CARBON monoxide plays an important role in the oxidizing capacity of the Earth’s atmosphere, and may thereby indirectly affect the concentrations of many man-made and natural trace gases, which in turn affect climate, atmospheric chemistry and the ozone layer(1). CO is produced in the atmosphere by the oxidation of methane and other hydrocarbons, and is released into the atmosphere from automobiles, agricultural waste and the burning of savanna(1-4). Recent estimates’ show that human activities such as these are presently responsible for more than half the annual emissions of CO. During the 1980s there was evidence that atmospheric CO concentrations were increasing at similar to 1.2 +/- 0.6% per year, leading to feedbacks that could amplify global warming. Here we present a continuation of these measurements which show that from 1988 to 1992 global CO concentrations have started to decline rapidly at a rate of about -2.6 +/- 0.8% per gear. A recent study(5) has verified our findings with data from the past 3-4 years. The rate of decrease is particularly rapid in the Southern Hemisphere; we hypothesize that this mag reflect a reduction in tropical biomass burning. The total amount of carbon monoxide in the atmosphere is less now than a decade ago.