Since the start of the Green Revolution in the mid-1960s, India has tripled its wheat yield. Yet air pollution also rose steadily during this period, with deleterious impacts on crops. In a recent paper drawing on published literature, I estimate that the increases in ozone and aerosol pollutants across the northern Indian wheat belt have held yield growth back by about one third.
In other words, India, which produced a record wheat harvest of 100 million tons in 2018-19, could have produced 40 million tons more if the atmosphere were as clean as it was 50 years ago.
India doesn’t really need that additional wheat; it is already close to self-sufficiency. But these findings suggest the economic constraints air pollution imposes; a clean atmosphere would have permitted many Indian farmers to replace wheat with more renumerative crops and to be less dependent on government subsidies, to the benefit of the overall economy.
India’s air pollution problem has grown into a major threat to public health, but its effects on plant life—including crops—are also significant. In recent decades, industrialization and population growth have driven rising energy production and consumption, particularly from coal-fired power plants, but also from burning wood, cow dung and crop waste, and the expanding use of transport vehicles.
Ozone is formed mainly from nitrogen oxide emissions from burning fossil fuels reacting with sunlight; aerosol particles such as soot are released directly from burning fuels. Only in the last 20 years have reasonably solid numbers on their concentrations been recorded; today they are continuously measured and/or estimated via satellite observation and other techniques. Atmospheric flows widely distribute these pollutants across India’s wheat belt.
Ozone disrupts the wheat plant’s biology. It enters through the open pore-like stomata in the leaves along with the CO2 needed for photosynthetis and growth. Ozone is highly chemically reactive and directly damages the photosynthetic machinery, reducing its output and eventually causing premature leaf senescence. Damage is linked to atmospheric ozone concentrations above 40 parts per billion during daylight hours.
Aerosols scatter sunlight and reflect some back into space; the net effect is negative for crop growth. Today aerosols block 16% of the total sunshine reaching the wheat crop in northern India.
The ozone effects are calculated from supplying controlled amounts of ozone to crops over the life of the crop; there are now a reasonable number of such field studies conducted under Indian conditions. Aerosol effects are more easily derived from well-validated models of crop photosynthesis.
Averaging the results from a number of credible studies, it appears that the total estimated negative effects on wheat yield is two thirds due to ozone and one third due to aerosols. These results were independently confirmed by modeling Indian states’ wheat yield change over the last 30 years as a function of known factors driving yield, including surrogates of atmospheric surrogates, like coal consumption.
How can agricultural research address this problem? For instance, could developing ozone-resistant wheat varieties be one possible solution? Some recent work in the United States points to this potential for soybeans. However, wheat and other crops have not seen significant spikes in ozone in their evolutionary history, so there is unlikely to be natural variation in tolerance that breeders can exploit. In addition, modern, high-yield wheats with irrigation and more open stomata could be experiencing greater ozone impacts than older varieties, a trade-off that may be impossible to reverse.
Using genetic engineering to increase antioxidant levels in leaves to detoxify ozone might be feasible, but that is a long-term project.
Countering the effects of reduced sunlight due to aerosol pollution via breeding or agronomy is also problematic.
One obvious solution is eliminating the burning of crop residue, which is especially common with rice straw ahead of wheat planting. This would reduce aerosols—but only early in the crop cycle. (After various attempts, the government now seems determined to curtail the practice.) Machines that seed wheat into the rice straw are helping in this regard, and provide additional benefits. There is also a major effort underway to replace manure as a cooking fuel with gas and/or electricity, and to use natural gas in public transport in cities like New Delhi.
These are good but modest moves towards solving the pollution problem—a goal that undoubtedly lies not with agricultural research but with policy and regulation.
Unlike carbon dioxide and other greenhouse gases, ozone and aerosols have lifetimes in the atmosphere of only weeks or months, so mitigation policies quickly show results. In North America, for example, ozone levels have fallen due to regulation, and the corresponding yield losses for maize and soybean were estimated to be only 4% and 2%, respectively, in 2010. North America has also seen its skies brighten since the mid 1980s as aerosol concentrations have fallen, with predicted benefits for crop yields.
These problems go beyond wheat, and beyond India. All crops, including the other staple, rice, are likely to suffer yield losses of the same order as those estimated for wheat. East Asia, meanwhile, suffers levels of atmospheric pollution similar to India’s (though aerosol levels may have leveled off in some areas, including China). Similar crop yield losses can thus be expected across the region.
The history of North America and Western Europe suggests that it is economic development that will ultimately provide the policy environment and funds for systematically reducing pollution. While the necessary technologies are available to India and other countries facing this problem, it is unclear whether this process can be short-circuited sooner in the development cycle. But the economic benefits in increased crop productivity would be very large (as well as the benefits of improved public health).
These challenges also exist against the backdrop of climate change. Interestingly, the crop yield increase from reducing ozone and aerosols would easily exceed the projected reductions due to climate change out to 2050 and beyond. Of course, that does not mean climate change mitigation and adaptation efforts should therefore be postponed or relaxed; greenhouse gas emissions continue to rise and persist in the atmosphere. But these findings suggest that cleaning up ozone and aerosol emissions in Asia could give the region vital breathing space.
Tony Fischer is an agricultural scientist and Honorary Fellow with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Canberra, Australia.
