A THEORETICAL ANALYSIS OF THE PAST VARIATIONS IN GLOBAL ATMOSPHERIC COMPOSITION AND TEMPERATURE STRUCTURE

Over the last decade, advances in observation techniques coupled with an improved understanding of atmospheric chemical and physical processes have demonstrated that concentrations are changing for a number of radiatively and chemically active trace gases. Many of the observed changes appear to be the result of human activities. Changing concentrations of these gases are of concern both because of their potential direct radiative effects on atmospheric temperature and climate and because of their potential effects on tropospheric and stratospheric ozone.

This study attempts a comprehensive analysis of how recognized anthropogenic and natural influences may have affected the atmosphere since the beginning of the industrial age. Within this context, model results for past and present atmosphere are compared with available observations of relevant atmospheric parameters. This report also examines whether recognized anthropogenic influences are sufficient to explain recent measured trends in atmospheric ozone and temperature. An attempt is made to evaluate the chemical composition and temperature structure of the atmosphere before major human influence, termed the pre-industrial atmosphere. A comparison is made between the calculated pre-industrial and present atmospheres to determine whether the ozone and temperature distributions might have been substantially different before human activities influenced the global atmosphere.

Results from this study suggest that man has altered the ozone distribution and temperature structure of the global atmosphere over this period. Increasing concentrations of CO(,2), CH(,4), N(,2)O and chlorocarbons, plus emissions of nitrogen oxides from aircraft, appear to be influencing the present atmosphere. The calculated pre-industrial (1850) atmosphere was found to be measurably different from the present atmosphere. Model calculated trends in ozone and temperature for the 1960s and 1970s are generally in agreement with the measured trends, but there are too many remaining theoretical and sampling uncertainties for causality to be established. The generally excellent comparison of observations with the calculated distribution of trace species lends confidence to the capability of current theory to represent atmospheric processes. The results presented here plus calculations studying potential anthropogenic influences on the future atmosphere suggest that total ozone measurements may not provide a very sensitive indication of the impact of human activities on the global atmosphere. Monitoring of changes in the distributions of ozone and temperature with altitude should be more useful.