Without human intervention, concentrations of many atmospheric gases would be expected to change slowly. Ice core measurements of the gases trapped in ancient ice bubbles indicate this was the case before the last century.
However, since the beginning of the industrial age, emissions associated with human activities have risen rapidly. Agriculture, industry, waste disposal, deforestation, and especially fossil fuel use have been producing increasing amounts of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and other important gases. Due to increasing emissions, atmospheric levels of these greenhouse gases have been building at an unprecedented rate, raising concerns regarding the impact of these gases on climate. Some of the gases, such as CFCs, are also responsible for large observed depletions in the natural levels of another gas important to climate, ozone. Of these gases, two, carbon dioxide and methane, are of special concern to climate change. These two gases are discussed in some detail in the sections below. Under the international Montreal Protocol and its amendments, emissions of CFCs and other halocarbons are being controlled and their atmospheric concentrations will gradually decline over the next century. Emissions leading to atmospheric concentrations of sulfate and other aerosol particles are also important to climate change and are further discussed below. Unless stated otherwise, most of the discussion below is based on the most recent IPCC and WMO international assessments (IPCC, 1996a; WMO, 1998) of global change, with concentrations and trends updated as much as possible, such as data available from NOAA CMDL (National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory).
Carbon dioxide has the largest changing concentration of the greenhouse gases. It is also the gas of most concern to analyses of potential human effects on climate. Accurate measurements of atmospheric CO2 concentration began in 1958. The annually averaged concentration of CO2 in the atmosphere has risen from 316 ppm (parts per million, molar) in 1959 to 364 ppm in 1997 (Keeling and Whorf, 1998), as shown in Figure 1.2. The CO2 measurements exhibit a seasonal cycle, which is mainly caused by the seasonal uptake and release of atmospheric CO2 by terrestrial ecosystems. The average annual rate of increase over the whole time period is about 1.2 ppm or 0.4% per year, with the rate of increase over the last decade being about 1.6 ppm/yr. Measurements of CO2 concentration in air trapped in ice cores indicate that the pre-industrial concentration of CO2 was approximately 280 ppm. This data indicates that carbon dioxide concentrations fluctuated by ±10 ppm around 280 ppm for over a thousand years until the recent increase to the current 360+ ppm, an increase of over 30%.
Why has the atmospheric concentration of CO2 increased so dramatically? Analyses with models of the atmosphere-ocean-biosphere system of the carbon cycle, in coordination with observational analyses of the isotopes of carbon in CO2, indicate that human activities are primarily responsible for the increase in CO2. Two types of human activities are primarily responsible for emissions of CO2: fossil fuel use, which released about 6.0 GtC into the atmosphere in 1990, and land use, including deforestation and biomass burning, which may have contributed about an additional 1.6 ± 1.0 GtC. Evaluations of carbon releases from vegetation and soils based on changes in land use indicate that land use decreased carbon storage in vegetation and soil by about 170 Gt since 1800. The added atmospheric CO2 resulting from human activities is redistributed within the atmospheric, oceanic, and terrestrial biospheric parts of the global carbon cycle, with the dynamics of this redistribution determining the corresponding rise in atmospheric CO2 concentration. In the future, as the amount of CO2 increases in the atmosphere and in the ocean, it is expected that the oceans will take up a smaller percentage of the new emissions. Analyses of the carbon budget previously had implied that a mismatch existed between observed levels of CO2 and known loss processes. This discrepancy suggested that a missing carbon sink has existed during recent decades. This sink now appears to be largely explained through increased net carbon storage by the terrestrial biomass stimulated by the CO2 fertilization effect (increased growth in a higher CO2 concentration atmosphere) (Kheshgi et al., 1996).
Carbon dioxide is emitted when carbon-containing fossil fuels are oxidized by combustion. Carbon dioxide emissions depend on energy and carbon content, which ranges from 13.6 to 14.0 MtC/EJ for natural gas, 19.0 to 20.3 for oil, and 23.9 to 24.5 for coal. Other energy sources such as hydro, nuclear, wind, and solar have no direct carbon emissions. Biomass energy, however, is a special case. When biomass is used as a fuel, it releases carbon with a carbon-to-energy ratio similar to that of coal. However, the biomass has already absorbed an equal amount of carbon from the atmosphere prior to its emission, so that net emissions of carbon from biomass fuels are zero over its life cycle.
Human-related emissions from fossil fuel use have been estimated as far back as 1751. Before 1863, emissions did not exceed 0.1 GtC/yr. However, by 1995 they had reached 6.5 GtC/yr, giving an average emission growth rate slightly greater than 3 percent per year over the last two and a half centuries. Recent growth rates have been significantly lower, at 1.8 percent per year between 1970 and 1995. Emissions were initially dominated by coal. Since 1985, liquids have been the main source of emissions despite their lower carbon intensity. The regional pattern of emissions has also changed. Once dominated by Europe and North America, developing nations are providing an increasing share of emissions. In 1995, non-Annex I (developing countries; includes China and India) nations accounted for 48 percent of global emissions.
Future CO2 levels in the atmosphere depend not only on the assumed emission scenarios, but also on the transfer processes between the major carbon reservoirs, such as the oceans (with marine biota and sediments) and the terrestrial ecosystems (with land use changes, soil and forest destruction). Recent work for the new IPCC assessment shows, based on projections of fossil-fuel use and land use changes, that the concentration of CO2 is expected to increase well above current levels by 2100 (75 to 220% over pre-industrial concentrations). As discussed later, none of these scenarios lead to stabilization of the CO2 concentration before 2100.
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