Understanding the complex air pollution-agriculture relationship

If I asked you to imagine a place where air pollution was a big problem, you’d probably think of a city. A place with lots of factories, cars, or large coal-fired power plants. You probably wouldn’t imagine a farm, but perhaps you should. Agriculture generates significant levels of air pollution and, in some parts of the world, it is now the single largest sectoral source of damages from particulate air pollution.

But agriculture is not just a source of particulate pollution. Agricultural output also suffers when pollution levels are higher—particulate pollution directly interferes with crops’ ability to photosynthesize sunlight, reducing yields, while other pollutants like ozone are toxic to crops.

In three recent papers, co-authors and I examine multiple aspects of this relationship: How choices about tillage practices can reduce particulate air pollution and generate substantial welfare gains; the role of technological upgrading in driving the contribution of agricultural burning to air pollution; and finally, the surprising ways that air pollution and climate change can influence crop yields.
 

Do tillage choices affect air pollution?

It has long been hypothesized that choices around tillage techniques (the practice of turning over the ground to prepare for planting) could play a role in fighting climate change. Soils can store carbon, sometimes more efficiently than forests, and so practices that reduce soil disturbance, often called no-till, might yield climate change benefits by increasing sequestration.

But David Lobell and I hypothesized that no-till might have an additional benefit—because it reduces soil disturbance, it also reduces dust generation. That, in turn, reduces harmful particulate pollution in downwind communities. We find that as no-till techniques are more widely adopted, there is a substantial reduction in air pollution. These reductions in pollution lead to reductions in mortality in downwind communities. 

We can monetize the reductions in mortality attributable to the reduction in pollution and compare those benefits to the potential carbon benefits of no-till approaches. When we do, we find, as has been found in other contexts, that the benefits of reducing local air pollution are of similar or larger magnitude than the carbon benefits of no-till. This suggests that adopting climate-friendly policies might make sense because of their pollution benefits alone, even if you ignore the climate benefits.
 

Agricultural mechanization can increase crop burning.

Air pollution in North India, including in the capital, Delhi, is the worst in the world. Levels of particulate pollution regularly exceed the recommended WHO limit by more than 1,000 percent. A major source of this pollution is the burning of harvest residue by farmers to clear their fields and prepare for the winter planting season after harvesting the monsoon crop. This practice has been engaged in for many years, but the number of fires has increased substantially in the last two decades. 

In recent work, I examine how the implementation of the Mahatma Gandhi National Rural Employment Guarantee Act (MNREGA) has contributed to this increase in fires. After implementation of MNREGA, the use of fires to clear residue increased by 21 percent, with a corresponding increase of particulate pollutants. This increase appears to be driven by farmers choosing to mechanize the harvest—which leaves more residue on the fields to be removed—in response to the labor market changes induced by MNREGA.
 

Particulate pollution’s surprising impact on crop yields.

In a third recently published paper, Sherrie Wang and I examine how wildfire smoke—an increasingly common form of particulate pollution—affects crop yields. We expected that smoke would reduce crop yields because it blocks sunlight and reduces photosynthesis. However, we discovered that, at low levels, smoke exposure actually increases yields because crops benefit from an increased amount of refracted (versus direct) sunlight.

At high levels of smoke exposure, however, the absorption effect we had anticipated dominates and smoke does reduce yields. Because smoke is likely to increase due to climate change, it is likely that, in the long-run, smoke exposure will reduce crop yields.
 

The relationship between agriculture and air pollution is complex.

Agriculture is a major source of air pollution in many parts of the world, and policy makers should carefully consider this fact in setting agricultural policy. However, air pollution also plays an important, and often overlooked, role in determining agricultural yields. Failure to consider both features of the air pollution-agriculture relationship will result in inefficient policies.