J. Ingram, M. Crucifix, D.M.H. Sexton, M.M. Joshi, B.W. Dong, H. Spencer, R.S.R. Hill, J.M. Gregory, A.B. Keen, A.K. Pardaens, J.A. Lowe, A. Bodas-Salcedo, S. Stark, and Y. Searl, 2006: The New Hadley Centre Climate Model (HadGEM1): Evaluation of Coupled Simulations. J. Climate, 19, 1327-1353.

Lin, J.L., G.N. Kiladis, B.E. Mapes, K.M. Weickmann, K.R. Sperber, W. Lin, M.C. Wheeler, S.D. Schubert, A. Del Genio, L.J. Donner, S. Emori, J.F. Gueremy, F. Hourdin, P.J. Rasch, E. Roeckner, and J.F. Scinocca, 2006: Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals. J. Climate, 19, 2665-2690.

Schmidt, G.A., R. Ruedy, J.E. Hansen, I. Aleinov, N. Bell, M. Bauer, S. Bauer, B. Cairns, V. Canuto, Y. Cheng, A. Del Genio, G. Faluvegi, A.D. Friend, T.M. Hall, Y. Hu, M. Kelley, N.Y. Kiang, D. Koch, A.A. Lacis, J. Lerner, K.K. Lo, R.L. Miller, L. Nazarenko, V. Oinas, J. Perlwitz, J. Perlwitz, D. Rind, A. Romanou, G.L. Russell, M. Sato, D.T. Shindell, P.H. Stone, S. Sun, N. Tausnev, D. Thresher, and M.S. Yao, 2006: Present-Day Atmospheric Simulations Using GISS ModelE: Comparison to In Situ, Satellite, and Reanalysis Data. J. Climate, 19, 153-192.

Stott, P.A., G.S. Jones, J.A. Lowe, P. Thorne, C. Durman, T.C. Johns, and J.C. Thelen, 2006: Transient Climate Simulations with the HadGEM1 Climate Model: Causes of Past Warming and Future Climate Change. J. Climate, 19, 2763-2782.

Vecchi, G.A., and B.J. Soden, 2007: Global Warming and the Weakening of the Tropical Circulation. J. Climate, 20, 4316-4340.


Community Climate System Model -

Geophysical Fluid Dynamics Laboratory - Run a Climate model on your home computer -


IPCC Fourth Assessment Report -

CCSP Synthesis and Assessment Report 3.1 -

American Institute of Physics -

All Websites accessed on or about November of 2007