Changes In Atmospheric Carbon Dioxide Concentration In Historical Times

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Remarkably, carbon dioxide was the first constituent of the earth's atmosphere to be identified. Joseph Black published the discovery during 1754 in a University of Edinburgh thesis. By 1870, quantitative measurements developed which compare in precision with today's measurements. Interpretation of early observations is made difficult not by inadequacies in analytical chemistry but because they were not undertaken to measure changes in the composition of the atmosphere. Sampling for this purpose is complicated owing to the extreme danger of contamination from, for example, the observer's breath. While numerous observations of the C02 content of the atmosphere are reported in the literature during the period 1870-1910, their interpretation remains ambiguous. After about 1910, few reports are available since the composition of the atmosphere was considered known and fixed and analytical chemists pursued other interests.

The situation changed dramatically in 1958 when C. D. Keeling of the Scripps Institution of Oceanography, at the suggestion of Roger Revelle, began a series of painstaking measurements of C02 concentration on a remote site at what is now the Mauna Loa Observatory in Hawaii. His observations, continued to the present, show an exponential growth of atmospheric carbon dioxide on which a periodic seasonal fluctuation is superimposed (Keeling et al., 1984). (See Figure 5.1 in Chapter 5.) The seasonal variation follows the biospheric uptake of C02 during the growing season and its discharge during winter. Keeling also established and maintained an observing regimen at the South Pole. The Antarctic observations show a similar exponential growth but a more diminished seasonal variation. Since the atmosphere at the South Pole is well removed from biological activity, the smaller seasonal fluctuations are expected. Keeling's observations have been duplicated at other stations in various parts of the world over shorter periods of time. In all sets of observations, the exponential increase is clear, though the amplitude of the seasonal terms varies with latitude. For example, a 20-year-long record taken from observations on a weather ship in the northeast Pacific (50°N, 145°W) shows greater seasonal fluctuations than those seen at Mauna Loa; the amplitude of these fluctuations reflects the strong seasonal dependence of the mid-latitude Northern Hemisphere biological activity.

The exponential growth in atmospheric C02 concentration correlates with the release of C02 by the burning of oil, coal, gas, and wood. Since 1945, the injection of carbon into the atmosphere by fuel combustion appears to be exponential, with a slowing in growth following the 1973 explosion in energy prices (see Figure 9.2). Exponential fits to the Mauna Loa and South Pole bracket the increase in carbon-based fuel use with the following exponential doubling times:

While the growth in atmospheric C02 parallels the release of carbon through energy use, the overall carbon budget is not well understood. The observed increase in atmospheric C02 accounts for only about half the carbon released through energy and industrial uses. Oceans are generally assumed to be a major sink for C02. While numerous models have been constructed to mimic ocean-wide diffusive and convective

Mauna Loa Fuels

South Pole

26.6 years 23.9 years 21.9 years

Figure 9.2 Anthropogenic C02 Production

Figure 9.2 Anthropogenic C02 Production

Historical Pics Global Warming

Year

The rate of production of carbon dioxide by man's activities (after Keeling, 1984). Units are 10'2 g yr~'. The major deviations from exponential growth between 1914 and 1945 reflect the effects of the two world wars and the worldwide economic chaos between the wars.

Year

The rate of production of carbon dioxide by man's activities (after Keeling, 1984). Units are 10'2 g yr~'. The major deviations from exponential growth between 1914 and 1945 reflect the effects of the two world wars and the worldwide economic chaos between the wars.

exchange processes, the ocean's capacity to remove C02 from the atmosphere over times on the order of decades remains largely uncertain. The carbon balance mystery is deepened by inadequate data on the net contribution of biospheric processes to the atmospheric carbon burden. Destruction of forest cover and cultivation of new land provides a source of atmospheric C02, but stronger growth in the many plants stimulated by increased C02 concentration should lead to greater biological productivity, and thus lead the biosphere to serve as a sink for C02. Both the sources and sinks associated with the biosphere are large numbers so that the net impact, being the difference of these large numbers, is most uncertain.

Estimates of future atmospheric carbon dioxide levels are clouded by questions regarding the overall carbon budget. A rough guess is provided by the observation that historically about half the fossil fuel carbon placed in the atmosphere remains there. More refined estimates will require a better definition of the role of the oceans and biosphere in the carbon cycle. Since oxygen is also removed from the atmosphere during the formation of C02, long-term measurements of the oxygen content of the atmosphere may be required to sort out the various sinks and sources for carbon.

The concentration of C02 in the atmosphere was about 315 ppm by volume when Keeling began his observations in 1958. During the period 1958-1986, C02 concentration increased by about 30 ppm, or 9.5% in 28 years. Central to understanding the total impact of intensive agriculture and energy use on the present C02 concentration is to establish the concentration 100 years ago, when these activities were just beginning. There are a number of ways to obtain a preindustrial value. The first is to assume that conditions prevailing over the last 30 years held during the preceding 70. If one-half the carbon released in combustion remains in the atmosphere and all other sources and sinks are relatively constant, then the concentration of C02 in the atmosphere during 1890 was about 293 ppm. Two ways of checking this value are available—analysis of air trapped in glaciers and a careful reanalysis of the older observations.

When snow is transformed into ice during glacier growth, air is trapped in the intergrain spaces. The air remains in the airtight glacial ice as the glacier continues to grow. Chemical analysis of this air provides an estimate of the C02 content of the atmosphere at the time the air was encased. A number of difficulties lessen the value of such determinations. The time at which a particular layer of ice was laid down is difficult to estimate. The rate of snow accumulation affects the quantity of air that is trapped. The seasonal melting of snow may incorporate new air, leading to confusion. Despite these uncertainties, the values obtained from analysis of the most favorable samples indicate an atmospheric C02 concentration in 1890 of between 280 and 295 ppm (Raynaud and Barrola, 1985). These values are in general agreement with the value obtained by extrapolating current C02 concentration backward, assuming constant relative sources and sinks of C02.

From and Keeling (1986) have carried out a detailed reexamination of early observations of C02 concentrations, particularly those made by Callendar, the discoverer of the significance of anthropogenic C02. Among the criteria From and Keeling used was whether early observations showed a seasonal cycle in accord with modern observations. The result of their analysis is that the mean annual concentration of atmospheric C02 circa 1880, in uncontaminated air at 50°N latitude, was 292 ppm. This value is consistent with backward extrapolation of current values as well as estimates derived from glacial air samples.

Historical observations coupled with extremely careful modern measurement show that the C02 concentration of the atmosphere has increased from about 290 ppm to 345 ppm in 100 years, or 19%. Future concentrations of C02 are difficult to estimate since they depend on unknown energy use rates, mixtures of fuels, patterns of deforestation and agriculture, and uptake of C02 by the oceans and atmosphere. Uncertainties in future C02 levels due to uncertainties in the carbon cycle may be small if past behavior provides a reliable clue; if this is the case, then about one-half of the C02 produced by fossil fuel combustion ends up in the atmosphere. There remains the difficult task of projecting energy use decades into the future if we are to determine the atmosphere of tomorrow and its associated climate.

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