Agriculture uses some 4 750 million ha of land for cultivating crops and animal husbandry. Cultivated temporary and permanent crops occupy over 1 500 million ha, while land under permanent meadows and pastures occupies almost 3 300 million ha. The overall change in agricultural land area since 2000 is small, but land under permanent and irrigated crops has increased, while land under permanent meadows and pastures has significantly declined. Rapid growth in urban areas has displaced all types of agricultural land use (Map S.1) (Table S.1).
The agroclimatic context for the pattern of land use is changing rapidly. Farming enterprises are adapting to new thermal regimes that can upset crop growth stages and their supporting soil ecologies, with specific implications for spreading crop disease and pests (Map S.2). Fundamental changes to the water cycle, particularly the patterns of rainfall and periods of drought, are forcing adjustment of rainfed and irrigated production. Under climate change, growing periods may become longer in boreal and arctic regions, but shorter in areas affected by extended drought periods when compared with current reference lengths (Map S.3).
Climate change impacts on the water cycle are expected to significantly affect agricultural output and the environmental performance of productive land and water systems. Climate models predict decreases in renewable water resources in some regions (mid-latitude and dry subtropical regions) and increases in others (mainly high latitudes and humid mid-latitude regions). Even where increases are projected, there may be short-term shortages due to changing streamflow caused by greater variability in rainfall.
As part of the global carbon cycle, forest cover is a valuable indicator of climate health. Global forest land cover is just over 4 billion ha, some 30 percent of the total land area (Map S.4). The net annual forest cover loss between 2010 and 2020 is estimated at 4.7 million ha/year compared with 5.2 million ha/year between 2000 and 2010 and 7.8 million ha/year between 1990 and 2000, taking account of forest expansion through regeneration and afforestation (Figure S.1).
Soils are an essential buffer or “regulator” of climate change. Soils under conventional agriculture continue to be a source of carbon dioxide emissions, but conservation techniques can halt, and in some instances, reverse the loss of soil organic carbon (SOC) (Map S.5). Peat soil degradation and drainage release large amounts of carbon through decomposition. Fires in drained peatlands accounted for about 4 percent of global fire emissions between 1997 and 2016. Agricultural practices also cause soils to emit other greenhouse gases (GHGs) in addition to carbon dioxide, and climate change exacerbates these emissions. Soils emit nitrous oxide when fertilizers are applied, and when nitrogen-fixing crops are planted. They also emit methane when flooded for rice cultivation.
The global distribution of salt-affected soils (Map S.6) reflects naturally saline and sodic soils and a build-up of salts through human-induced soil water processes. Soil salinity is estimated to take up to 1.5 million ha of cropland out of production each year. Higher rates of evapotranspiration are expected to exacerbate the accumulation of salts in the surface horizons, but the extent of subsoil salinity at the 30–100 cm depth range is much more pronounced.
Pressures on land and water resources have never been so intense, and their accumulation is pushing the productive capacity of land and water systems to the limit. Cropland increased by 4 percent (63 million ha) between 2000 and 2019. Growth in arable land, mainly for irrigated crops, doubled, while that for rainfed cropping increased by only 2.6 percent over the same time period. Population increases have meant agricultural land available per capita for crops and animal husbandry declined by 20 percent between 2000 and 2017, to 0.19 ha/capita in 2017.
The impacts of climate change, from severe floods and droughts to persistent heat domes, are producing predicted and also surprising changes. Increasing evapotranspiration from cropland is anticipated, as is variable rainfall, leading to changes in land/crop suitability and reduced yields where temperature stresses attenuate carbon assimilation. Greater variations in river run-off and groundwater recharge are expected, affecting rainfed and irrigated agriculture. Absorbing extreme floods on previously drained agricultural land presents a dilemma for urban and rural flood disaster planning when nature-based solutions (NbSs) are deployed.
In 2019, global anthropogenic emissions were 54 billion tonnes of carbon dioxide equivalent (CO2-eq), of which 17 billion tonnes CO2-eq, or 31 percent, came from agrifood systems. In terms of single gases, agrifood systems generated 21 percent of carbon dioxide emissions, 53 percent of methane emissions and 78 percent of nitrous oxide emissions. Emissions from agricultural land (farm gate) were the largest component of agrifood systems with around 7 billion tonnes CO2-eq, followed by pre- and post-production processes (6 billion tonnes CO2-eq) and land-use change (4 billion tonnes CO2-eq). While emissions from agrifood systems increased globally by 16 percent between 1990 and 2019, their share in total emissions decreased, from 40 percent to 31 percent, as did the per capita emissions, from 2.7 to 2.1 tonnes CO2-eq per capita (Figure S.2).
Future climate change scenarios point to the need for changing cropping patterns and management practices to adapt to changes in crop/land suitability. Agricultural systems are already adapting with more-precise use of technology and inputs, partly as a response to climate change, but mainly as a response to the more-sophisticated demands of the global food system. For this reason, the significance of traditional measures of land and water productivity has declined as more factors of production are taken into account. Indeed, while growth in agricultural land use and irrigated areas has stagnated, total factor productivity in agriculture has increased by 2.5 percent each year over the past few decades, reflecting greater efficiency in the use of agricultural inputs. It has replaced resource intensification as the primary source of growth in world agriculture (Figure S.3). This gain has raised awareness of the need for sustainable agriculture and efficient use of limited natural resources. While the use of agricultural inputs has intensified to meet current demand, the resulting environmental impacts have accumulated to the point where a wide range of environmental services are affected, limiting agriculture’s capacity to respond. At the same time, intersectoral competition for land and water resources is intense, so the scope to extend irrigated areas and convert new land to agriculture is extremely constrained.