The role of croplands in the terrestrial biosphere: Past, present, and future

Human activities over the last three centuries have significantly transformed the Earth's environment, primarily through the conversion of natural ecosystems to croplands and pastures. In this dissertation, I examine the role of croplands in the terrestrial biosphere using a combination of historical data, satellite imagery, and numerical modeling.; A major accomplishment of this study has been to reconstruct spatially explicit changes in global croplands from 1700 to 1992. The reconstructed changes are consistent with the history of human civilizations and patterns of economic development. Cropland areas increased from about 4 Million km 2 in 1700, to 8.2 Million km2 in 1850, to 18 Million km2 in 1992. In the last century, there has also been significant cropland abandonment. Most of the cropland area came at the expense of forests/woodlands (21% loss) and savannas/grasslands/steppes (20% loss).; By examining existing relationships between croplands, climate, and soils, I have constructed a global map of land suitability for cultivation. This map indicates that there is an additional 67% potentially cultivable land in the current climate, primarily in Tropical Africa and northern South America. Vulnerability analysis indicates that cultivated land in the Canadian Prairie Provinces and the northern margins of cultivation in Europe and the Former Soviet Union are especially vulnerable to decreases in temperature. The western boundary of cultivation in the Midwestern U.S.A., and cultivated land in large parts of India, and portions of northeastern China are particularly vulnerable to increases in temperature or decreases in precipitation.; Finally, I have incorporated the dynamics of land transformation between natural vegetation and croplands within the IBIS global dynamic ecosystem model and examined the influence of changing atmospheric CO₂ concentrations, climate, and croplands over 1860–1992 on the global carbon cycle. Terrestrial ecosystems have been a small source of CO₂ to the atmosphere from 1860 to 1960 and been a sink since then. Cropland changes contributed to a net release of 109 Gt-C over the whole period. During 1980–1992, the NH extra tropics have been a large sink of carbon, owing to the CO₂ fertilization effect. This result is consistent with inversion based estimates of the global carbon budget.