In recent years, ozone pollution has caused an annual loss of USD 63 billion in wheat, rice and maize harvests across East Asia, according to a study recently published in Nature Food. The region has become a global hotspot of air pollution in recent decades.
China alone loses a third of its wheat, nearly a quarter of its rice and 9 percent of its maize due to ozone pollution, translating into annual harvest losses of USD 21.8 billion, 30.8 billion and 7.8 billion respectively.
Most of the world’s ozone exists in the stratosphere as the ozone layer, where it protects the Earth from a vast majority of the sun’s ultraviolet light and the potential damage this radiation could cause. It is in the troposphere – the lowest level of the atmosphere – where ozone can become a pollutant. Here, fossil fuel emissions mix with sunlight to create ozone, a major component of smog that is particularly prevalent during the summer.
Although ozone pollution levels have stagnated or declined in North America and Europe over the last two decades, they have been rising in East Asia. Industrializing economies, urbanizing populations and growing transportation sectors have all contributed to a steep rise in energy use that still largely comes from burning fossil fuels.
The region is also a center of economic activity that depends heavily on burning fossil fuel. East Asia is a major manufacturing hub; China in particular produced more than 28 percent of the world’s manufactured goods in 2018. Eight of the world’s 10 largest ports by volume of shipped goods are also located in the region.
At the same time, East Asia is also an important center for agriculture, with the three countries studied by the researchers – China, Japan and South Korea – producing roughly 43 percent of the world’s rice, 22 percent of its wheat and 29 percent of its maize in 2020, according to the Food and Agriculture Organization of the UN.
“A problem with the ozone impacts on cereal crops is that [farmers] will never know that the yield was reduced by ozone,” said Kazuhiko Kobayashi, a professor at the University of Tokyo and an author of the study, in an email. “There are no visible damages in leaves identifiable as caused by ozone.” He also noted that ozone damage happens across entire regions, so crop losses due to ozone pollution may be more difficult to identify within individual farms.
In the study, the researchers used hourly measurements of ozone levels for six months, then averaged them across the latest three years when data was available. They then referenced research in the region on the sensitivity of the staple crops to ozone to calculate the amount of yield lost to the pollutant. These results were then compared with the yield of crops that were applied with ethylene diurea – an anti-ozonant, or a substance that protects plants against ozone damage that is often used in research to measure the impact of ozone on plant health – to confirm the scale of crop damage from ozone.
The researchers found wheat crops to suffer the most under ozone pollution. The North China Plain, China’s main wheat-producing region and a major ozone pollution hotspot, suffers some of the worst losses of wheat yields in the country. South Korea also loses 28 percent and Japan 16 percent of their wheat harvests yearly.
Rice is another crucial crop in the region. Hybrid rice – which is a cross between two genetically distinct rice strains – was found to be far more susceptible to yield losses due to ozone pollution. From 2017 to 2019, China lost nearly 30 percent of its hybrid rice yields, whereas only 12 percent of its “inbred” rice harvest was lost to ozone in that time period. More than half of all rice paddies in China grow hybrid rice, which are favored because of their high productivity.
The researchers note that breeding more ozone-tolerant cultivars and using anti-ozonants such as ethylene diurea could help blunt the impact of ozone pollution. Although ethylene diurea is mainly used for research for now, the study authors found that it could increase wheat yields by 40 percent and rice yields by 20 percent. They add, however, that there must be proof that ethylene diurea is non-toxic and economical for farmers before it can be widely used in agriculture.
Zhaozhong Feng, a professor at Nanjing University of Information Science & Technology and another author of the study, said in an email that reducing ozone pollution could result in “more staple food, [a] lower need to increase the cultivated land area, more harvested product for farmers and high contribution to national economies.”
To reduce ozone pollution, the Climate & Clean Air Coalition recommends cutting down on the emissions of methane, which is one of the gases that can form ozone when in contact with sunlight. Reducing methane emissions is also a major goal for halting climate change, and more than 100 countries pledged to cut these emissions by 30 percent by 2030 during last year’s climate talks in Glasgow. Improving the management of livestock manure, fixing methane leaks on gas pipelines, and separating out biodegradable municipal waste to use as compost or bioenergy are a few ways to lower methane emissions.