Rising carbon dioxide levels: Win, lose or draw for yields?

No matter where you fall on the climate change belief spectrum, one trend is now fact. Carbon dioxide levels in the atmosphere are significantly higher today than they were for centuries. Sharp increases began in the late 20th century and continue. 

The late Jim Newman, a pioneering 20th century ag climatologist at Purdue University, noted that by 2000, carbon dioxide levels had risen high enough to declare that climate change, at least based on CO₂ levels, was real.

What is less clear is whether increasing carbon dioxide levels help or hurt crop yields. Or does it make any difference? That’s because the impacts of environment on crops are complicated, and because major crops — including corn, soybeans and wheat — have built-in differences that determine how changing carbon dioxide levels might affect them.

According to the Indiana Climate Change Impacts Assessment Report issued by Purdue University ag researchers in 2018: “Higher concentrations of CO₂ can lead to increased sugar production, and in turn, increased plant growth and higher yields. Research also shows that plants use water more efficiently when grown in high-CO₂ environments, also improving yields.” 

However, the report states that corn and soybeans, as noted, are different, and that higher CO₂ may help soybeans more than corn. In fact, higher average temperatures, also forecast for the future, could offset any gain for corn and result in reduced corn yields.

In 2023, Federal Reserve Banks began studying possible impacts of climate change on farm profitability. A 2023 report from the Chicago Federal Reserve Bank indicated that negative impacts of climate change — led by more storms, drought, floods and increased extremes in weather — would likely offset any positive changes of climate change, and lead to negative yield impacts.

Closer look at carbon dioxide

Against this backdrop of uncertainty, Mark Jeschke, agronomy manager with Pioneer, attempted to drill down to what farmers really need to know about carbon dioxide. He penned “C3 and C4 Photosynthesis — Implications for Crop Production” in the 2025 Pioneer Agronomy Research Summary.

“Plant breeders and researchers are trying to increase photosynthetic efficiency at the same time that CO₂ levels are rising dramatically,” Jeschke says. He notes that for an estimated million years, CO₂ levels fluctuated between 180 and 300 parts per million. Over the past 150 years, and especially the past 50 years, levels have risen quickly to 420 ppm.

“The real question is, how will continued rising CO₂ levels impact photosynthetic activity in crops like corn and soybeans going forward?” Jeschke says. “Since soybeans are C3 plants and corn plants are C4, we must consider them separately.”

How C3 and C4 plants differ. Soybeans, wheat and about 90% of all known 300,000 plant species fall into the C3 class. They incorporate CO₂ into carbohydrates using the Calvin cycle. The key enzyme, known as RuBisCo, reacts with oxygen instead of CO₂ about 25% of the time, producing a toxic substance that plants must detoxify through photorespiration. It is a wasteful process that consumes energy.

About 3% of all plant species, including corn and grain sorghum, evolved a modification that greatly reduces energy losses to photosynthesis, Jeschke explains. In these plants, another enzyme, PEPC, carries out a second step. The result is that C4 plants are more efficient and productive.

Higher CO₂ and C3 plants. Plants like soybeans benefit from higher CO₂ levels because photorespiration goes down and photosynthesis efficiency goes up, Jeschke says. However, studies show that extra CO₂ increases vegetative growth by over 20% but yield by less than 10%. Plus, a multiyear study showed that the yield kick from more CO₂ disappeared under drought stress.

Higher CO₂ and C4 plants. With adequate moisture, field trials show no significant yield benefit in corn from higher CO₂ levels. “We expected it since C4 plants already suppress photorespiration on their own,” Jeschke says. However, under drought stress, higher CO₂ levels confer an advantage to C4 plants: Stomata stay closed because they have adequate CO₂. That conserves water.

Net effect on crops. “Higher CO₂ concentrations can benefit yields of both crops under certain conditions,” Jeschke notes. “However, the changes to climate brought about by higher CO₂ levels also bring other changes which are negative to yield.”

These changes include higher average temperatures, more weather extremes, and more floods and droughts. These are all things noted in both the Purdue climate assessment report and the Chicago Federal Reserve Bank assessment. Carbon sequestration, through changes in farming methods including using cover crops and no-till, helps but likely won’t offset bigger global forces at play impacting climate change and weather patterns.

“Globally, the net effect of higher CO₂ levels and climate change on crop production is expected to be close to zero over coming decades as positive and negative effects roughly cancel each other out,” Jeschke says. 

He adds, though, that important changes might be made at regional or local levels.