We present new estimates of global nitrous oxide (N2O) emissions for the period 1500--1994 based on revised Intergovernmental Panel on Climate Change guidelines [Intergovernmental Panel on Climate Change (IPCC), 1997; Mosier et al., 1998]. Use of these estimates as input to a simple atmospheric box model resulted in a closed N2O budget over time, showing that increases in atmospheric N2O can be primarily attributed to changes in food production systems. We hypothesize that before the ninetheenth century conversion of natural land to agriculture had no net effect on N2O. During the twentieth century a fast expansion of agricultural land coupled with intensification of land use may have caused a net increase in N2O. In our base scenario the total N2O emissions increased from 11 TgNyr-1 in 1850 to 15 TgNyr-1 in 1970 and to 18 TgNyr-1 in 1994. ¿ 1999 American Geophysical Union