Carbon Dioxide Emissions
The primary greenhouse gases in the earth’s atmosphere are water vapor (H2O), carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFC-11, 12, 113, CCl4), nitrous oxide (N2O), ozone (O3), and aerosols. After water vapor, which is not directly affected by human activities, carbon dioxide is the greenhouse as most prevalent in the atmosphere. Because carbon dioxide circulates throughout the biosphere in such large volumes, it plays a primary role in the thermal regulation of the earths atmosphere. Methane, though it is the second most prevalent gas by volume, is four times as powerful as a heat trapping gas, and has more than doubled its pre-industrial concentration, has a shorter residence time in the atmosphere and is generated in much smaller quantities than carbon dioxide. Scientific interest in climate change has therefore focused primarily on CO2: its behavior in the atmosphere, its past and present concentrations, and its relation to human industrial activity.
Records of relative atmospheric CO2 concentrations constitute one of the most basic building blocks of climate change science. Evidence for the increase of man-made carbon dioxide in the atmosphere is well established. Because carbon derived from the combustion of fossil fuels and organic matter (associated with deforestation) contains fewer carbon isotopes than would be found in carbon normally circulating through the carbon cycle, it is possible to determine the ratio of anthropogenic (human-made) to naturally produced carbon. Measurements to this effect, drawn from atmospheric samplings at the research station at Mauna Loa, Hawaii, where measurements have been taken continuously since 1957, and the U.S. research station at Point Barrow, Antarctica establish a trend of rising carbon dioxide emissionsdue to human activity during the second half of the 20th century.
When brought into relation with the next most substantial body of instrumental data—gas concentrations frozen in air bubbles taken from the Greenland and Antarctic ice sheets—researchers have been able to make long-term, historical comparisons of carbon dioxide levels. This paleoclimatological evidence, corroborated by ice cores drilled at a number of different sites around the world, establishes that the present concentrations of carbon dioxide in the atmosphere are the highest in nearly half a million years (much longer than any individual cycle of glaciation and deglaciation), up 31 percent since the beginning of the industrial revolution circa 1750.
Further, “The rate of increase over the past century is unprecedented, at least during the past 20,000 years.” Similar evidence has been obtained for the other greenhouse gases (though some, such as chlorofluorocarbons (CFCs), have actually begun to diminish at global levels). Thus, based on historical evidence and given the known properties of carbon dioxide as a heat trapping gas, steadily rising levels of carbon dioxide should lead to a detectable rise in average global temperature over a long enough time span.