Calories up, nutrition down

It's a familiar photo. Rows of crops under a greenhouse roof, a CO2 injection line running along the gutter, a claim underneath: "CO2 is plant food — more of it makes plants grow bigger." It's usually posted as a rebuttal to climate concern, carrying an unstated implication: that rising CO2, whatever else it does, isn't something to worry about where the food supply is concerned.

The claim is true. The implication is not.

Most of the world's crops aren't grown in greenhouses. And in an open field, more CO2 comes with a catch.

Crop yield increases, nutrient density decreases

Elevated CO2 stimulates photosynthesis, which is precisely why commercial greenhouse growers pump extra CO2 into their tomato and cucumber houses: it measurably increases growth and yield. And it does the same in C3 crops (the term botanists use for plants including staples like wheat, rice, and soy) — most of what feeds the planet.

Two things happen inside a single plant at the same time as that growth boost, though.

The first is dilution, laid out in a 2014 meta-analysis by Irakli Loladze. A plant fixes carbon faster than it takes up nitrogen and minerals from the soil, so the same pool of protein and micronutrients ends up spread through a larger mass of starch and sugar. The plant isn't losing nutrients so much as gaining carbohydrate around them.

The second is subtler and, on its own, arguably the more interesting mechanism, described in a 2013 study by McGrath and Lobell. Elevated CO2 causes plants to partially close their stomata, which reduces transpiration — the flow of water pulled up from the soil through the plant and out through the leaves. A lot of mineral nutrition doesn't move into the plant under its own power. It rides along dissolved in that water flow. Less transpiration means less of those minerals arrive at all, independent of how big the plant has become.

Faster carbon fixing dilutes every nutrient; reduced transpiration cuts the delivery of the minerals that travel in the water on top of that. Yield goes up, but nutrient density goes down.

Field trials confirm the pattern

Free-air CO2 enrichment experiments — FACE trials — show the same effect in crops grown outdoors: increased size, reduced nutrient density. FACE trials use open-air rings to raise CO2 over crops growing in ordinary fields, exposed to real weather, real soil, and real water limits, rather than an artificial growing environment.

The biggest FACE dataset on this question comes from Myers et al.'s 2014 study, which compared crops grown at roughly 200 ppm CO2 above the ambient baseline with the same crops grown at baseline levels. Wheat grain showed lower zinc, iron, and protein than the same cultivars grown at today's CO2 levels — a 2008 meta-analysis by Taub, Miller and Allenhad already found protein concentration falling across major food crops under elevated CO2. C3 legumes lost zinc and iron too, though not protein.

More recently, a November 2025 meta-analysis from Leiden University — the largest to date, drawing on roughly 59,000 samples across 43 crops — put a number on the average effect: a 3.2% decline in nutrient concentration across all crops and nutrients, with individual crop-nutrient pairs losing far more. Zinc in chickpeas came out down by 37.5%, the single largest drop in the dataset. And the same study found calorie content rising even as nutrient density fell — record yield and reduced nutrient density moving in the same study, in the same direction, at the same time.

A greenhouse is not a wheat field

The greenhouse photo is irrelevant to a wheat field. Commercial CO2-enriched greenhouses are fertigated: growers deliver nutrient solution straight to the root zone on a controlled schedule, sidestepping the transpiration bottleneck. Dilution still operates — but these crops are tomatoes, cucumbers, peppers, and leafy greens, grown for size, shelf life, and yield, not nutrient density, under conditions sharing almost nothing with a wheat field that depends on rain and soil chemistry. Nobody's measuring the zinc in a greenhouse cucumber.

Pointing to the greenhouse doesn't disprove the mechanism — it demonstrates what it takes to hide it.

More food, less nutrition

More CO2 delivers on the photo's promise: yields increase. And intuitively, higher yields should mean less hunger. But a diet isn't just calories. Grain that's heavier on the scale but lighter in zinc, iron, and protein doesn't leave people any better nourished.

The stakes aren't abstract, either. Around two billion people already depend on C3 grains and legumes for most of their dietary zinc and iron, deficiencies of which already cost an estimated 63 million life-years a year, before any of this. Nutritionists have a name for this: hidden hunger — a shortfall in vitamins and minerals that persists even when someone is eating enough calories. This isn't some marginal group at the edge of the food system. It's the people already closest to the deficiency line.

The same CO2 that's increasing yields is, paradoxically, increasing hunger.

Written in collaboration with Claude (Anthropic).