Showing papers by "James S. Gerber published in 2018"

Increasing importance of precipitation variability on global livestock grazing lands

Abstract: Pastures and rangelands underpin global meat and milk production and are a critical resource for millions of people dependent on livestock for food security1,2. Forage growth, which is highly climate dependent3,4, is potentially vulnerable to climate change, although precisely where and to what extent remains relatively unexplored. In this study, we assess climate-based threats to global pastures, with a specific focus on changes in within- and between-year precipitation variability (precipitation concentration index (PCI) and coefficient of variation of precipitation (CVP), respectively). Relating global satellite measures of vegetation greenness (such as the Normalized Difference Vegetation Index; NDVI) to key climatic factors reveals that CVP is a significant, yet often overlooked, constraint on vegetation productivity across global pastures. Using independent stocking data, we found that areas with high CVP support lower livestock densities than less-variable regions. Globally, pastures experience about a 25% greater year-to-year precipitation variation (CVP = 0.27) than the average global land surface area (0.21). Over the past century, CVP has generally increased across pasture areas, although both positive (49% of pasture area) and negative (31% of pasture area) trends exist. We identify regions in which livestock grazing is important for local food access and economies, and discuss the potential for pasture intensification in the context of long-term regional trends in precipitation variability. Satellite measures of vegetation greenness, together with animal stocking data and key climatic factors, reveal interannual precipitation variability to be a significant constraint on global pasture productivity.

Progress towards sustainable intensification in China challenged by land-use change

Abstract: China is experiencing rapid land-use change and shifts in farm management. However, the interactive effects of these drivers on cropping system sustainability are unclear. Here, we evaluate spatio-temporal trade-offs among crop production and five key environmental indicators, including land use, water consumption, excess nitrogen and phosphorous use, and greenhouse gas emissions in China. From 1987 to 2010, as crop kilocalorie production increased (+66%), so did the total environmental impact of all indicators (+1.3–161%) except greenhouse gas emissions (−18%). Concurrently, environmental intensity—impact per kilocalorie produced—decreased for all indicators (−51–−13%) except excess phosphorus (+57%). Despite substantial loss and displacement of cropland to urban expansion, counterfactual scenario analysis indicates that farm management explained >90% of changes in crop production and environmental impact. However, cropland is expanding in regions of relatively high land and irrigation intensity. Although efficiency gains partly compensated for increased environmental pressures, continued geographic shifts in cropland could challenge progress towards agricultural sustainability in China. China’s agricultural output is growing rapidly, but the environmental impacts are unclear. This study finds this impact has risen, but much more slowly than output due to improved farm management, though ongoing shifts in cropland location may challenge this development.

An attainable global vision for conservation and human well-being

Abstract: A hopeful vision of the future is a world in which both people and nature thrive, but there is little evidence to support the feasibility of such a vision. We used a global, spatially explicit, systems modeling approach to explore the possibility of meeting the demands of increased populations and economic growth in 2050 while simultaneously advancing multiple conservation goals. Our results demonstrate that if, instead of “business as usual” practices, the world changes how and where food and energy are produced, this could help to meet projected increases in food (54%) and energy (56%) demand while achieving habitat protection (>50% of natural habitat remains unconverted in most biomes globally; 17% area of each ecoregion protected in each country), reducing atmospheric greenhouse-gas emissions consistent with the Paris Climate Agreement (≤1.6°C warming by 2100), ending overfishing, and reducing water stress and particulate air pollution. Achieving this hopeful vision for people and nature is attainable with existing technology and consumption patterns. However, success will require major shifts in production methods and an ability to overcome substantial economic, social, and political challenges.

Progress towards sustainable intensification in China challenged by land-use change

Abstract: China is experiencing rapid land-use change and shifts in farm management. However, the interactive effects of these drivers on cropping system sustainability are unclear. Here, we evaluate spatio-temporal trade-offs among crop production and five key environmental indicators, including land use, water consumption, excess nitrogen and phosphorous use, and greenhouse gas emissions in China. From 1987 to 2010, as crop kilocalorie production increased (+66%), so did the total environmental impact of all indicators (+1.3–161%) except greenhouse gas emissions (−18%). Concurrently, environmental intensity—impact per kilocalorie produced—decreased for all indicators (−51–−13%) except excess phosphorus (+57%). Despite substantial loss and displacement of cropland to urban expansion, counterfactual scenario analysis indicates that farm management explained >90% of changes in crop production and environmental impact. However, cropland is expanding in regions of relatively high land and irrigation intensity. Although efficiency gains partly compensated for increased environmental pressures, continued geographic shifts in cropland could challenge progress towards agricultural sustainability in China.China’s agricultural output is growing rapidly, but the environmental impacts are unclear. This study finds this impact has risen, but much more slowly than output due to improved farm management, though ongoing shifts in cropland location may challenge this development.

Quantifying the Limitation to World Cereal Production Due To Soil Phosphorus Status

Abstract: Phosphorus (P) is an essential element for plant growth. Low P availability in soils is likely to limit crop yields in many parts of the world, but this effect has never been quantified at the global scale by process-based models. Here we attempt to estimate P limitation in three major cereals worldwide for the year 2000 by combining information on soil P distribution in croplands and a generic crop model, while accounting for the nature of soil-plant P transport. As a global average, the diffusion-limited soil P supply meets the crop's P demand corresponding to the climatic yield potential, due to the legacy soil P in highly fertilized areas. However, when focusing on the spatial distribution of P supply versus demand, we found strong limitation in regions like North and South America, Africa, and Eastern Europe. Averaged over grid cells where P supply is lower than demand, the global yield gap due to soil P is estimated at 22, 55, and 26% in winter wheat, maize, and rice. Assuming that a fraction (20%) of the annual P applied in fertilizers is directly available to the plant, the global P yield gap lowers by only 5-10%, underlying the importance of the existing soil P supply in sustaining crop yields. The study offers a base for exploring P limitation in crops worldwide but with certain limitations remaining. These could be better accounted for by describing the agricultural P cycle with a fully coupled and mechanistic soil-crop model.

Uncertainties of potentials and recent changes in global yields of major crops resulting from census- and satellite-based yield datasets at multiple resolutions.

Abstract: Global agriculture is under pressure to meet increasing demand for food and agricultural products There are several global assessments of crop yields, but we know little about the uncertainties of their key findings, as the assessments are driven by the single best yield dataset available when each assessment was conducted Recently, two different spatially explicit, global, historical yield datasets, one based on agricultural census and the other largely based on satellite remote sensing, became available Using these datasets, we compare the similarities and differences in global yield gaps, trend patterns, growth rates and changes in year-to-year variability We analyzed maize, rice, wheat and soybean for the period of 1981 to 2008 at four resolutions (0083°, 05°, 10° and 20°) Although estimates varied by dataset and resolution, the global mean annual growth rates of 17-18%, 15-17%, 11-13% and 14-16% for maize, rice, wheat and soybean, respectively, are not on track to double crop production by 2050 Potential production increases that can be attributed to closing yield gaps estimated from the satellite-based dataset are almost twice those estimated from the census-based dataset Detected yield variability changes in rice and wheat are sensitive to the choice of dataset and resolution, but they are relatively robust for maize and soybean Estimates of yield gaps and variability changes are more uncertain than those of yield trend patterns and growth rates These tendencies are consistent across crops Efforts to reduce uncertainties are required to gain a better understanding of historical change and crop production potential to better inform agricultural policies and investments

Balancing tradeoffs: Reconciling multiple environmental goals when ecosystem services vary regionally

Abstract: As the planet’s dominant land use, agriculture often competes with the preservation of natural systems that provide globally and regionally important ecosystem services. How agriculture impacts ecosystem service delivery varies regionally, among services being considered, and across spatial scales. Here, we assess the tradeoffs between four ecosystem services—agricultural production, carbon storage, biophysical climate regulation, and biodiversity—using as a case study the Amazon, an active frontier of agricultural expansion. We find that the highest values for each of the ecosystem services are concentrated in different regions. Agricultural production potential and carbon storage are highest in the north and west, biodiversity greatest in the west, and climate regulation services most vulnerable to disruption in the south and east. Using a simple optimization model, we find that under scenarios of agricultural expansion that optimize total production across ecosystem services, small increases in priority for one ecosystem service can lead to reductions in other services by as much as 140%. Our results highlight the difficulty of managing landscapes for multiple environmental goals; the approach presented here can be adapted to guide value-laden conservation decisions and identify potential solutions that balance priorities.

Sustainability Evaluation of the Maize–Soybean Intercropping System and Maize Monocropping System in the North China Plain Based on Field Experiments

Abstract: Monocropping systems, which currently dominate China’s major grain production regions, contribute to resource scarcity and environmental pollution. Intercropping has the potential to improve resource use efficiency. However, prior studies of intercropping systems have generally focused on ecological, economic, and social consequences. Here, we make a comparative ecological sustainability analysis on energy capture and efficiency of maize monocropping and maize–soybean intercropping systems through emergy evaluation based on field experiments performed from 2012 to 2014. We find that maize monocropping shows higher sustainability than maize–soybean intercropping in the North China Plain at present. Quantitative results indicate that for maize monocropping, the emergy yield ratio (EYR) and emergy sustainability index (ESI) are 13.7% and 21.1% higher than that of intercropping systems, and the environmental loading ratio (ELR) is 7.3% lower than that of intercropping systems. To further test, we applied three levels of nitrogen fertilizer in intercropping systems (120 kg ha−1, 180 kg ha−1, 240 kg ha−1), and find that a reduced rate of N fertilizer for intercropped system leads to higher sustainability (ESI 5.3% higher) but still lower sustainability than maize monocropping. Key drivers of the different sustainability outcomes are decreased energy output and a larger proportion of labor input associated with intercropping systems.