We present a 650-year Antarctic ice core record of concentration and isotopic ratios (δ13C and δ18O) of atmospheric carbon monoxide. Concentrations decreased by ∼25% (14 parts per billion by volume) from the mid-1300s to the 1600s then recovered completely by the late 1800s. δ13C and δ18O decreased by about 2 and 4 per mil (‰), respectively, from the mid-1300s to the 1600s then increased by about 2.5 and 4‰ by the late 1800s. These observations and isotope mass balance model results imply that large variations in the degree of biomass burning in the Southern Hemisphere occurred during the last 650 years, with a decrease by about 50% in the 1600s, an increase of about 100% by the late 1800s, and another decrease by about 70% from the late 1800s to present day.