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OCR for page 4
Carbon Dioxide and Climate: A Scientific Assessment
2
Carbon in the Atmosphere
A brief account of the key features of the exchange of carbon between the atmosphere, the living and dead organic matter on land (the terrestrial biosphere), and the oceans is essential as a basis for the discussion that follows. The intermediate layers (100–1000 m) of the oceans also play a central role both as a sink for excess atmospheric CO2 and for heat. For these reasons some basic features of the carbon cycle will be outlined, based primarily on the recently published review by the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (Bolin et al., 1979).
The CO2 concentration in the atmosphere has risen from about 314 ppm (parts per million, volume) in 1958 to about 334 ppm in 1979, i.e., an increase of 20 ppm, which is equivalent to 42×109 tons of carbon. During this same period, about 78×109 tons of carbon have been emitted to the atmosphere by fossil-fuel combustion. It has further been estimated that more than 150×109 tons of carbon have been released to the atmosphere since the middle of the nineteenth century, at which time the CO2 concentration in the atmosphere most likely was less than 300 ppm, probably about 290 ppm.
By reducing the extent of the world forests (at present about 30 percent of the land surface) and increasing the area of farmland (at present about 10 percent of the land surface) man has also transformed carbon in trees and in organic matter in the soil into CO2. The magnitude of this additional emission into the atmosphere is poorly known. Estimates range between 40×109 tons and more than 200×109 tons for the period since early last century.
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Carbon Dioxide and Climate: A Scientific Assessment
Since these emissions are not known with any degree of accuracy during the period for which accurate observations of atmospheric CO2 are available (1958–1979), we know only approximately the ratio between the net increase of CO2 in the atmosphere and the total man-induced emissions. However, at least 50 percent of the emissions and perhaps more than 70 percent have been transferred into other natural reservoirs for carbon. We need to consider three possible sinks for this transfer:
The remaining forests of the world (because of more effective carbon assimilation as a result of higher CO2 levels in the atmosphere);
The surface and intermediate waters of the oceans (above about 1000 m);
The deep sea (below about 1000 m).
The distribution of past emissions of CO2 between these sinks is not entirely clear. On the basis of the radiocarbon concentration in the deep sea, it has been concluded that only a rather small part of the emissions so far have been transferred into the deep sea. However, the proper role of the deep sea as a potential sink for fossil-fuel CO2 has not been accurately assessed. As indicated in Section 3.3 on the oceans, theoretical estimates of mass transfer from the mixed layer into the intermediate waters indicate that this part of the ocean may have been a more important sink for carbon dioxide emitted into the atmosphere than has so far been considered. This conclusion is also in accord with observations of the penetration of radioactive trace substances produced by nuclear-weapons testing into the intermediate waters. Whether some increase of carbon in the remaining world forests has occurred is not known.
Our limited knowledge of the basic features of the carbon cycle means that projections of future increases of CO2 in the atmosphere as a result of fossil-fuel emissions are uncertain. It has been customary to assume to begin with that about 50 percent of the emissions will stay in the atmosphere. The possibility that the intermediate waters of the oceans, and maybe also the deep sea, are in more rapid contact with the atmosphere may reduce this figure to 40 percent, perhaps even to a somewhat smaller figure. On the other hand, a continuing reduction of the world forests will further add to any increase due to fossil-fuel combustion. The ability of the oceans to serve as a sink for CO2 emissions to the atmosphere is reduced as the concentrations increase because of the chemical characteristics of the carbonate system of the sea.
If all the fossil-fuel reserves were used for combustion, the airborne fraction would increase considerably above the values of 30 to 50 percent mentioned above. Global fossil-fuel resources contain at least 5000×109 tons of carbon, of which oil and gas together represent about 10 percent. The
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Carbon Dioxide and Climate: A Scientific Assessment
maximum conceivable amount of future releases from the land biosphere due to deforestation and other changes in land use is of the order of 500×109 tons. An emission of 5000×109 tons of carbon as CO2 (i.e., about eight times the pre-industrial amount of CO2 in the atmosphere) during the next few centuries probably would lead to four to six times higher CO2 concentration than at present, i.e., 1300–2000 ppm. In view of the huge amounts involved, it seems unlikely that increases in carbon stored in the terrestrial biosphere could reduce these values substantially.
Decline of CO2 levels in the atmosphere will take centuries because of the slow turnover of the deep sea. However, as the more CO2-rich waters reach the calcium carbonate deposits on the continental slopes, dissolution may increase the capacity of the oceans to absorb CO2. Since this process fundamentally depends on the rate with which ocean water can get in contact with the bottom sediments, it is not likely to proceed quickly, although our knowledge is inadequate to assess the role of this process more than qualitatively at present.
Considering the uncertainties, it would appear that a doubling of atmospheric carbon dioxide will occur by about 2030 if the use of fossil fuels continues to grow at a rate of about 4 percent per year, as was the case until a few years ago. If the growth rate were 2 percent, the time for doubling would be delayed by 15 to 20 years, while a constant use of fossil fuels at today’s levels shifts the time for doubling well into the twenty-second century.
There are considerable uncertainties about the future changes of atmospheric CO2 concentrations due to burning of fossil fuels. It appears, in particular, that the role of intermediate waters as a sink for CO2 needs careful consideration. Predictions of CO2 changes on time scales of 50 to 100 years may be significantly influenced by the results of such studies. However, considerable changes of atmospheric CO2 levels will certainly occur as a result of continuing use of fossil fuels. This conclusion is a sufficient basis for the following discussion of possible climatic changes.