The Climate Benefits of Better Nitrogen and Phosphorus Management
Pursuing more efficient use of these elements has clear environmental, socioeconomic, and national security benefits. It would also help reduce some of the risks of a warming climate.
Nearly four decades have passed since the phrase “global warming” first appeared in a scientific journal. Writing in Science in 1975, geochemist Wallace Broecker warned that rising atmospheric carbon dioxide (CO2) levels would result in a world climate unprecedented in modern human history. Now, as Broecker’s forecast is becoming a reality, we can no longer just debate ways to slow climate change; we must figure out how to live with it. Although much of the work in this area has focused on the carbon cycle, expanding our focus to other elements, especially nitrogen and phosphorus, can make a positive contribution. By providing more fertilizers to farmers in some of the world’s poorest nations and reducing nitrogen and phosphorus losses to the environment in developed and rapidly developing ones, we could reduce some of the risks of a changing climate. At the same time, a more efficient, less polluting relationship with the global nitrogen and phosphorus cycles would mitigate a host of other environmental challenges, increase food security, improve human welfare, lessen some national security concerns, and probably save money.
A century ago, world leaders were asking how they would be able to feed a fast-growing population. At the time, the potential for food growth was constrained by finite reserves of nitrogen and phosphorus that could be readily accessed for crop fertilizers. Only two generations later, the situation was entirely different. Widespread implementation of the Haber-Bosch process—an industrial means for converting the limitless pool of atmospheric N2 into usable forms of nitrogen, including fertilizer—had released much of the world from nitrogen constraints on crop growth. In parallel, the ability to locate and mine reserves of phosphorus rose markedly. In combination with revolutions in plant breeding and genetics, these developments formed the foundation for the Green Revolution, rapidly increasing world food production.
Feeding people is a good thing, but our ability to transform the nitrogen and phosphorus cycles has had startling and unsustainable consequences. In just the past two generations, humans have shifted from modestly to dominantly affecting global nitrogen and phosphorous cycles. Although N2 in the atmosphere is unreactive and phosphorous in rocks is unavailable to organisms, we now create more reactive nitrogen every year than all natural processes on land combined, and we have tripled the rate at which biologically available phosphorus enters ecosystems. Much though not all of that new nitrogen and phosphorus becomes fertilizer. Substantial amounts are also used for making other industrial goods, such as plastic and nylon. Further, billions of pounds of additional reactive nitrogen are created inadvertently as a byproduct of fossil fuel combustion. These changes represent massive and unprecedented reorganizations of two element cycles on which all life depends.
Not surprisingly, these reorganizations have brought unintended consequences. Excess nitrogen and phosphorus in the environment cause diverse environmental ills, many of which directly affect human health and welfare. Beyond the effects on climate discussed below, these include air pollution, acid rain, marine and freshwater eutrophication, biodiversity loss, and the stimulation of some invasive species. Freshwater eutrophication carries a multibillion-dollar price tag in the United States alone. Some estimates suggest that safe planetary levels of nitrogen and phosphorus have already been exceeded, with long-term consequences for humanity.
Of course, there is a major upside to our domination of the global nitrogen and phosphorus cycles. Billions of people depend on the ability to make and disseminate mineral fertilizers, and a reasonable future for humanity must include the continued creation of such fertilizers at substantial rates. However, we are a long way from achieving an equitable, efficient, and sustainable use of nitrogen and phosphorus in agriculture, and we are not close to reducing nitrogen and phosphorus pollution to tolerable levels.
In wealthier countries with modern forms of intensive agriculture, large fractions of nitrogen and phosphorus applied to fields, often more than half, never make it into the crop itself. Concentrated animal feeding operations create additional inefficiencies as nitrogen and phosphorus in animal feed are transported to feed lots, but the nitrogen and phosphorus in animal excreta are not returned to crops. The explosion of the biofuel industry has caused a similar redistribution of these elements to areas near refineries in a way that has nothing to do with food production. Inefficient nitrogen and phosphorus use in agriculture, along with industrial pollution, underpin the environmental challenges listed above.
Although the sources of excess nitrogen and phosphorus in the environment are similar, phosphorus, unlike nitrogen, remains a finite, diminishing, and irreplaceable resource, and one that is concentrated in just a few countries. Although the extent of readily accessible phosphorous reserves remains debated, it is clear that in a business-as-usual scenario, the United States will become increasingly dependent on foreign sources of phosphorus, many of which lie in nations that may be unstable and/or pose challenges to foreign policy and national security. Sooner or later, the United States and the world will need to become far more efficient in their use of phosphorus or lose the ability to maintain high rates of food production at reasonable cost.
Thus, even without the prospect of climate change, we need to shift the ways we interact with and manage the nitrogen and phosphorus cycles. Climate change multiplies this concern, because many of the environmental effects of excess nitrogen and phosphorus, as well as food insecurity in poorer countries, are likely to worsen under a rapidly changing climate. Fortunately, maintaining the benefits of our nitrogen and phosphorus use while greatly reducing the unwanted consequences does not require phantom technologies or massive social upheaval. We can begin to improve agricultural nutrient-use efficiencies and reduce industrial forms of nitrogen and phosphorus with current knowledge and technology and without suffering major economic blows. Similarly, we know that increased access to nitrogen and phosphorus fertilizers in regions such as sub-Saharan Africa can lessen food scarcity and initiate cascading social, economic, and environmental benefits.
Pursuing more equitable and efficient nitrogen and phosphorus use has clear environmental, socioeconomic, and national security benefits. Could improving our management of the nitrogen and phosphorus cycles also contribute to climate change mitigation or adaptation? For the most part, climate mitigation is a question about nitrogen: Because reactive nitrogen exists in many atmospheric forms, it has multiple and counteracting effects on the radiative balance of the atmosphere. The major warming effect is via increased emissions of nitrous oxide (N2O), a greenhouse gas that is 300 times more potent than CO2. On the cooling side, human-created reactive nitrogen can form aerosols that reflect the Sun’s energy back to space. Moreover, airborne nitrogen compounds that are produced by agriculture, transportation, and other industrial sectors can fertilize nearby forests, thereby removing CO2 from the atmosphere.
Nitrogen’s cooling effects have prompted some observers to say that human acceleration of the nitrogen cycle may be beneficial. However, when all of the warming and cooling effects of nitrogen are calculated, they appear to largely cancel each other out in the short term. At best, recent estimates suggest a small net cooling effect, but such effects will diminish as any boost in forest production saturates with time, and because the effective contribution of N2O to climate warming is forecast to double or more by 2100. Thus, continued release of excess nitrogen to the environment will probably accelerate climate change with time and will also lead to the formation of more ozone holes in this century.
However, this focus on the rate of climate change misses the larger picture. Because climate change is already a reality and is certain to continue under any scenario, an assessment of its risks must include not only the pace of change but the inevitable effects. When viewed in this fashion, excess nitrogen and phosphorus in the environment add to the risks and clearly provide opportunities for mitigation and adaptation.
Consider air pollution. Tropospheric ozone (O3,or smog) is a pollutant with widespread negative consequences for human health and crop production; it is also a greenhouse gas. Atmospheric nitrogen oxide (NOx) concentrations regulate the formation of tropospheric O3, and so does temperature. Using business-as-usual scenarios for reactive nitrogen creation and CO2 emissions, several projections suggest that O3 -related human mortality and crop damage will rise sharply in the next few decades, especially in tropical and subtropical regions where rising temperatures and rising NOx concentrations will interact synergistically to produce more O3.
But what if we reduced NOx emissions? U.S. experience shows this can be done. During the past decade, NOx concentrations have fallen, largely because of Clean Air Act regulation of industrial and transportation emissions. So far, those reductions have reduced but not eliminated O3 risks in the United States, because NOx is emitted by other sources and because rising temperatures have erased some of the gains associated with NOx reductions. But if we can continue to reduce NOx —by targeting industrial emissions and improving agricultural efficiencies—then at some point the effect of temperature won’t matter, because high O3 levels cannot occur in the absence of substantial NOx concentrations. Thus, in the case of O3, the best way to reduce or remove the threat that warming-enhanced O3 poses to human health—its climate change risk—is almost certainly via the mitigation of nitrogen pollution.
Managing the nitrogen cycle to reduce smog mitigates some climate forcing and reduces the risk that climate change will worsen air quality. Other examples primarily involve risk reduction. For example, excess nitrogen in the atmosphere also forms another regulated class of air pollution known as fine particulate matter (PM). Not only do the chemical reactions that can lead to PM formation go faster at higher temperatures, PM’s lifetime in the air depends on rainfall. Shift toward a drier climate, as is forecast for significant portions of the United States, including most of those with the highest rates of population growth, and PM risks will worsen. In this case, nitrogen is not the only contributor to PM formation, but reducing nitrogen emissions could help. And although dollars are a poor metric for evaluating the benefits of improved health, they still offer perspective: In 2002 terms, the annual costs of nitrogen-related air pollution in the United States were conservatively estimated at $17 billion, with most of the cost attributed to a shortening of human lives.
Move from air to water and similar examples can be found. Nitrogen- and phosphorus-driven freshwater and marine eutrophication has major socioeconomic consequences that include lost livelihoods, reduced property values, damage to fisheries, loss of recreational opportunities, and several health risks. As with air pollution, evidence suggests that human-driven climate change will, on average, worsen eutrophication in freshwater and marine systems. The reasons are complex and system-specific, but in general a warmer climate means increased stratification of water bodies, decreased oxygen-holding capacities, higher potential loading of nitrogen and phosphorus, greater concentration of nitrogen and phosphorus in regions that will become hotter and drier, shifts in biological processes that can elevate the risks, or all of these. Thus, without changing current trajectories, the effects of eutrophication will spread and worsen in the coming decades. But widespread eutrophication cannot occur without enough nitrogen and phosphorus loading to aquatic systems. Lower that input and another climate risk is reduced or removed, even with the climate warming rapidly.
The potential benefits extend to food security. The link between ground-level ozone and crop damage mentioned above is one example, but there are many others. Not everything about a warmer world will be negative; in agriculture, for example, there are likely to be winners and losers across nations and regions. However, most forecasts suggest that the biggest hazards affect those who can least afford increased risk: developing nations in tropical and subtropical regions that are already struggling to secure an adequate food supply. Here, a concerted effort to extend the benefits of the Green Revolution to those who have missed out—Africa being the most notable example—could be a rapid and substantial counterweight to the growing threat of climate change. That goal is not just a moral imperative. The Defense Department, the State Department, and other U.S. entities focused on foreign policy and national security list climate change as a growing concern. Climate-related threats to basic human needs such as clean water and food can interact with social unrest and conflict, with consequences that spread well beyond the borders of the affected nations. Enhancing access to nitrogen and phosphorus fertilizers in Africa and other regions that do not now have enough is one step toward increasing food security and reducing the risk of social disruption. Although too much nitrogen and phosphorus poses threats to environments and society in much of the world, not enough nitrogen and phosphorus is a major threat and a significant offset to climate risk in the poorest countries.
In addition, a warming climate poses threats to biodiversity, clean water, and the health of coral reefs and other near-shore marine ecosystems, as well as accelerating the spread of parasitic and infectious diseases. Pollution with nitrogen, phosphorus, or both also carries risks for all of these sectors. Unlike ground-level O3 or eutrophication, nitrogen and phosphorus are generally not the major agents of risk, but lowering their release to the environment would lessen the multiple stresses that alter ecosystems and affect human well-being.
Overall, the connections between nitrogen, phosphorus, and climate are not just about net effects on rising temperatures or changing precipitation patterns. When expanded to consider the need to adapt to a changing climate, it is clear that business as usual with nitrogen and phosphorus will enhance our risks and make adaptation to climate change more difficult. However, concerted efforts to reduce nitrogen and phosphorus pollution from industry, improve the efficiency of their use in agriculture, and enhance their availability for use in fertilizer in food-insecure regions would have multiple benefits, including a reduction of climate risks. We believe that the second scenario is feasible and would provide multiple benefits to society, from local to global scales.
Admittedly, the threats and opportunities of altered nitrogen and phosphorus cycles pose some unique challenges for human society. Unlike the risks from fossil fuel CO2, where it is possible and ultimately necessary to envision a shift to energy systems that are carbon-free, food production requires nitrogen and phosphorus, and we must enhance natural supplies of these nutrients to meet world food demands. Thus, managing the nitrogen and phosphorus cycles sustainably becomes a classic optimization problem, one that emphasizes waste reduction while enhancing food quality, quantity, and accessibility.
Fortunately, the opportunities for doing so abound and in many cases are already under way. In the case of U.S. agriculture, yields have continued to rise in recent decades while fertilizer use has remained steady; the efficiency of nitrogen and phosphorus use has improved. Better efficiencies have been achieved in multiple ways, ranging from the use of precision agriculture technology to optimally timed fertilizer additions and crop demand, to comparatively low-tech solutions such as the use of cover crops that reduce nutrient losses. More can be achieved still. Using known management and technology solutions, nitrogen and phosphorus losses could be cut dramatically in some U.S. food systems without altering yields. Reaching very high efficiencies would not be easy, because it would require a combination of strategies that mix better on-field management with changes in incentive structures, crop types, and dietary preferences. However, loss reductions in the range of 30 to 50% could be achieved in many systems without these more significant transformations in the food system. That level of cuts would make a substantial dent in the downstream and downwind effects of excess nitrogen and phosphorus. The opportunities for improvement are even greater in rapidly developing economies such as China, which now uses much more nitrogen and phosphorus fertilizer much less efficiently than either the United States or Europe, and at a much higher cost in pollution and human health. As fertilizer, especially phosphorous fertilizer, becomes scarce, greater efficiency in its use will only sharpen a country’s competitive edge in the global economy.
On the industrial side, the Clean Air Act demonstrates that regulatory policies can reduce pollution without any compelling evidence for the kinds of economic trauma sometimes anticipated. The need to regulate NOx and other pollutants spawned the development of new technologies that can scrub emissions at a higher rate and lower cost. In some cases, combining regulatory with market-based solutions may achieve even greater reductions, but regardless of the instruments used, the mechanisms for further improvement clearly exist and should be pursued. Given the well-demonstrated health consequences of nitrogen-related air pollution, along with the additional risks posed by a changing climate, now is not the time to relax emission controls.
In Africa, work by the Millennium Villages Project (MVP) and others has shown that improving access to nitrogen and phosphorus fertilizers can make a substantial difference in human well-being. Perhaps most notably, the village-scale improvements first demonstrated by the MVP catalyzed the government of Malawi to enhance access to nitrogen fertilizer and improved seed varieties in a policy targeted at its poorest farmers. The result was a jump in food production that took Malawi from years of food shortage to being a net exporter of grain. The United States, other countries of means, and private entities should focus more of their food security efforts on helping such policies become widespread.
But progress in nitrogen and phosphorus management will need to go beyond just implementing already known policies. This is an interdisciplinary challenge that requires better communication among natural and social scientists, economists, engineers, policymakers, and a host of stakeholders. Those who understand the issues best also must do a better job of educating the public. Many of the impediments to progress on nitrogen and phosphorus issues come from a lack of public understanding. There is a substantial role for personal choice, but without effective communication, we can’t expect informed choices.
Overall, we suggest that improving the management of nitrogen and phosphorus will bring multiple benefits to humanity. Climate change can provide an additional incentive for improving management. At the same time, focusing on the multiple compelling reasons for improving nitrogen and phosphorus management may represent an opportunity to make progress on climate policy in ways that are less politically divisive. Particularly in the United States, movement on either climate mitigation or adaptation has been notoriously difficult, especially when framed as a response to climate change alone. However, when other more tangible benefits exist, ranging from economic to national security to other forms of environmental protection or repair, the barriers to progress may be less daunting. Whether progress comes under a climate change banner, or whether the climate benefits ride along behind other incentives, the United States and the world must move down this path.
S. R. Carpenter and E. M. Bennett, “Reconsideration of the Planetary Boundary for Phosphorus,” Environmental Research Letters, DOI: 10.1088/1748-9326/6/1/014009 (2011).
J. E. Compton, J. A. Harrison, R. L. Dennis, T. L. Greaver, B. H. Hill, S. J. Jordan, H. Walker, and H. V. Campbell, “Ecosystem Services Altered by Human Changes in the Nitrogen Cycle: A New Perspective for US Decision Making.” Ecology Letters 14 (2011): 804–815.
J. N. Galloway, A. R. Townsend, J. W. Erisman, M. Bekunda, Z. C. Cai, J. R. Freney, L. A. Martinelli, S. P. Seitzinger, and M. A. Sutton, “Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions,” Science 320 (2008): 889–892.
G. K. MacDonald, E. M. Bennett, P. A. Potter, and N. Ramankutty, (Agronomic Phosphorus Imbalances Across the World’s Croplands,” Proceedings of the National Academy of Sciences; DOI: 10.1073/pnas.1010808108 (2011).
G. P. Robertson and P. M. Vitousek, “Nitrogen in Agriculture: Balancing the Cost of an Essential Resource,” Annual Review of Environment and Resources, DOI: 10.1146/annurev.environ.032108.105046 (2009).
A. R. Townsend and R. W. Howarth, “Fixing the Nitrogen Problem,” Scientific American, (Feb. 2010): 64–71.
P. M. Vitousek, R. Naylor, T. Crews, M. David, L. Drinkwater, E. Holland, J. Katzenberger, L. Martinelli, P. Mat-son, G. Nziguheba, D. Ojima, C. Palm, G. P. Robertson, P. Sanchez, A. R. Townsend, and F. S. Zhang, “Nutrient Imbalances Along Trajectories of Agricultural Development,” Science 324 (2009): 1519–1520.
Alan R. Townsend ([email protected]) is professor of ecology and evolutionary biology and director of the Environmental Studies Program at the University of Colorado, Boulder. Peter M. Vitousek ([email protected]) is professor of biology and director of the Emmett Interdisciplinary Program in Environment and Resources at Stanford University. Benjamin Z. Houlton ([email protected]) is assistant professor of global ecology and biogeochemistry at the University of California, Davis.