2 Agriculture’s Impacts on Biodiversity, the Environment, and Climate
Agriculture has had major impacts on biodiversity and the environment. Inputs to agriculture in the form of fertilizers, pesticides, and the mechanization of farming have grown, which has produced higher yields but also has increased pollution of the air, water, and soil. The expansion of agriculture to previously wildland and the more intense harvesting of seafoods have placed new pressures on wild habitats and fisheries. Greenhouse gases from agriculture are contributing to higher temperatures and changing precipitation patterns, which have further stressed many plant and animal species.
Biodiversity and the state of the environment also have had major impacts on agriculture. Changes in climate are already affecting which crops can be grown where and the productivity of farmlands and pasturelands. Declining biodiversity is reducing the numbers and variety of pollinators, loosening natural checks on pests, and eliminating species that could have benefited humans in the future.
These and other interconnections among agriculture, biodiversity, and the environment add complexity to efforts to change food systems. However, these interactions also increase the number of ways in which agricultural production, biodiversity, and environmental resilience can be enhanced. For example, improving agricultural production while reducing the environmental impacts of agriculture requires considering broader ecological processes and ecosystem services as well as the wider social and cultural consequences of farmers’ knowledge and actions.
AGRICULTURE AND CLIMATE CHANGE
According to the Intergovernmental Panel on Climate Change, reflecting a consensus of both scientists and governments, higher temperatures, changing precipitation patterns, and greater frequency of extreme events are already affecting food security.1 For example, fruit and vegetable production, a key component of healthy diets, is particularly vulnerable to climate change. Livestock is also vulnerable, with increasing atmospheric carbon dioxide and temperature expected to degrade the productivity, species composition, biogeochemistry, and the quantity and the quality of forage available to herbivores in pastoral systems.
Agriculture is already a major contributor to greenhouse gas emissions, and it is likely to become a proportionately greater contributor as other sectors engage in mitigation. Agriculture’s contributions
1 IPCC (Intergovernmental Panel on Climate Change). 2019. Climate Change and Land: An IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse Gas Fluxes in Terrestrial Ecosystems. P. R. Shukla, J. Skea, E. Calvo Buendia, V. Masson-Delmotte, H.-O. Pörtner, D. C. Roberts, P. Zhai, R. Slade, S. Connors, R. van Diemen, M. Ferrat, E. Haughey, S. Luz, S. Neogi, M. Pathak, J. Petzold, J. Portugal Pereira, P. Vyas, E. Huntley, K. Kissick, M. Belkacemi, and J. Malley, eds. Geneva, Switzerland: IPCC.
to greenhouse gas emissions comes not only from fossil fuel use but also from land clearing for cropping and grazing; methane emissions from ruminant livestock, rice cultivation, and burning of manure and biomass; and nitrous oxide emissions to the atmosphere as a result of fertilizer use.2 Agriculture uses more inputs of natural resources per unit of value added than any other sector of the economy, including manufacturing, construction, and transportation (see Figure 2-1). Furthermore, even with these outsized inputs, the current incremental rate of improvement in agricultural production is only a few percent per year. Even if all non-agricultural fossil fuel use was to stop, future greenhouse gas emissions solely from agriculture because of land clearing, ruminants, manure, rice, burning, and nitrous oxide from fertilized soils would, in total, accumulate so as to exceed the emissions limit set by the Paris Agreement for staying below a 2o Celsius global temperature increase.3
Challenges from climate change are often multiple and linked, like drought and saltwater intrusion for farmers, or losses due to insect pests in a warming climate. Climate change also produces tradeoffs that have to be accommodated within food systems. For example, increased carbon dioxide can cause crops to grow faster, but it can also lower the nutritional quality of the crops.
2 Smith, P., M. Bustamante, H. Ahammad, H. Clark, H. Dong, E. A. Elsiddig, H. Haberl, R. Harper, J. House, M. Jafari, O. Masera, C. Mbow, N. H. Ravindranath, C. W. Rice, C. Robledo Abad, A. Romanovskaya, F. Sperling, and F. Tubiello. 2014. Agriculture, Forestry and Other Land Use (AFOLU). In Climate Change 2014: Mitigation of Climate Change. Contribution to Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, pp. 811–922, O. Edenhofer, R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel, and J. Minx, eds. Cambridge, UK: Cambridge University Press.
3 Clark, M. A., N. G. G. Domingo, K. Colgan, S. K. Thakrar, D. Tilman, J. Lynch, I. L. Azevedo, and J. D. Hill. 2020. Global food system emissions could preclude achieving the 1.5° and 2°C climate change targets. Science 370(6517):705–708.
THE USE OF NITROGEN IN AGRICULTURE
The use of nitrogen in agriculture is a good example of the complex interactions among agriculture, climate, and other environmental issues. Since the 1960s, global agricultural use of fertilizer on cropland has increased more than fivefold (see Figure 2-2).4 This has resulted in large increases of nitrogen in the environment, including nitrates in groundwater, runoff into rivers and coastal areas, and increases in the level of nitrogen oxides and nitrous oxide in the atmosphere. Nitrogen oxides and ammonium contribute to air pollution, with an estimated 19,000 people dying prematurely every year in the United States because of particulate matter caused by these and other agricultural emissions.5 Nitrous oxide generated largely by agriculture is already responsible for about 8 percent of anthropogenic greenhouse gas warming, a number that will go up as nitrogen use continues to increase.6
4 Lassaletta, L., G. Billen, J. Garnier, L. Bouwman, E. Velazquez, N. D. Mueller, and J. S Gerber. 2016. Nitrogen use in the global food system: Past trends and future trajectories of agronomic performance, pollution, trade, and dietary demand. Environmental Research Letters 11(9):095007.
5 Thakrar, S. K., S. Balasubramanian, P. J. Adams, I. M. L. Azevedo, N. Z. Muller, S. N. Pandis, S. Polasky, C. Arden Pope III, A. L. Robinson, J. S. Apte, C. W. Tessum, J. D. Marshall, and J. D. Hill. 2020. Reducing mortality from air pollution in the United States by targeting specific emission sources. Environmental Science and Technology Letters 7:639−645.
6 Tian, H., C. Lu, P. Ciais, A. M. Michalak, J. G. Canadell, E. Saikawa, D. N. Huntzinger, K. R. Gurney, S. Sitch, B. Zhang, J. Yang, P. Bousquet, L. Bruhwiler, G. Chen, E. Dlugokencky, P. Friedlingstein, J. Melillo, S. Pan, B. Poulter, R. Prinn, M. Saunois, C. R. Schwalm, and S. C. Wofsy. 2016. The terrestrial biosphere as a net source of greenhouse gases to the atmosphere. Nature 531:225–228.
Climate change will further boost the amount of nitrous oxide released into the environment.7 As the use of nitrogen-intensive crops expands and moves into areas previously too cold to support such crops, nitrogen losses to the atmosphere will increase. Extreme events such as droughts and intense rainfall will depress the uptake of nitrogen by plants, causing more nitrogen to enter the broader environment. Longer growing seasons and warmer winters will lead to more mineralization of nitrogen by microorganisms from insoluble organic forms to soluble and biologically available forms. Where fertilization rates exceed what crops need, the emissions of nitrous oxide into the atmosphere from unused nitrogen increase.8 For example, when growing switchgrass for biofuels, the production of nitrous oxide from excess fertilizer application could halve the climate benefits.9
Tailoring fertilizer rate to crop type and productivity could help avoid such losses.10 For example, crop yield maps can reveal areas within fields that routinely have lower productivity than other areas. These areas could be fertilized at lower rates so that unused nitrogen does not pollute water and the atmosphere. Alternatively, fields could be subdivided so that low-yielding cropland is used for conservation and bioenergy production. Winter cover crops could be used to scavenge nitrogen, which is especially effective given that most nitrogen is lost in the off season. However, none of these solutions is sufficient to solve the problem, and all require incentivization if they are to take place.
Barriers to reducing the use of nitrogen are largely social and economic rather than technical. Today, agricultural production is aimed toward high yields, not toward greenhouse gas mitigation or nitrate conservation. However, evidence indicates that agriculture could be managed to maximize environmental benefits with only a minimal reduction in yields.11 For example, when the European Union introduced a directive in the 1990s that requires farmers to show how much fertilizer they need to produce their crops, farmers began using less nitrogen while yields continued to increase.
7 Robertson, G. P., and P. M. Vitousek. 2009. Nitrogen in agriculture: Balancing the cost of an essential resource. Annual Review of Environment and Resources 34:97–125.
8 McSwiney, C. P., and G. P. Robertson. 2005. Non-linear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system. Global Change Biology 11(10):1712–1719.
9 Ruan, L., A. K. Bhardwaj, S. K. Hamilton, and G. P. Robertson. 2016. Nitrogen fertilization challenges the climate benefit of cellulosic biofuels. Environmental Research Letters 11:064007.
10 Millar, N., A. Urrea, K. Kahmark, I. Shcherbak, G. P. Robertson, and I. Ortiz-Monasterio. 2018. Nitrous oxide (N2O) flux responds exponentially to nitrogen fertilizer in irrigated wheat in the Yaqui Valley, Mexico. Agriculture, Ecosystems & Environment 261:125–132.
11 Snapp, S. S., R. G. Smith, and G. P. Robertson. 2015. Designing cropping systems for ecosystem services. Pp. 378–408 in S. K. Hamilton, J. E. Doll, and G. P. Robertson, eds. The Ecology of Agricultural Landscapes: Long-Term Research on the Path to Sustainability. New York: Oxford University Press.
AGRICULTURE AND BIODIVERSITY
In addition to its effects on climate, the expansion of agriculture has caused massive losses in biodiversity around the world: natural habitats have been converted to farms and pastures, pesticides and fertilizers have polluted the environment, and soils have been degraded. Many plant and animal populations will face extinction in future decades as land clearing and agricultural production increase.12 Agricultural ecosystems have also become less diverse as the use of crop monocultures has expanded. Even in developed countries such as the United States, directives to use more land for biofuels have caused millions of acres to be converted to monoculture crops, like corn, that had not been grown on that land before.
As with climate change, the interactions of agriculture and biodiversity run both ways. Greater biodiversity benefits agriculture through such effects as an increase in pollinators, the presence of species that reduce pests, and better soil quality. For example, work in ecology has demonstrated a strong link between biodiversity and the stability and productivity of ecosystems.13,14,15
Similarly, a greater diversity of crop types within agricultural systems can improve national food security and stability.16 At a national level, some crops do better in warm years while others do better in cool years, or in wetter and drier years. By averaging across crop yields, greater crop diversity increases the year-to-year stability of national yields and the reliability of food production. Box 2-1 looks at some of the issues involved in protecting biodiversity while maintaining agricultural yields.
12 Tilman, D., M. Clark, D. R. Williams, K. Kimmel, S. Polasky, and C. Packer. 2017. Future threats to biodiversity and pathways to their prevention. Nature 546:73–81.
13 Tilman, D., and J. Downing. 1994. Biodiversity and stability in grasslands. Nature 367:363–365.
14 Isbell, F., D. Craven, J. Connolly, M. Loreau, B. Schmid, C. Beierkuhnlein, T. M. Bezemer, C. Bonin, H. Bruelheide, E. de Luca, A. Ebeling, J. N. Griffin, Q. Guo, Y. Hautier, A. Hector, A. Jentsch, J. Kreyling, V. Lanta, P. Manning, S. T. Meyer, A. S. Mori, S. Naeem, P. A. Niklaus, H. W. Polley, P. B. Reich, C. Roscher, E. W. Seabloom, M. D. Smith, M. P. Thakur, D. Tilman, B. F. Tracy, W. H. van der Putten, J. van Ruijven, A. Weigelt, W. W. Weisser, B. Wilsey, and N. Eisenhauer. 2015. Biodiversity increases the resistance of ecosystem productivity to climate extremes. Nature 526:574–577.
15 Cardinale, B. J., K. Gross, K. Fritschie, P. Flombaum, J. W. Fox, C. Rixen, J. van Ruijven, P. B. Reich, M. Scherer-Lorenzen, and B. J. Wilsey. 2013. Biodiversity simultaneously enhances the production and stability of community biomass, but the effects are independent. Ecology 94(8):1697–1707.
16 Renard, D., and D. Tilman. 2019. National food production stabilized by crop diversity. Nature 571:257–260.
THE CURRENT TRAJECTORY
As global population and per capita incomes continue to grow, demand for food will increase.17 Growing more crops for consumption by both people and livestock will require increasing yields on existing land or converting more wildland to cropland. Increasing yields on existing land implies decreasing the gaps between how much a given area of land is capable of producing and how much it produces today, which in the past has usually entailed increasing the use of fertilizer, irrigations, new kinds of cultivars, and other inputs to agriculture. But more inputs to agriculture have historically resulted in greater negative impacts on biodiversity and the environment. In addition, the yields of some crops appear to be reaching their limits as previous increases have leveled off.18
Agriculture will continue to adapt to new environmental conditions, as it has in the past. For example, the planting of maize has moved away from the hottest regions and toward cooler regions, which has reduced the negative effects of temperature increases.19 In contrast, soybean production has moved toward warmer regions that boost yields. Since the early 1980s, planting of maize begins more than 10 days earlier on average and grain filling is more than 1 week longer, with a resulting increase in yields.20
17 Tilman, D., C. Balzer, J. Hill, and B. L. Befort. 2011. Global food demand and the sustainable intensification of agriculture. Proceedings of the National Academy of Sciences 108(50):20260–20264.
18 Grassini, P., K. M. Eskridge, and K. G. Cassman. 2013. Distinguishing between yield advances and yield plateaus in historical crop production trends. Nature Communications 4:2918.
19 Sloat, L. L., S. J. Davis, J. S. Gerber, F. C. Moore, D. K. Ray, P. C. West, and N. D. Mueller. 2020. Climate adaptation by crop migration. Nature Communications 11:1243.
20 Butler, E. E., N. D. Mueller, and P. Huybers. 2018. Peculiarly pleasant weather for US maize. Proceedings of the National Academy of Sciences 115(47):11935–11940.
Farmers will continue to manage crops to minimize or avoid harms to yields caused by changed conditions and take advantage of new opportunities. They will choose different crops to grow, which will result in the migration of crops across landscapes. Plants and animals will be bred to be more resistant to warmer temperatures, increased humidity, and other environmental changes. Many practices can be optimized and scaled up to advance such adaptations, including investments in infrastructure, capacity building, decision systems, market connectivity, and supply chains. Given that the drivers of change are global, adaptation will also need to take place from the perspective of the global food system.
However, it will be important to understand the limits of agricultural systems to adapt to changing environmental conditions. Farmers, for example, face severe and increasing economic pressures that are not directly related to environmental changes. Agriculture is a powerful system, but post-farm industries are even more powerful. The economic value added to a nation’s gross domestic product from farming is typically a small percentage of the value added by the entire food system, including processing, distribution, and retail. In the United Kingdom, agriculture accounts for only about 8 percent of the value added in the country’s overall food system.21 Food and drink manufacturing, wholesaling, and retailing, in contrast, account for 59 percent, while cafés, restaurants, and other food sales venues account for another 29 percent. Similarly, in the United States, farms receive about 12 cents of every dollar spent by U.S. consumers. At the same time, prices for major commodities like corn, soybeans, and wheat in the United States have dropped to historical lows. While these low prices benefit consumers, the lack of revenue flowing to farmers is a major reason why farmers in these countries and elsewhere rely heavily on subsidies, while conservation receives substantially less governmental support.
The dynamics of the global food system are further challenged by imbalances among urban and rural areas, among countries, and among regions. Countries such as the United States produce more food than they require, while other countries must rely on imports to meet their needs. Mega-cities in the global South have become reliant on the global commodity trade, and their environmental footprints are rising at an even faster rate than their populations. Many aspects of the global food system have not been stress tested against environmental or social shocks that can be expected to occur in the future.
in addition to the demands made of land to produce food, pressure will grow to use land for the production of bioenergy and to sequester carbon. This pressure could increase the conversion of land to agriculture, the degradation of already farmed land, and food insecurity. Integrating bioenergy production and carbon sequestration into sustainably managed landscapes could produce fewer adverse side effects and have other positive co-benefits, such as salinity control, enhanced biodiversity, and reduced eutrophication. However, it will be necessary to figure out how contrasting land uses can work together in complementary ways.
21 Department for Environment, Food & Rural Affairs. 2019. Agriculture in the United Kingdom 2019. London, UK: Department for Environment, Food & Rural Affairs.
An underlying question is how land ought to be used.22 Land can provide food, habitat, bioenergy, climate change mitigation, amenities, housing, timber, and other services and resources. Agriculture is part of much broader systems that have many actors, many sectors, and many needs. Food security can mean many different things, including food nationalism, self-sufficiency, defense, control, resilience, risk management, capacity, and sovereignty. Analyzing and rationalizing these many services will require a multidisciplinary approach and inclusive consultation with stakeholders.
The 2009 Royal Society report Reaping the Benefits: Science and the Sustainable Intensification of Global Agriculture, adapting an earlier analysis,23 defined sustainability as having four attributes:24
- Persistence: the capacity to continue to deliver desired outputs over long periods of time (human generations), thus conferring predictability
- Resilience: the capacity to absorb, utilize, or even benefit from perturbations (shocks and stresses) and thus persist without qualitative changes in structure
- Autarchy: the capacity to deliver desired outputs from inputs and resources (factors of production) acquired from within key system boundaries
- Benevolence: the capacity to produce desired outputs (e.g., food, fiber, fuel, oil) while sustaining the functioning of ecosystem services and not causing depletion of natural capital (e.g., minerals, biodiversity, soil, clean water)
Similarly, the Food and Agriculture Organization of the United Nations established five principles that must be pursued to make agriculture sustainable:25
- Improve efficiency in the use of resources
- Conserve, protect, and enhance natural resources
- Protect and improve rural livelihoods, equity, and social well-being
- Strengthen the resilience of people, communities, and ecosystems to climate change and market volatility
- Promote responsible and effective governance mechanisms
Given these objectives, the challenge of sustainable agriculture is how to produce sufficient and nutritious food for all people with low environmental impacts. As discussed in the remainder of this summary, many deliberative levers of change can be used to address this challenge, including increased agricultural efficiency and yields, smarter land use, better use of markets and trade, reductions of
22 Lang, T. 2020. Feeding Britain: Our Food Problems and How to Fix Them. London, UK: Penguin.
23 Pretty, J. N. 2008. Agricultural sustainability: Concepts, principles and evidence. Philosophical Transactions of the Royal Society B 363(1491):447–465.
24 Royal Society. 2009. Reaping the Benefits: Science and the Sustainable Intensification of Global Agriculture. London, UK: Royal Society.
25 FAO (Food and Agriculture Organization of the United Nations). 2014. Building a Common Vision for Sustainable Food and Agriculture: Principles and Approaches. Rome, Italy: FAO.
waste, and shifts in diets. In addition, forced levers of change, such as the coronavirus pandemic that gripped the world in 2020, can be expected to change food systems, though often in ways that are difficult to predict.
THE NEED FOR CHANGE
Under a business-as-usual scenario, the deleterious environmental effects of current food systems will continue to increase. Higher levels of food production will require more fertilizer, pesticides, and irrigation and more extensive resource extraction from the land and sea. This will be the case in places where growing populations, increased demand, and existing yield gaps will exert pressures to convert more wildland to cropland in particular (see Figure 2-3). Such an approach would bring more air and water pollution, increases in greenhouse gas emissions, greater degradation and erosion of soils, more conversion of natural habitats to agriculture, greater threats to biodiversity, and intensified competition for land and other resource inputs. Given the already substantial effects of agriculture on biodiversity and the environment, such a future is not sustainable.