While it is true that plants need carbon dioxide, it is equally true that excessively high levels can be dangerous. In rice, they reduce the content of vitamins and minerals, threatening the future of billions of people.
Italians are very fond of rice, but for more than two billion people it is the primary food source. Inhabitants of countries such as Bangladesh, Madagascar and Vietnam derive half of their daily calories from it. And not just calories, but also vitamins and minerals that, because of climate change, could become scarce.
High levels of carbon dioxide (CO2) in the air, in fact, reduce the nutritional quality of rice. This was revealed by an international research team in a study published in the journal Science Advances. Last month, CO2 in the atmosphere reached its highest level in 800,000 years, with an average exceeding 440 ppm. By the end of the century it is expected to reach 570 ppm: scientists wanted to understand what the consequences will be.
From 2010 to 2014, researchers subjected 18 varieties of rice to CO2 levels of between 560 and 590 ppm, releasing the gas into fields through pipes installed at two sites in China and Japan. As a result of the exposure, the proteins in rice fell by 10 per cent, iron by 8 per cent and zinc by 5. B vitamins dropped even further; vitamin B9 in particular declined by 30 per cent: also known as folic acid, it is essential during pregnancy to ensure the correct development of the foetus's nervous system.
According to scientists, the poor nutritional quality of rice could have repercussions on children's growth, but could also cause intestinal diseases and even increase the risk of contracting malaria. The most severe consequences are set to be felt in the poorest countries.
It was already known that harvests of cereals such as rice and maize could decrease by as much as 40 per cent by 2100 due to global warming in tropical and subtropical regions, without even factoring in the impact of increasingly frequent extreme weather events. A further piece of the puzzle is now added, one that "is an underestimated risk of the use of fossil fuels and deforestation," as one of the study's authors, Professor Kristie Ebi of the University of Washington (Seattle), commented.
One of the reasons why nutrients in rice decline as carbon dioxide increases — which, incidentally, we know to be indispensable in the plant world — could be that plants grow more quickly. With regard to vitamins, the researchers suggest that it is those containing nitrogen that decrease: for this very reason, the solution to the problem could be to find or develop cereal varieties that respond better to climate change. It is therefore essential to continue research, both to understand which crops are threatened and to find ways to protect them — and thereby save billions of people.
