THE CHANGING ATMOSPHERE.

The Earth's atmosphere has never been free of change, but change is now occurring at a pace several orders of magnitude more rapid than has ever occurred in the past, cometary impacts and major volcanic eruptions excepted. The impacts that will result from this high rate of change may well turn out to be dramatic, although they cannot be predicted with accuracy or certainty. In this paper, written on invitation as part of a single-topic issue of Scientific American entitled "Managing Planet Earth", evidence for atmospheric change is presented and discussed. Although in a few instances there have been recent decreasing concentration trends on local and regional scales, the global outlook is for continued increases in the concentrations and impacts of many atmospheric trace species, with the possibility of major, but still relatively unpredictable, effects on climate and on the stability of the atmospheric photochemical system.