A reconstruction of global agricultural areas and land cover for the last millennium

Abstract

[1] Humans have substantially modified the Earth's land cover, especially by transforming natural ecosystems to agricultural areas. In preindustrial times, the expansion of agriculture was probably the dominant process by which humankind altered the Earth system, but little is known about its extent, timing, and spatial pattern. This study presents an approach to reconstruct spatially explicit changes in global agricultural areas (cropland and pasture) and the resulting changes in land cover over the last millennium. The reconstruction is based on published maps of agricultural areas for the last three centuries. For earlier times, a country-based method is developed that uses population data as a proxy for agricultural activity. With this approach, the extent of cropland and pasture is consistently estimated since AD 800. The resulting reconstruction of agricultural areas is combined with a map of potential vegetation to estimate the resulting historical changes in land cover. Uncertainties associated with this approach, in particular owing to technological progress in agriculture and uncertainties in population estimates, are quantified. About 5 million km 2 of natural vegetation are found to be transformed to agriculture between AD 800 and 1700, slightly more to cropland (mainly at the expense of forested area) than to pasture (mainly at the expense of natural grasslands). Historical events such as the Black Death in Europe led to considerable dynamics in land cover change on a regional scale. The reconstruction can be used with global climate and ecosystem models to assess the impact of human activities on the Earth system in preindustrial times.Citation: Pongratz, J., C. Reick, T. Raddatz, and M. Claussen (2008), A reconstruction of global agricultural areas and land cover for the last millennium, Global Biogeochem. Cycles, 22, GB3018,

Temperature variability in China in an ensemble simulation for the last 1,200 years

“…Anthropogenic land use change prior to 1700 is reconstructed by Pongratz et al (2008) and after 1700 by Foley et al (2003). In China where crop areas were already large at 800 AD, natural vegetation is almost completely replaced by agricultural land use after 1700 in Southeast China.…”

An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes

“…These follow the protocol of the Paleoclimate Modeling Intercomparison Project, Phase 3 (PMIP3) (Schmidt et al 2011;Braconnot et al 2012). Prescribed external forcing factors are: variations in volcanic aerosols (Crowley and Unterman 2012), in total solar irradiance (TSI) (Vieira et al 2011) and in the atmospheric concentration of the most important well-mixed greenhouse gases and anthropogenic aerosols (Schmidt et al 2011); changes in global land-cover and agricultural areas (Pongratz et al 2008); and annual values of the orbital parameters, that is, eccentricity, obliquity, and perihelion. In addition, we use an 1156-year-long control simulation under constant preindustrial (1850) boundary conditions (hereafter PiControl) as a reference for the forced experiments.…”

Potential climate forcing of land use and land cover change

“…In the development of the RCP4.5 scenario, international carbon trading incentivizes preservation of forests and reforestation, which reduces CO 2 emissions and the resulting CO 2 RF from LULCC, increasing the enhancement factor. The grey shaded region is bounded by the annual rate of forest area change required to completely reforest to the estimated prehistoric forest area (Pongratz et al, 2008), or remove all forests by year 2100. Reported and projected forest area change from Meyfroidt and Lambin (2011) (purple) and FAO (2010) and Hansen et al (2013) (green) are depicted as constant rates through year 2100 to show the result if these rates were sustained.…”