Measurements have indicated a steady increase of CO2 content since the late 1800s, most of which is due to increased use of fossil fuels. If trends in fossil fuel use persist, the atmospheric CO2 content will be double the preindustrial levels of about 300 ppm by the mid 21st century. Recent atmospheric models predict a global temperature increase of 2-3°C for a doubling of atmospheric CO2, and increases near the poles of the earth may be considerably larger. Of the technologies considered, only coal combustion will result in substantial emissions of CO2. Space vehicle launches for SPS construction will emit some CO2, but these emissions should be two orders of magnitude smaller than the coal emissions per unit of energy produced. Coal combustion can contribute substantially to global CO2 levels. Table 3 compares the current and predicted CO2 emissions from U.S. coal combustion alone to current and projected global CO2 emissions. This comparison indicates that coal- fired energy generation may have a major impact on climate. Other Contributions to Climate Change In addition to the waste heat, particle, and CO2 influences, other parameters can affect climate. Natural climatic fluctuations may either augment or mask an anthropogenic effect. The current cooling trend of the earth is probably due to natural fluctuations and may obscure CO2 wanning Table 3. Projected Annual Emissions of CO2 from U.S. Utility Coal Combustion Compared with Projected Annual World Emissions of CO2
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