Samuel Myers is a principal research scientist at the Harvard Chan School of Public Health and director of the Planetary Health Alliance.

Feeding a planet inhabited by 10 billion people by mid-century — already a daunting task — is getting harder due to a little-known impact of global warming: the decline of essential nutrients in the world’s staple foods that exist in almost every single person’s diet around the world.

The mechanism by which rising carbon dioxide saps nutrients from our food crops remains somewhat unclear, but the effect is consistent across most plant types from trees to grasses to edible crops: It is reducing the availability of zinc, iron, protein and key vitamins in wheat, rice and several other fundamental grains and legumes.

The implications are huge: By 2050, hundreds of millions of people could slip below the minimum thresholds of these nutrients needed for good health, and more than 2 billion already deficient could see their conditions worsen. And it extends well beyond human nutrition as every animal in the biosphere depends, directly or indirectly, on plant consumption for nutrients.

These findings, which will appear this week as part of the most comprehensive review ever compiled on the two-way relationship between global warming and land use, highlight the urgent need to slash the greenhouse gas emissions that drive climate change. Human activity has increased atmospheric carbon more than 40 percent since the mid-19th century, enough to unleash a deadly onslaught of extreme weather made more destructive by rising seas. Without a drastic drop in emissions, those levels will climb even more quickly over the coming decades.

Scientists from the United Nations' Intergovernmental Panel on Climate Change are meeting in Geneva this week to validate a 30-page summary for policymakers of a 1,000-page underlying report. Food security is high on the agenda.

Nutritional deficiencies continue to take a heavy toll. Zinc deficiency affects the immune system and increases vulnerability to malaria, lung infections and deadly diarrheal diseases, claiming the lives of some 30,000 children younger than 5 each year. Protein deficiency causes stunting and increases infant mortality. Iron deficiency is linked to nearly 60,000 deaths and 34 million “life years” lost to disability or premature death every year, and can also result in decreased work capacity, reduced IQ and anemia.

Humans are deeply vulnerable to reductions in the nutrient content of staple food crops. We get 60 percent of dietary protein, 80 percent of iron and 70 percent of zinc requirements from plants, most of which are losing these nutrients in response to rising carbon dioxide levels.

Research I have co-written indicates that as a result of these emissions, nearly 2 percent of the global population — an extra 175 million people — could become zinc-deficient, and 122 million would no longer get enough protein. Some 1.4 billion women and children younger than 5 would find their iron intake reduced by 4 percent or more. Half a billion in this group risk developing iron-deficiency-related disease.

By 2050, the vitamin B content of rice is expected to drop 17 to 30 percent, upping the risk of deficiencies in folate (B9), thiamine (B1) and riboflavin (B2) for tens of millions of people, especially in regions dependent on rice. All these vitamins are crucial for normal and healthy development.

The reason for this is still a bit of a mystery. There are theories, such as that more carbon dioxide causes plants to produce more starch, which could have a diluting effect whereby plants become carbohydrate-rich and nutrient-poor. But that’s not the case for all nutrients; the science has a long way to go before we have sound answers.

We do know, however, that when the carbon dioxide effect is combined with the impact of climate change on crop yields, we see even larger reductions in the availability of nutrients in the global diet. Compared with a world without these effects, we anticipate a 14 to 20 percent reduction in the global availability of iron, zinc and protein by 2050, which would threaten large segments of the global population with nutrient deficiencies.

Supplements and vitamins could temporarily alleviate some of the health consequences, but these options have existed for decades and have not protected the billions of people who already suffer from nutrient deficiencies, in part because they are difficult to distribute and do not address the underlying cause of malnutrition.

The countries hit hardest are primarily those that have contributed the least to global carbon emissions, particularly nations in South Asia, the Middle East, sub-Saharan Africa and North Africa, and the former Soviet Union. India would also be hit especially hard, and there would be dramatic increases in zinc and protein deficiencies in China, Indonesia, Bangladesh, Brazil, Kenya and other emerging economies.

The bottom line is frighteningly clear: Unless governments dramatically step up their emissions-reduction efforts, nutritional deficiencies and their associated burdens are set to become even more severe and widespread. We cannot wait to act any longer.

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