4.1.2.3 Land use changes
Humans have been modifying their environment for millennia, in particular through deforestation. Before 1950, this mainly occurred in Europe, North America, India and China, leading to a high fraction of cropland in these areas (Fig. 4.5). In the last 50 years, the extension of cropland has been stabilised in many places, some regions even showing an increase in the surface covered by forest. By contrast, deforestation has occurred rapidly over this period in many countries in the tropics.
Deforestation has a direct impact on emissions of CO2 and CH4 (see section 4.1.2.1) as well as on the production of dust aerosols and of aerosols due to biomass burning (see section 4.1.2.2). Furthermore, the anthropogenic changes in land use have altered the characteristics of the Earth’s surface, leading to changes in the energy and moisture budgets. For instance, it has been estimated that past deforestation in tropical areas has led to a warming there of about 0.2°C, the changes in evapotranspiration being a significant contributor to this temperature rise. Several of these surface changes cannot be adequately represented by radiative forcing. However, it is possible to compute a radiative forcing for the modifications of albedo associated with land use changes, as this directly affects the radiative balance of the surface (although it is not always easy to separate this forcing from the feedbacks between vegetation and climate, see section 4.3.3). Forests have a lower albedo than crops or pasture, in particular when snow is present (see sections 1.5 and 4.3.3). The deforestation since 1750 has thus induced a radiative forcing which has been estimated to average around –0.2 Wm–2 across the globe (Fig. 4.2). However, the forcing is much higher in regions where deforestation has been the most severe, reaching several Wm–2 in some places.
: The fraction of land occupied by crops in 1750 and 1992. Figure from Ramankutty and Foley (1999). Copyright American Geophysical Union 1999.
