Estimating the world's potentially available cropland using a bottom-up approach
Introduction
Land is a scarce resource at a global scale. Increases in both human population and per capita consumption of material goods have increased demand for commodities produced from the land (Lambin and Meyfroidt, 2011). Meanwhile, land degradation and climatic change threaten the current base of productive land. Intensification of land use has the potential to satisfy the bulk of future increases in demand (Godfray et al., 2010). However, additional conversion to croplands will be unavoidable. By 2030, an additional 81 to 147 million hectares (Mha) of cropland will be needed compared to the 2000 baseline. Rapid urbanization, bioenergy policy mandates, forest plantations, and new protected areas are also competing for land access (Meyfroidt and Lambin, 2011). Total additional land demand is likely to range from 285 to 792 Mha between 2000 and 2030 (Lambin and Meyfroidt, 2011). The perception that we are approaching a limit in available productive land is growing, highlighting the need for improved information on land availability.
We define potentially available cropland (PAC) (sometimes referred to as land reserve, underutilized, or spare land) as the moderately to highly productive land that could be used in the coming years for rainfed farming, with low to moderate capital investments, and that is neither under intensive use, legally protected, nor under intact mature forest cover. Our definition excludes areas that could only be put into cultivation with major investments – e.g., irrigation or costly soil reclamation – and for which the ecological cost of conversion in terms of biodiversity and carbon storage is known to be very high – i.e., protected areas and intact or little disturbed forests (Gibson et al., 2011).
PAC, while underutilized in terms of agricultural production, is used for a variety of purposes, often as extensive grazing lands or long-term fallows. PAC may also have ecological or social functions such as ecological corridors, hunting-and-gathering grounds, recreation areas, sacred land, or protection against landslides. Decision-makers must consider the tradeoffs between the gains in agricultural production from expanding croplands and the associated losses of ecosystem and social services (DeFries et al., 2004). We therefore also define potentially available cropland that accounts for trade-offs (PACt), as the subset of PAC that could be converted at relatively low ecological and social costs – recognizing that this cost is never nil and will vary with differing perspectives and priorities (Fig. 1). Our reconceptualization of PAC therefore shifts away from a “finite stock” view to a “gradient” or “continuum” view – i.e., how social and ecological costs of land conversion increase with land use expansion.
Past estimates of PAC have adopted either a “residual approach” – i.e., estimating the total area that is agro-ecologically suitable and then excluding cultivated areas and, in some cases, intact forests, protected areas and densely populated areas (Ramankutty et al., 2002, IIASA/FAO, 2012) – or a “categorical approach” – i.e., identifying specific categories of land use/cover that could be converted to croplands (Campbell et al., 2008, Cai et al., 2011). Historically, vast amounts of forests and other lands fulfilling valuable social or ecological functions have been converted for agriculture, and still are in some parts of the world. Assuming that this trend will continue led to estimates of PAC ranging from 1600 to 1900 Mha, an area that is larger than the 1500 Mha of land already devoted to crops. Young, 1998, Young, 1999, Young, 2000 criticized these estimates, suggesting that cultivable land was overestimated; land already cultivated was underestimated; and the needs for land for alternative uses were insufficiently taken into account. A recent FAO report (Alexandratos and Bruinsma, 2012) still identifies a generous 1400 Mha of prime and good land that could be brought into cultivation if needed. Our more conservative approach integrates more comprehensive social and environmental constraints, in a context where indigenous rights and the value of ecosystem services are better acknowledged.
IIASA/FAO (2012) has produced a global database of agro-ecological zones that is widely used to identify land suitable for different cropping systems. IIASA/FAO (2012) identified 3100 Mha as good to very suitable for production of five key crops with mixed input levels, accounting for terrain, soils, temperature, and water availability. Of these lands, 1000 Mha are already cultivated. [Of the 1510 Mha under cultivation worldwide, 510 Mha are on land with moderate or lower suitability according to IIASA/FAO (2012).] This leaves 2100 Mha of land suitable for agricultural expansion including forests and protected areas. A recent World Bank report (2010) estimated that the non-cultivated area that is suitable for cropping, while being non-forested, non-protected, and populated with less than 25 persons/km2, amounts to 445 Mha globally. Using the most recent release of the Global Agro-ecological Zones database (GAEZ version 3.0, IIASA/FAO, 2012), we estimated 598 Mha, considering the same constraints. A large fraction of this area is concentrated in a few countries – e.g., Brazil, Argentina, Democratic Republic of Congo, Mozambique, and Russia.
PAC estimates have great policy significance – e.g., for bioenergy policies, standards of sustainable agricultural production, or regulations on large-scale land acquisitions by foreign actors (The World Bank, 2010, Nalepa and Bauer, 2012). The objective of this study is to evaluate the current size and geographic distribution of PAC and PACt accounting for the constraints and tradeoffs associated with their conversion. In contrast to previous exercises, we adopt a “bottom-up” approach by looking for PAC based on detailed, fine scale observations and expert knowledge for regions where previous analyses have identified large PAC reserves. Our time horizon focuses on the next five to ten years, thus allowing us to ignore potential future changes in demand, technology, infrastructure, labour force distribution, and climate.
Section snippets
Social and physical constraints to land conversion
There are several physical and social challenges to land conversion for agriculture within a short time horizon (Table 1). Ignoring these constraints leads to an overestimation of PAC. Even where broad-scale assessments would consider agro-ecological conditions to be suitable for agriculture, other biophysical factors may limit the potential to expand croplands. Climate variability affects crop production in drylands and susceptibility to soil erosion or salinization render some lands marginal
Environmental tradeoffs of PAC categories
In addition to constraints to land conversion for agriculture, there are social and ecological tradeoffs. Impoverished and food-insecure people often occupy marginal lands, and they could be displaced if large-scale agribusiness moves in. Beyond protected areas’ contribution to biodiversity protection, other habitats still have value as buffer zones and ecological corridors. Many land cover types contribute to terrestrial stocks and sinks of carbon. PAC falls under various categories of land
Case studies
The constraints and tradeoffs associated with cropland expansion play out differently in each region. We evaluated the availability, constraints, and tradeoffs entailed in cropland expansion for several case study regions that are often assumed to hold substantial PAC: the Chaco region, the Brazilian Cerrado, the Brazilian and Bolivian Amazon, the Congo basin, Indonesia, and Russia (Fig. 1). PAC was estimated in each of these regions based on the best available and most recent spatial data, and
Discussion
Consideration of constraints and trade-offs for a potential expansion of croplands in a bottom-up approach led, in all cases but one, to substantially more conservative estimates of PAC compared to global-scale approaches that only account for agro-ecological suitability. As our estimation of PACt only included carbon and biodiversity trade-offs, our final estimates still include land whose conversion would be associated with other significant social and ecological tradeoffs. We thus provide
Conclusion
The key messages from this study are that: (i) in the near future there is less potential additional cropland than is generally assumed, and (ii) converting land is always associated with social and ecological tradeoffs. Widely-cited sources tend to grossly overestimate potentially available cropland at the global scale, at least on a short-term horizon. There are few remaining places with “free and easy” lands, and multiple land uses – not just cropland expansion – are targeting productive
Acknowledgements
This paper is based on a workshop funded by the Francqui Foundation and Academia Belgica in Rome. Their support is gratefully acknowledged. Matthew Clark and Mitchell Aide provided the land cover shape files generated by NSF Grant CNHS 0709598 and 0709645 for the analysis of Chaco. We are grateful to T. Searchinger and two anonymous reviewers for their critical comments that have contributed to improve this paper. All the inadequacies of this study remain our responsibility.
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