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MIT Better World

By Leda Zimmerman

Many scientists also project that the pressure on food will be all the greater due to global warming. But in a study published last July in Nature Climate Change, Colette L. Heald identified another threat to food security: the interaction of rising temperatures and air pollution.

“We’ve known that the warming climate damages crops and reduces yields, and that ozone is toxic to plants, but ours is the first study that brings the impact of climate change and air pollution together,” says Heald, Mitsui Career Development Associate Professor, Department of Civil and Environmental Engineering and the Department of Earth, Atmospheric and Planetary Sciences.

Ozone, the primary component of smog, is photochemically produced from two key pollutants (nitrogen oxides and volatile organic compounds) emitted from sources such as vehicles and industry. In high enough concentrations, the compound proves destructive to plants, entering through pores in leaves called stomata and reducing the plant’s ability to feed itself and resist environmental stress.

There have already been “huge economic consequences” to high ozone levels around the world, Heald notes. She points to research suggesting that due to ozone exposure, crop yields since preindustrial times are 10% lower than they should be, at a cost of $11–18 billion per year.

Heald’s paper focused on crop yields in coming decades. She quantified the individual and combined effects of mean temperature and ozone pollution levels through 2050, focusing on four of the world’s most significant food crops—wheat, rice, maize, and soybeans. Her study offered some surprises.

“We learned that the effect of ozone pollution can go in either direction,” says Heald. Higher levels of ozone exacerbate the impact of warming to damage crops, lowering yields even more. But in areas with lower ozone levels, crop yields may increase—even in the presence of warmer temperatures.

Rising temperatures are a given globally, with geographic variations, because once in the atmosphere, the greenhouse gas CO2 sticks around for decades. But ozone only persists for a month or two, and levels vary from region to region. “This is because ozone is very connected to domestic air quality management,” says Heald.

For example, in the US and Europe, which impose strict regulations on tailpipe emissions, steady or declining ozone levels should limit harm to crop yields even while the planet warms. But Heald’s research on China detailed two scenarios: one showing an air quality trajectory with improvement, and another with continued degradation, with matching impacts on crop production.

In one of Heald’s worst-case global scenarios—modeling a rise in ozone levels across a number of regions—she predicted undernourishment rates in developing countries would increase by nearly 50% by 2050 due to the effects of air pollution and a warming climate.

Yet Heald believes much can be done to head off even the most dire impacts suggested by her study. Since some crops weather temperature and pollution impacts better than others, Heald says “farmers should understand exactly what is affecting their crops and make informed choices about what to grow.” This requires accurate pollution and climate information, something Heald hopes might come from innovative, small-scale sensors that could provide local ozone measurements around the world, as well as new geostationary satellite observations.

But ultimately, she says, “limiting the environmental risk to food production worldwide requires government intervention to reduce emissions, and society needs to think about tackling CO2 sooner rather than later.”