Humans have converted more than three quarters of the terrestrial biosphere into croplands, rangelands, villages and other agricultural biomes. In most regions, the transformation of forests, grasslands and other wild ecosystems into agricultural land has generated heterogeneous landscape mosaics in which croplands intermingle with a wide array of different land uses including settlements and forestry along with patches of disturbed, recovering, and remnant ecosystems. This mosaic structure has significant implications when assessing the climate impacts of agricultural land, which include altered carbon balance and greenhouse gas emissions together with changes in surface energy balance caused by altered surface albedo and fluxes of moisture and sensible heat. We used global data for population, land use, land cover, climate and terrain to characterize global patterns of agricultural landscape structure and its changes in relation to their potential influence on global climate.
Results/Conclusions
In contrast with historical trends, most agricultural regions have recently undergone small but significant contractions in cropped area caused by the abandonment of the least productive lands and by the expansion of housing into croplands, usually accompanied by intensified management of remaining croplands to increase their productivity. As a result, both housing density and tree cover are increasing in agricultural regions of North America, Europe and China, without notable declines in agricultural productivity. While trees and other woody vegetation usually increase both carbon sequestration and evaporative cooling relative to pre-existing croplands, their albedo is much lower, resulting in a strong net warming effect in most regions. Replacing crops with settlements also produces warming by eliminating evaporative cooling, even when accompanied by increases in surface albedo. Thus, the net climate impact of both cropland abandonment and settlement increase would appear to be surface warming, especially in the densely populated agricultural biomes, which incorporate built surface areas several times greater than all of the world’s urban areas combined. Yet these effects depend on climatic zone and can be balanced by global cooling caused by increased carbon sequestration in woody biomass and soils. And while intensified management of remaining croplands has likely increased fertilizer loading and nitrous oxide emissions, generating greenhouse warming, its effects on soil carbon balance and emissions of carbon dioxide and methane are less well understood. As a result, the net climate impacts of recent changes in agricultural landscape structure remain a challenge to measure. Given their potential impacts on global and regional climate, these certainly merit greater study.
