R EPORTS Forecasting Agriculturally Driven Global Environmental Change David Tilman,1* Joseph Fargione,1 Brian Wolff,1 Carla D’Antonio,2 Andrew Dobson,3 Robert Howarth,4 David Schindler,5 William H. Schlesinger,6 Daniel Simberloff,7 Deborah Swackhamer8 During the next 50 years, which is likely to be the final period of rapid agri- cultural expansion, demand for food by a wealthier and 50% larger global population will be a major driver of global environmental change. Should past dependences of the global environmental impacts of agriculture on human population and consumption continue, 109 hectares of natural ecosystems would be converted to agriculture by 2050. This would be accompanied by 2.4- to 2.7-fold increases in nitrogen- and phosphorus-driven eutrophication of terrestrial, freshwater, and near-shore marine ecosystems, and comparable increases in pesticide use. This eutrophication and habitat destruction would cause unprecedented ecosystem simplification, loss of ecosystem services, and species extinctions. Significant scientific advances and regulatory, technolog- ical, and policy changes are needed to control the environmental impacts of agricultural expansion. During the first 35 years of the Green Revolu- ecosystems by the use and release of limiting tion, global grain production doubled, greatly resources that influence ecosystem functioning reducing food shortages, but at high environ- (nitrogen, phosphorus, and water), release of mental cost (1–5). In addition to its effects on pesticides, and conversion of natural ecosys- greenhouse gases (1, 6, 7), agriculture affects tems to agriculture. These sources of global change may rival climate change in environ- 1Department of Ecology, Evolution and Behavior, Uni- mental and societal impacts (2, 8). Population versity of Minnesota, 1987 Upper Buford Circle, St. size and per capita consumption are assumed to Paul, MN 55108, USA. 2Department of Integrative be the two greatest drivers of global environ- Biology, University of California, Berkeley, CA 94720, mental change. Humans currently appropriate USA. 3Department of Ecology & Evolutionary Biology, Princeton University, Princeton, NJ 08540, USA. 4The more than a third of the production of terrestrial Oceans Program, Environmental Defense, and the ecosystems and about half of usable freshwa- Ecosystems Center, Marine Biological Lab, Woods ters, have doubled terrestrial nitrogen supply Hole, MA 02543, USA. 5University of Alberta, Z-811 and phosphorus liberation, have manufactured Biological Sciences Building, Edmonton, Alberta, T6G and released globally significant quantities of 2E9, Canada. 6The Phytotron, Duke University, Durham, NC 27708, USA. 7Department of Ecology & pesticides, and have initiated a major extinction Evolutionary Biology, University of Tennessee, Knox- event (2–4, 8–10). Global population, which ville, TN 37996, USA. 8Environmental and Occupa- increased 3.7-fold during the 20th century, to 6 tional Health, University of Minnesota, 420 Delaware billion people (11), is forecast to increase to 7.5 Street SE, Minneapolis, MN 55455, USA. billion by the year 2020 and to about 9 billion *To whom correspondence should be addressed. E- by 2050 (12). Constant-dollar global per capita mail: [email protected] www.sciencemag.org SCIENCE VOL 292 13 APRIL 2001 281 R EPORTS gross domestic product (GDP) increased 4.6- jectories of the past 35 or more years. Because ognizing that substantial changes in future pop- fold in the 20th century (13) and is projected to these trajectories include in them the impacts of ulation and economic growth, agricultural poli- be 1.3 times current levels by 2020 and 2.4 past technological developments, changes in cies, climate, and other factors would affect our times current levels by 2050 (14, 15). How consumer choices, and environmental regula- results. Detailed regional forecasts and forecasts might projected increases in population and tions, our forecasts implicitly assume similar based on mechanistic models that couple re- wealth influence the global environment? The technological, regulatory, and behavioral gional economies, agriculture, and the environ- prospects of climate change are widely recog- changes in the future. Shifts in these could ment are also needed and would complement nized (16). Here, we explore the nonclimatic cause major deviations from our forecasts. our simpler global approach. global environmental impacts of agricultural We use univariate and multiple regressions Four forecasts were made for each variable: expansion during the coming 20 to 50 years. to forecast future global trends for each of seven by a linear fit to its temporal trend (Fig. 1), We use past global trends and their dependence environmental variables related to agriculture extrapolated to the years 2020 and 2050; by the on global population and GDP to empirically (Table 1). Because of the exponential nature of fitted dependence of each variable on population forecast the potential global environmental im- past global population and economic growth, we size (13, 17, 18), combined with the global pacts of agriculture. Like economic forecasting, had anticipated exponential temporal trends for population size projected (12) for 2020 and ecological forecasting is notoriously difficult these variables. Surprisingly, each was a linear, 2050; by the linear dependence of each variable and imprecise. Our forecasts are not predic- and almost equally strong, function of time, on GDP (13, 17), combined with global GDP tions, but rather are estimates of environmental population, and GDP (Fig. 1 and Table 1). We projections (14, 15) for 2020 and 2050; and by impacts should agriculture continue on the tra- thus use linear fits in our forecasts, while rec- multiple regression fitting each variable to year, Fig. 1. (A) Trends in annual rates of application of ni- trogenous fertilizer (N) ex- pressed as mass of N, and of phosphate fertilizer (P) ex- pressed as mass of P2O5, for all nations of the world ex- cept the former USSR (18, 19), and trends in global to- tal area of irrigated crop land (H2O) (18). (B) Trends in global total area of land in pasture or crops (18). (C) Trend in global pesticide production rates, measured as millions of metric tons per year (30). (D) Trend in ex- penditures on pesticide im- ports (18) summed across all nations of the world, trans- formed to constant 1996 U.S. dollars. All trends are as dependent on global popula- tion and GDP as on time (Table 1). Table 1. Univariate and multivariate forecasts for years 2020 and 2050, based on **P Ͻ 0.0001; *P Ͻ 0.01; NS, P Ͼ 0.05. The value in 2000 is based on temporal trends observed in the past 35 to 40 years and their dependence on population extrapolation from the latest available data, generally 1998. Mean projections are and GDP. Parentheses show R2 values for each regression. Levels of significance: means of the three univariate and the one multivariate projection. Fertilizer (106 MT) Pesticide Irrigated land Crop land Pasture land Imported (106 ha) Produced (109 ha) (109 ha) NP (109 1996 (106 MT) U.S.$) Value in 2000 87.0 34.3 280 3.75 11.8 1.54 3.47 Mean projections Forecast 2020 135 47.6 367 6.55 18.5 1.66 3.67 Forecast 2050 236 83.7 529 10.1 32.2 1.89 4.01 Individual projections for 2050 Univariate By year 186 62.0 465 7.33 25.8 1.79 3.90 (0.986**) (0.927**) (0.998**) (0.946*) (0.957**) (0.976**) (0.977**) By population 166 56.2 417 8.02 22.2 1.73 3.79 (0.980**) (0.910**) (0.996**) (0.990*) (0.951**) (0.974**) (0.979**) By GDP 343 98.3 761 18.1 48.8 2.20 4.59 (0.964**) (0.904**) (0.992**) (0.995*) (0.955**) (0.973**) (0.977**) Multivariate 249 118 473 7.06 32.0 1.83 3.75 (0.989**) (0.979**) (0.998**) (0.994NS) (0.960**) (0.977**) (0.982**) 282 13 APRIL 2001 VOL 292 SCIENCE www.sciencemag.org R EPORTS population, and per capita GDP, combined with 26). Eutrophication is the biggest pollution could lead to the loss of about a third of remain- projected values for these in 2020 and 2050. We problem in most coastal waters (23), and, with ing tropical and temperate forests, savannas, and present all four forecasts for 2050 to illustrate overfishing and aquaculture (27), is a major grasslands and of the services, including carbon similarity and variability, and mean forecasts for threat to marine biodiversity. Agricultural nutri- storage (33), provided by these ecosystems. Ad- 2020 and 2050 (Table 1). The averages for 2020 ent pollution has led to increased blooms of ditional natural habitat would be lost worldwide allow a mid-course evaluation of the 50-year toxic algae in many coastal systems and to the to urban and suburban development, to road- forecasts. large hypoxic (“dead”) zone in the Gulf of Mex- ways, and to the rotation of low-quality lands The doubling of global food production dur- ico (24, 28). In total, projected increases in N through agriculture. Species extinction is an ir- ing the past 35 years was accompanied by large and P fertilization and irrigation would cause reversible impact of habitat destruction. Interac- increases in global nitrogen (N) and phosphorus significant losses of biodiversity, as well as tions between climate change, species invasions, (P) fertilization and irrigation [Fig. 1A and (5)]. marked changes in the composition and func- and habitat fragmentation could cause further If past trends in N and P fertilization (18, 19) tioning of both terrestrial and aquatic ecosys- diversity losses, because many species may be and irrigation (18) and their dependence on tems (2, 3, 8, 23, 25, 26, 28). unable to migrate through fragmented habitats population and GDP continue, our mean fore- Although society benefits from pesticides, to reach regions with suitable climates and soils cast is for global N fertilization to be 1.6-fold some cause environmental degradation or affect (34).