CO2: Friend or Foe to Agriculture?


Rep. Lamar Smith said climate change “alarmists” ignore the “positive impacts” of more carbon dioxide in the atmosphere, such as increased food production and quality. But the impact of increased CO2 levels on agriculture is more complicated than that — and, on balance, likely negative, particularly in the future.

Other factors aside, an atmosphere with more CO2 does boost crop yield in the short term via increased rates of photosynthesis. In the long term, multiple experts told us the positive effect of increased CO2 on crops will diminish and the negative impacts of climate change, such as higher temperatures and extreme rainfall, will grow.

Smith, the chairman of the House Committee on Science, Space & Technology, made his claim in a July 24 op-ed published in the Daily Signal, a news website created by the conservativeHeritage Foundation.

Smith, July 24:  A higher concentration of carbon dioxide in our atmosphere would aid photosynthesis, which in turn contributes to increased plant growth. This correlates to a greater volume of food production and better quality food. Studies indicate that crops would utilize water more efficiently, requiring less water. And colder areas along the farm belt will experience longer growing seasons.

In making his claim, Smith also argued, “The American people should be made aware of both the negative and positive impacts of carbon dioxide in the atmosphere,” adding, “Without the whole story, how can we expect an objective evaluation of the issues involving climate change?”

We agree. Below, we take a look at both the pros and cons of increased CO2 on agriculture.

Carbon Dioxide’s Diminishing Return

Let’s take a look at Smith’s claims one by one. First, does a “higher concentration of carbon dioxide in our atmosphere … aid photosynthesis, which in turn contributes to increased plant growth,” as Smith said?

Yes, but to a point.

During photosynthesis, plants use energy from sunlight to convert CO2 and water into oxygen and glucose, a sugar molecule. Plants then release oxygen from their leaves, but they also combine oxygen with glucose to produce energy for growth through a different process called respiration.

The United Nations Intergovernmental Panel on Climate Change’s 2014 report does say that increased atmospheric CO2 has “virtually certainly enhanced [crop] water use efficiency and yields.” So, Smith is right that more CO2 leads to more photosynthesis, which correlates to increased crop yields. And he’s also right that “[s]tudies indicate that crops would utilize water more efficiently” in an atmosphere with more CO2.

But the IPCC adds that the CO2 effect has a greater impact on wheat and rice, than on corn and sugarcane.

Photosynthesis in wheat and rice relies more on CO2 in the atmosphere, while corn and sugarcane rely more on “internal cycling” during photosynthesis, Jerry Hatfield, the director of the U.S. Department of Agriculture’s National Laboratory for Agriculture and The Environment, explained to us over the phone.

In other words, increased CO2 doesn’t boost crop yield equally across the board.

Hatfield, who was also part of the IPCC process that received the 2007 Nobel Peace Prize and who currently serves on an IPCC special committee, also explained to us that the positive impacts of CO2 may “reach a point of diminishing return,” or “saturation,” in the future. What does that mean?

Right now, the concentration of CO2 in the atmosphere is just over 400 parts per million, according to NASA. (For comparison, before 1950, the level of CO2 hadn’t surpassed 300 ppm for hundreds of thousands of years.)

Hatfield told us that plants would reach CO2 saturation at around 550 to 600 ppm, at which point the more gas “won’t be as beneficial.”

In an email, Frances Moore, an assistant professor studying climate change’s impact on agriculture at the University of California, Davis, put it this way: “My research does show that higher CO2 concentrations are beneficial to crops, but this effect quickly declines at higher and higher concentrations because plant growth becomes limited by other nutrients.”

Higher levels of COwouldn’t necessarily be harmful to crops, added Hatfield. Still, “we know so little about the effects of super high concentrations of CO2 on plant growth,” he said.

At an increase of 3 ppm per year, the rate in 2015 and 2016, according to the National Oceanic and Atmospheric Administration, the Earth would reach saturation well before the end of the century. Since 1960, the rate has fluctuated, so it could decrease, but the trend generally shows an increasing rate.

Better Quality Food?

In his op-ed, Smith also said increased CO2 correlates to “better quality food.” We reached out to his office to get some clarification on what the chairman meant by “better quality.”

Alicia Criscuolo, a press assistant for the House science committee, told us by email, “Chairman Smith uses ‘quality’ as a term to encompass a wide range of benefits,” such as a “rise in production and size of plants grown in a CO2 enhanced environment” and an “increased concentration of vitamin C that results from increased CO2 exposure.”

Specifically, his office pointed us to two papers, one about strawberries and another concerning sour oranges.

The paper about strawberries, published in Photosynthesis Research in 2001, didn’t exactly conclude that increased CO2 “leads to an increase in biomass and overall production of strawberries,” as Criscuolo said in an email to us.

Rather, the study, authored by USDA collaborator James A. Bunce, investigated how other factors, such as temperature and soil quality, affected a strawberry plant’s propensity to increase its photosynthesis rate in an environment with elevated CO2 levels. While the study did show that strawberries photosynthesize more with increased CO2 levels, it didn’t look at strawberry quantity or quality.

The paper about sour oranges, published in the journal Agriculture, Ecosystems & Environment in June 2002, found that when a 75 percent increase in CO2 levels — from 400 ppm to 700 ppm — doubles fruit production, it also increases the vitamin C concentration of the fruit’s juice by 7 percent.

It’s important to note two things about this study. First, its primary author, Sherwood B. Idso, is the president of the Center for the Study of Carbon Dioxide and Global Change, a nonprofit that denies that increased CO2 causes global warming. Second, sour oranges shouldn’t be confused with juicing oranges. Sour oranges are mostly used to make marmalade.

We also asked Samuel B. Myers, a senior research scientist at Harvard studying the human health impacts of climate change, what he thought of the idea that increased atmospheric CO2will lead to “better quality food,” as Smith said.

“Rep. Smith’s claim about better quality food is pure fabrication,” he told us by email. “All our research shows that rising concentrations of CO2 reduce the nutritional value of staple food crops,” such as wheat, barley and rice. “We have shown … that staple food crops lose significant amounts of iron, zinc, and protein (critical nutrients for human health) when grown in open-field conditions” at elevated CO2 levels, he said, though scientists aren’t sure why increased CO2 leads to decreased nutrients in staple crops.

In fact, earlier this month, Myers and colleagues published a paper in Environmental Health Perspectives that found that “an additional 1.6% or 148.4 million of the world’s population may be placed at risk of protein deficiency” because of elevated CO2 levels.

Longer Growing Seasons?

In his op-ed, Smith also claimed that, due to increased CO2, “colder areas along the farm belt will experience longer growing seasons.” This is true, but warmer regions, such as the southern states, will also experience negative effects because of climate change.

To support his claim, Smith’s office pointed us to a June 2014 paper in Nature by Melissa Reyes-Fox, a technician at the USDA, and others. The paper explains that scientists have previously found evidence to suggest that global warming has caused a lengthening of the growing season in temperate and polar regions of the Earth.

Reyes-Fox and her group found that a longer growing season, especially when water is a limiting factor, “is not due to warming alone, but also to higher atmospheric CO2concentrations.” However, the researchers didn’t look at food crops, but a grassland in Wyoming.

Still, the IPCC’s 2014 report does say with “high confidence that warming has benefitted crop production in some high-latitude regions, such as northeast China or the UK,” and that “high-latitude locations will, in general, become more suitable for crops.” This is due, in part, to the fact that “declines in frost occurrence will lead to longer growing seasons,” the report says.

However, this “latitudinal expansion of cold-climate cropping zones polewards … may be largely offset by reductions in cropping production in the mid-latitudes as a result of rainfall reduction and temperature increase,” the IPCC adds. “For tropical systems where moisture availability or extreme heat rather than frost limits the length of the growing season, there is a likelihood that the length of the growing season and overall suitability for crops will decline.”

Fewer frost days may also negatively impact fruit and nut trees, Hatfield, at the USDA, told us. The IPCC and the U.S. Global Change Program make similar conclusions in their reports.

The Global Change report explains, for example, that fruit and nut trees “have a winter chilling requirement,” or a number of hours a year where temperatures are between 32 and 50 degrees F, ranging from 200 to 2,000 hours depending on the type of tree. These temperatures signal fruiting trees to develop flower buds in the spring.

But not all crops and not all regions will be affected in the same way.

“Projections show that chilling requirements for fruit and nut trees in California will not be met by the middle to the end of this century,” the Global Change report says. However, the report adds that scientists expect apples in the Northeast to have sufficient chilling hours for the rest of the century, though this might not be the case for plums and cherries in the region.

The IPCC report also points out, “Several studies have projected negative yield impacts of climate trends for perennial trees, including apples in eastern Washington … and cherries in California … although CO2 increases may offset some or all of these losses.”

The projections for wine and coffee are even less favorable. Increasing temperatures associated with rising COemissions are likely to reduce the area suitable for grapes used to produce the highest-quality wines “by more than 50% by late this century,” the Global Change report says.  And coffee production in Costa Rica, Nicaragua and El Salvador “will be reduced by more than 40%,” according to the IPCC report.

Unreliable Rainfall

Smith didn’t address how changes in rainfall might affect agriculture in the future. But all the experts we spoke with emphasized the importance of reliable water availability, in addition to temperature and CO2, for crop production and quality. For this reason, it’s worth outlining how climate change will change precipitation patterns.

First, as we’ve written before, scientists are more confident when linking temperature-related weather to global warming than they are linking precipitation changes to global warming. But there is still plenty of evidence to suggest global warming will affect rainfall patterns across the globe.

Hatfield, at the USDA, explained to us that crops generally prefer steady rainfall during the summer, when the most growth occurs. But climate change, due to increased CO2, is causing the U.S. to see more precipitation in the form of spring storms.

The Global Change report also makes a note of this.

The Midwest, for example, is seeing “increasing intensity of storms and the shifting of rainfall patterns toward more spring precipitation,” the report says. In Iowa in particular, there hasn’t been an increase in total precipitation per year, but there has been a “large increase in the number of days with heavy rainfall,” the report adds.

Extreme rainfall is bad for crops for a number of reasons, one being that it leads to soil erosion. During these weather events, the nutrients from the soil are washed away into nearby lakes and rivers, polluting them. The extreme rainfall then leaves the soil less capable of supporting crop growth, the Global Change report adds.

Yet More Cons to CO2

Increased CO2 can also negatively impact crop production by disproportionately benefiting weeds, says Global Change report. Hatfield explained to us that weeds are genetically diverse and, as a result, can adapt to changing environments. Crops, on the other hand, are, by default, inbred and genetically uniform. For this reason, they aren’t as adaptable to changing environments.

There are also other negative effects of burning fossil fuels — such as an increase in ground-level ozone, which hinders photosynthesis and other important plant functions, as the IPCC explains in its report“This results in stunted crop plants, inferior crop quality, and decreased yields … and poses a growing threat to global food security,” the report adds.

Overall, every expert we spoke with said the net impact of CO2 and climate change will leave crop production and quality worse off in the future, not better.

For example, Myers, at Harvard, told us, “While there may be a small fertilization effect of elevated CO2 on plant growth, this increase will be more than offset by climate change which is causing increased temperatures, changes in precipitation, and complex changes in agricultural pests, pathogens and pollinators.” 

Moore, at the University of California, Davis, also told us: “Considering just CO2 fertilization and the effect of higher temperatures, we find that at very small amounts of warming (i.e. one degree C) the net effect might be a slight increase in crop yields.” (Since 1880, the Earth has warmed nearly 1 degrees C already, according to NASA.)

But Moore added that “at higher levels of warming, the negative effect of higher temperatures rapidly comes to dominate the positive effect of CO2 fertilization, causing crop yields to decline markedly, including in the United States.” And that doesn’t even take into account other negative effects, such as “disruptive rainfall patterns” and benefits to weeds, she said.

So Smith is right that there are some positive sides to increased CO2 in the atmosphere, but the net impact is likely negative, especially in the future.

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