Decline in atmospheric carbon monoxide raises questions about its cause

Carbon monoxide plays an important role in the global environment. In urban areas, high concentrations of CO pollute the air and threaten our health, while in unpopulated areas, CO is present at such low concentrations that it has no known health or direct environmental effect. Following a sharp increase in the early 1980s, probably due to a rise in man-made sources (Table I), atmospheric concentrations of CO are now declining. This trend is an indicator of important global changes probably caused by human activities and may reflect a slowdown in tropical biomass burning.