A Reassessment of Atmospheric Pollution as a Cause of Long-Term Changes of Global Temperature

Abstract

Two globally extensive forms of atmospheric pollution (carbon dioxide and particulate loading) are each considered from the viewpoint of long-term changes in their total abundance, and the impact of such changes on the equilibrium temperature of the Earth.

A comparison of the observed levels of atmospheric CO₂ since 1958 with estimates of the fossil CO₂ input to the atmosphere from human activities indicates that between 50 and 75% of the latter has remained in the atmosphere. The present-day CO₂ excess (referred to 1850) is estimated at 11%; the excess is conservatively projected to increase to 15% by 1980, 22% by 1990, and 32% by 2000 A.D. Changes of mean atmospheric temperature due to C02, calculated by Manabe et al. [20] as about 0.3°C per 10% change of CO₂, are sufficient to account for only about one-third of the observed warming of the Earth between 1880 and 1940, but would appear capable of contributing a further warming of about 0.6°C between the present time and the end of the century.

The total global atmospheric loading by small particles is estimated at about 4 × 10⁷ tons at present, of which about 1 × 10⁷ tons is derived directly or indirectly from human activities. If the anthropogenic fraction should grow in the future at about 4% yr^-1, the total loading would increase to a level at the end of this century about double that of the 19th century, and 60% above present-day levels. At present, the total anthropogenic loading is estimated to exceed the average stratospheric loading by volcanic dust during the past 120 yr, and to equal about one-fifth of the stratospheric loading following the 1883 eruption of Krakatoa. The impact of anthropogenic particle loading changes on mean temperature cannot be reliably determined from present information although a cooling effect is likely.

Of the two forms of pollution, it appears that the carbon dioxide increase is more influential in raising planetary temperatures than the anthropogenic particle increase is in lowering planetary temperatures. (If, however, both the CO₂ and particle inputs to the atmosphere should grow at equal rates in the future, the relative importance of particle effects will increase and could eventually become dominant.) In balance, the net thermal impact of all global-scale pollution (including thermal pollution) is likely to be one of warming, perhaps increasingly so after 2000 A.D. It is concluded that the cooling of climate since 1940, apparently still in progress, is a natural phenomenon plausibly related to an enhanced stratospheric loading by volcanic dust in the period. Natural variations of climate have been larger than those probably induced by human activities during the past century, but the rapidity with which human impacts on atmospheric quality threaten to grow in the future has disturbing climatic overtones that demand better understanding.