Estimating historical changes in global land cover: Croplands from 1700 to1992

Human activities over the last three centuries have significantly transform ed the Earth's environment, primarily through the conversion of natural eco systems to agriculture. This study presents a simple approach to derive geo graphically explicit changes in global croplands from 1700 to 1992. By cali brating a remotely sensed land cover classification data set against cropla nd inventory data, we derived a global representation of permanent cropland s in 1992, at 5 min spatial resolution [Ramankutty and Foley, 1998]. To rec onstruct historical croplands, we first compile an extensive database of hi storical cropland inventory data, at the national and subnational level, fr om a variety of sources. Then we use our 1992 cropland data within a simple land cover change model, along with the historical inventory data, to reco nstruct global 5 min resolution data on permanent cropland areas from 1992 back to 1700. The reconstructed changes in historical croplands are consist ent with the history of human settlement and patterns of economic developme nt. By overlaying our historical cropland data set over a newly derived pot ential vegetation data set, we analyze our results in terms of the extent t o which different natural vegetation types have been converted for agricult ure. We further examine the extent to which croplands have been abandoned i n different parts of the world. Our data sets could be used within global c limate models and global ecosystem models to understand the impacts of land cover change on climate and on the cycling of carbon and water. Such an an alysis is a crucial aid to sharpen our thinking about a sustainable future.