Keeling and Bacastow^O developed a model that predicts future CO2 levels in the atmosphere, assuming that fossil fuel reserves can produce 8.2 times the preindustrial amount of CO2 • This model takes into account the amount of carbon currently present in each reservoir, exchanges between reservoirs, and a growth factor of biota up to the year 2010 based on increased CO2 levels. Rather than predict a single fossil fuel combustion pattern, they chose to look at the response to four different patterns. All of the patterns showed an initial rapid rise in CO2 levels consistent with current trends, a peak level in the period between 2100 and 2300, and then a slow decline over many centuries. The four patterns indicate peak CO2 concentrations of six to eight times preindustrial levels. Because the residence time of CO2 in the atmosphere is of the order of centuries, CO2 levels are predicted to remain at more than five times the preindustrial levels for several centuries after the peak level is reached. 2.3.4 Climatic Response to Increased CO2 Levels It seems to be well established that CO2 levels have increased since the late 1800s and will continue to increase in the future as the use of fossil fuels increases. Thus, the important issue is how the earth's climate will respond to increasing atmospheric concentrations of CO2. The major primary climatic impact of increased CO2 is a cooling of the stratosphere and an increase in surface temperature.$3 Carbon dioxide is virtually transparent to incoming solar radiation but absorbs outgoing terrestrial infrared radiation in several wave bands. This outgoing radiation would normally escape to space and result in a heat loss from the lower atmosphere. Numerous different atmospheric models have been used to simulate the response of the atmosphere to changing CO2 levels. Three basic types of models have been used. Initial efforts were made with one-dimensional, radiative-convective models that yield vertical temperature profiles based on assumptions concerning solar input, surface albedo, humidity, cloudiness, and the nature of absorption, scattering, and reflection of radiation in the atmosphere.6^ A second type of model is a two-dimensional energy balance model that is similar to the one-dimensional radiative-convective model but also allows for north-south fluxes of heat.^^ The most sophisticated type of
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