Throughout the Yangtze River Delta, a region in southern China famed for its widespread rice production, farmers grow belts of slender green stalks. Before they reach several feet tall and turn golden brown, the grassy plants soak in muddy, waterlogged fields for months. Along the rows of submerged plants, levees store and distribute a steady supply of water that farmers source from nearby canals.
This traditional practice of flooding paddies to raise the notoriously thirsty crop is almost as old as the ancient grain’s domestication. Thousands of years later, the agricultural method continues to predominate in rice cultivation practices from the low-lying fields of Arkansas to the sprawling terraces of Vietnam.
As the planet heats up, this popular process of growing rice is becoming increasingly more dangerous for the millions of people worldwide that eat the grain regularly, according to research published Wednesday in the journal Lancet Planetary Health. After drinking water, the researchers say, rice is the world’s second largest dietary source of inorganic arsenic, and climate change appears to be increasing the amount of the highly toxic chemical that is in it. If nothing is done to transform how most of the world’s rice is produced, regulate how much of it people consume, or mitigate warming, the authors conclude that communities with rice-heavy diets could begin confronting increased risks of cancer and disease as soon as 2050.
“Our results are very scary,” said Donming Wang, the ecological doctorate student at the Institute of Soil Science, Chinese Academy of Sciences who led the paper. “It’s a disaster … and a wake-up call.”
Back in 2014, Wang and an international team of climate, plant, and public health scientists started working together on a research project that would end up taking them close to a decade to complete. Wading through rice paddies across the Yangtze Delta, they sought to find out just how projected temperatures and levels of atmospheric CO2 in 2050 would interact with the arsenic in the soil and the rice crops planted there. They knew, from past research, that the carcinogen was a problem in rice crops, but wanted to find out how much more of an issue it might be in a warming world. The team didn’t look at just any rice, but some of the grain varieties most produced and consumed worldwide.
Although there are an estimated 40,000 types of rice on the planet, they tend to be grouped into three categories based on length of the grain. Short-grain rice, or the sticky kind often used in sushi; long-grain, which includes aromatic types like basmati and jasmine; and medium-grain, or rice that tends to be served as a main dish. Of these, the short-to-medium japonica and long-grain indica are the two major subspecies of cultivated rice eaten across Asia. Wang’s study modelled the growth of 28 varieties of japonica, indica, and hybrid rice strains central to cuisine for seven of the continent’s top rice consuming and producing countries: Bangladesh, China, India, Indonesia, Myanmar, Philippines, and Vietnam. India, Vietnam, and China are among the group of eight nations that lead the rest of the world in rice exports.
After nearly a decade of observing and analyzing the growth of the plants, the researchers discovered that the combination of higher temperatures and CO2 encourages root growth, increasing the ability of rice plants to uptake arsenic from the soil. They believe this is because climate-related changes in soil chemistry that favor arsenic can be more easily absorbed into the grain. Carbon-dioxide enriched crops were found to capture more atmospheric carbon and pump some of that into the soil, stimulating microbes that are making arsenic.
The more root growth, the more carbon in the soil, which can be a source of food for soil bacteria that multiply under warming temperatures. When soil in a rice paddy is waterlogged, oxygen gets depleted, causing the soil bacteria to rely further on arsenic to generate energy. The end result is more arsenic building up in the rice paddy, and more roots to take it up to the developing grain.
These arsenic-accumulating effects linked to increased root growth and carbon capture is a paradoxical surprise to Corey Lesk, a Dartmouth College postdoctoral climate and crop researcher unaffiliated with the paper. The paradox, said Lesk, is that both of these outcomes have been talked about as potential benefits to rice yields under climate change. “More roots could make the rice more drought-resistant, and cheaper carbon can boost yields generally,” he said. “But the extra arsenic accumulation could make it hard to realize health benefits from that yield boost.”
Arsenic comes in many different forms. Notoriously toxic, inorganic arsenic — compounds of the element that don’t contain carbon — is what the World Health Organization classifies as a “confirmed carcinogen” and “the most significant chemical contaminant in drinking-water globally.” Such forms of arsenic are typically more toxic to humans because they are less stable than their organic counterparts and may allow arsenic to interact with molecules that ramp up exposure. Chronic exposure has been linked to lung, bladder, and skin cancers, as well as heart disease, diabetes, adverse pregnancy, neurodevelopmental issues, and weakened immune systems, among other health impacts.
Scientists and public-health specialists have known for years that the presence of arsenic in food is a mounting threat, but dietary exposure has long been considered much less of a risk in comparison to contaminated groundwater. So policy measures to mitigate the risk have been slow-going. The few existing standards that have been enacted by the European Union and China, for example, are considered inconsistent and largely unenforced. No country has formally established regulations for organic arsenic exposure in foods. (In the U.S., the Food and Drug Administration has established an action level of 100 parts per billion of inorganic arsenic in infant rice cereal, but that recommendation for manufacturers isn’t an enforceable regulation on arsenic in rice or any other food.)
Wang hopes to see this change. The levels of inorganic arsenic commonly found in rice today fall within China’s recommended standards, for example, but her paper shows that lifetime bladder and lung cancer incidences are likely to increase “proportionally” to exposure by 2050. Under a “worst case” climate scenario, where global temperatures rise above 2 degrees Celsius and are coupled with CO2 levels that increase another 200 parts per million, the levels of inorganic arsenic in the rice varieties studied are projected to surge by a whopping 44 percent. That means that more than half the rice samples would exceed China’s current proposed limit, which limits 200 parts per billion for inorganic arsenic in paddy rice, with an estimated 13.4 million cancers linked to rice-based arsenic exposure.
Because these health risks are in part calculated based on body weight, infants and young children will face the biggest health burdens. Babies, in particular, may end up facing outsize risks through the consumption of rice cereals, according to the researchers.
“You’re talking about a crop staple that feeds billions of people, and when you consider that more carbon dioxide and warmer temperatures can significantly influence the amount of arsenic in that staple, the amount of health consequences related to that are, for lack of a better word, enormous,” said study coauthor Lewis Ziska, a plant biologist researching climate change and public health at Columbia University.
But everyone should not suddenly stop eating rice as a result, he added. Though the team found the amount of inorganic arsenic in rice is higher than a lot of other plants, it’s still quite low overall. The key variable is how much rice a person eats. If you are among the bulk of the world that consumes rice multiple times a week, this looming health burden could apply to you, but if you do so more sporadically, Ziska says, the inorganic arsenic you may end up exposed to won’t be “a big deal.”
In that way, the study’s projections may also deepen existing global and social inequities, as a big reason rice has long reigned as one of the planet’s most devoured grains is because it’s also among the most affordable.
Beyond mitigating global greenhouse gas emissions — what Ziska calls “waving my rainbows, unicorns, and sprinkles wand” — adaptation efforts to avoid a future with toxic rice include rice paddy farmers planting earlier in the season to avoid seeds developing under warmer temperatures, better soil management, and plant breeding to minimize rice’s propensity to accumulate so much arsenic.
Water-saving irrigation techniques such as alternate wetting and drying, where paddy fields are first flooded and then allowed to dry in a cycle, could also be used to reduce these increasing health risks and the grain’s enormous methane footprint. On a global scale, rice production accounts for roughly 8 percent of all methane emissions from human activity — flooded paddy fields are ideal conditions for methane-emitting bacteria.
“This is an area that I know is not sexy, that doesn’t have the same vibe as the end of the world, rising sea levels, category 10 storms,” said Ziska. “But I will tell you quite honestly that it will have the greatest effect in terms of humanity, because we all eat.”
