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General Circulation Model (GCM)

Introduction

A general circulation model (GCM) is a complex mathematical model that attempts to provide information about the global climate. GCMs usually include equations that describe the energy changes that occur when regions of different temperature, pressure, chemical composition, velocities, and accelerations interact with each other. Some GCMs focus on modeling the atmosphere (AGCMs) while others model the ocean (OGCMs). More advanced versions couple the atmosphere and ocean (AOGCMs), but become increasingly complex.

Historical Background and Scientific Foundations

In the 1960s, scientists began developing models of circulation patterns and heat transfer in the atmosphere. Led by the work of Syukuro Manabe (1931–) at the Program in Atmospheric and Oceanic Science at Princeton University, a group of theoretical meteorologists developed models of the atmospheric dynamics. In collaboration with Kirk Bryan (1929–), Manabe coupled the AGCM to a model of ocean dynamics. Predictions from this complex system formed the basis for some early reports of the Intergovernmental Panel on Climate Change (IPCC).

AOGCMs are four-dimensional models. Three of the dimensions are spatial: they mathematically slice the ocean and atmosphere into depth layers and further subdivide these layers by latitude and longitude. The models calculate changes that happen in each of these small regions with regard to the fourth dimension, time. Significant research has gone into understanding how changes in resolution and numerical techniques used to calculate changes in the models affect the stability and reliability of the models.

WORDS TO KNOW

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

CARBON CYCLE: All parts (reservoirs) and fluxes of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of Earth, but most of that pool is not involved with rapid exchange with the atmosphere.

HYDROLOGY: The science that deals with global water (both liquid and solid), its properties, circulation, and distribution.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

MATHEMATICAL MODEL: Used to develop descriptions of systems using numerical data and to make predictions of future states or events based upon current data and known changes in data over time.

METEOROLOGY: The science that deals with Earth's atmosphere and its phenomena and with weather and weather forecasting.

GCMs usually consider variables that affect temperature, in particular energy input and energy dissipation. Examples of dynamics that might affect these variables are velocity, pressure, radiation, albedo, convection, and water processes like evaporation of water vapor, precipitation, and hydrology. The next generation of GCMs, called Earth System Models of Intermediate Complexity (EMICs), will specifically incorporate the role of the carbon cycle in climate change into simulations.

Impacts and Issues

In 2007, the IPCC released its fourth assessment. The panel placed significant reliance on predictions made by some of the more advanced AOGMs. The major prediction from the AOGMs studied by the IPCC is that Earth will continue to warm at a rate of 0.4°F (0.2°C) per decade.

The panel reports that AOGCMs can reproduce observed measurements of the climate and that they are able to make quantitative estimates of climate change. In particular, confidence in the temperature predictions made by the models is very strong. The models have been shown to be very effective at simulating extreme weather events, like hot and cold spells and the frequency and distribution of tropical cyclones. Some of the variables that are not as well represented in AOGCMs are precipitation and the effects of cloud cover.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Web Sites

“Estimating the Circulation and Climate of the Ocean.” Massachusetts Institute of Technology, 2007. < http://www.ecco-group.org/index.htm> (accessed October 28, 2007).

“General Circulation Models.” NASA Goddard Institute for Space Studies. < http://www.giss.nasa.gov/research/modeling/gcms.html> (accessed October 28, 2007).

“Intergovernmental Panel on Climate Change.” World Meteorological Organization and the United Nations Environmental Programme. < http://www.ipcc.ch> (accessed October 28, 2007).

“The Next Generation of Climate Models.” HPC Wire, July 21, 2006. < http://www.hpcwire.com/hpc/732506.html> (accessed October 28, 2007).

Juli Berwald