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Yes, plants need carbon dioxide. No, that doesn’t mean it’s harmless.


Carbon Dioxide is critical for plants. It’s a key part of photosynthesis, the process by which plants turn CO2, water and sunlight into the sugars they need to grow – and the oxygen life on Earth needs to survive.

But narratives that the elevated levels of atmospheric CO2 we’re seeing today – just over 414 parts per million on average as of 2021, the highest they’ve been in the past 800,000 years[1] – are beneficial to plants oversimplify plants’ relationship with CO2, while ignoring the adverse impacts of climate change as a whole. These claims seek to promote alleged benefits of elevated atmospheric CO2 concentrations while downplaying climate-related harms, often asserting that, because plants need CO2 to survive, the high atmospheric CO2 levels we’re seeing today are harmless – or even beneficial.

“It’s looking at one little mechanism while missing the broader picture,” Thomas Bytnerowicz, a postdoc scholar at the University of Texas at Austin, said of the claims.

Scientists have shown, of course, that the climate change caused by elevated atmospheric CO2 concentrations we’re observing today is not harmless; as the IPCC notes in its 2022 Working Group II report: “Human-induced climate change, including more frequent and intense extreme events, has caused widespread adverse impacts and related losses and damages to nature and people, beyond natural climate variability.”[12] Those “widespread adverse impacts” are being felt by plants too – despite their reliance on CO2.

“The benefit[s] of increasing CO2 concentrations for plant growth are increasingly being outweighed by the negative impacts, especially of global warming,” said Sara Vicca, assistant professor at the University of Antwerp. “This is true for natural as well as agricultural ecosystems.”

We spoke to experts to parse out plants’ relationship to CO2 and how higher levels of CO2 and the accompanying climate change are affecting plants – including key crops – now and in the future.

The limits of the land carbon sink

Many claims of CO2’s benefit to plants hinge on the finding that more CO2 in the atmosphere stimulates higher levels of plant growth. The science does support that claim: A 2016 study, which looked at long-term satellite records, found that from 1982–2009, the globe showed “a persistent and widespread increase of growing season integrated LAI [Leaf Area Index] (greening) over 25% to 50% of the global vegetated area.”[2] This greening trend has helped slow climate change – as NASA notes, global greening since the 1980s “may have reduced global warming by as much as 0.2° to 0.25° Celsius…In other words, the world would be even warmer than it is if not for the surge in plant growth.”[3]

Figure 1 – Map shows the “greening” trend across much of the globe between 2000-2018. Green areas show increased greenness, while brown areas show decreased. “Leaf area index” is the amount of leaf area relative to ground area. Credit: NASA.

This cycle of CO2 in the atmosphere being taken up by plants and soil is known as the land carbon sink. It – as well as the ocean carbon sink – is helpful to the planet, as it means not all the excess CO2 humans emit is going towards warming the planet. But it doesn’t mean that the CO2 we’re emitting is harmless.

“The ocean sink, the land carbon sink – they’re only taking up a fraction of the emissions that are being created by humans,” said Bytnerowicz. “So it’s helping us, but the more we put out there, there’s still going to be more in the atmosphere, even if more and more gets taken up by forests and oceans.”

What might happen to the land carbon sink in the future is something that scientists are actively studying, particularly as warming melts permafrost, a major carbon sink.[20]

“We are already seeing the first signs of a decline in the land CO2 sink and increasing extreme heatwaves and droughts seem to be a key reason behind this,” Vicca said. “This means that, as warming progresses, the climate change buffer on land is decreasing, and with unabated warming, there’s a risk that the land would become a source of CO2 to the atmosphere in the long run.”

Additionally, greening can come with its own challenges. The Arctic is one of the regions that has been experiencing greening, as temperatures in the region rise three times faster than the rest of the planet. In addition to the challenges snow cover declines are causing for Arctic communities, a 2022 study found that “greening of the Arctic might not necessarily translate into more net CO2 uptake, as early and peak season carbon gains might be offset by a late-season CO2 loss, and respiration might counterbalance the increase in plant productivity.”[13] That means that, even if Arctic plants continue to grow bigger, denser or stretch into areas that previously had low vegetation, they may not offset climate change as much as scientists had previously hoped.

And when increased greenness comes from agriculture, it also doesn’t have as much of a CO2-storing impact. Whereas trees are long-term carbon storers, intensively-farmed crops are much quicker to release the carbon they’ve captured back into the atmosphere. Agriculture plays a significant role in the greening scientists have observed in recent years: a 2019 study found that “cropland greening contributes the most to the net increase in leaf area globally since 2000 (33%).”[25]

More growth, less nutrition

Crops are sometimes invoked when making claims about CO2’s benefit to plants and the planet. In April, a blog post in the American Thinker claimed that “higher CO2 levels with slightly warmer temperatures increase the productivity of most plants…As the planet greens, dry climates become fertile, supporting plant life which in turn feeds both humans and animals.”

The key part of that claim is “slightly,” said Frances Moore, assistant professor at the University of California Davis. “We find that at warming above ~ 1 degree, the adverse effects of warming outweigh the beneficial effects of CO2 to produce net negative effects on agricultural yields,” Moore wrote in an email.

Figure 2 – Impacts of temperature change on yields of four major crops. Darkest, middle, and lightest lines show responses at the 75th, 50th, and 25th quantiles of baseline growing-season temperature, respectively. Dashed lines show the 95% confidence interval based on 750 block bootstraps, blocking at the study level. Plotted response curves are for temperature only and do not include CO2 fertilization or adaptation. Temperature changes are relative to a local 1995–2005 baseline.[9] Source: Moore et al.

Increased levels of CO2 have been shown to have a complicated relationship with many crops, sometimes leading to more growth, but less nutrition. A 2016 study found that elevated CO2 levels led to “consistent declines in vitamins B1, B2, B5, and B9 and, conversely, an increase in vitamin E” in rice, a key food source for more than 2 billion of the world’s people.[6] The study also confirmed previous studies’ findings of declines in “protein, iron, and zinc.”[7]

“Hidden hunger, that is, the insufficient supply of vitamins and minerals like zinc or iron in diets with sufficient calorie content, currently affects about two billion people and the problem is amplified by food price volatility,” notes a 2014 article published in Nature Climate Change.[19] “Both CO2 fertilization and climate change — which is expected to increase food prices and volatility — will presumably exacerbate hidden hunger.”[19]

A 2014 study also warned of widespread threats to human nutrition from rising CO2 levels, finding that “elevated [CO2] is associated with significant decreases in the concentrations of zinc and iron in all C3 [C3 refers to the type of photosynthesis exhibited by these plants] grasses and legumes.”[8] The study goes on to state:

“The global CO2 [concentration] in the atmosphere is expected to reach 550 ppm in the next 40–60 years, even if further actions are taken to reduce emissions. At these concentrations, we find that the edible portions of many of the key crops for human nutrition have decreased nutritional value when compared with the same plants grown under identical conditions but at present ambient [CO2].”

In addition, climate change could lead to more stress on crops from agricultural pests such as insects. Though the impacts of climate change can vary among geographic locations and insect species – with some impacts, such as changes in temperature and precipitation, being detrimental to insects, and others beneficial – a 2022 study notes that “most climate change scenarios tend to favour pest proliferation worldwide.”[23]

“This is particularly valid in temperate regions, where the cold season currently remains a limiting factor for pest development,” the study continues. “Further, invasive species are predicted to proliferate and expand more easily in temperate than in tropical regions. Overall, the increasing impact of climate change on insect pests is expected to extend into the future, especially as mean global temperature is predicted to increase during the next decades according to all available climatic scenarios.”

These findings have economic impacts as well: a 2017 study found that, even when accounting for the benefits CO2 has on plants, the cost CO2 has on agriculture leads the social cost of carbon to more than double.[9] This adds up significantly, considering a total of about 43 billion tons of CO2 were emitted from human activities in 2019.

And impacts are expected to worsen as the climate warms. One 2021 study found that in the future, heat waves could cause 5-10 times the amount of damage to crops than previously thought.[22] The findings of another 2021 study that used models to predict climate impacts on agriculture suggested that “major breadbasket regions will face distinct anthropogenic climatic risks sooner than previously anticipated.”[18] And a 2022 study projects that, by 2100, “clusters” of extreme weather events – such as extreme rainfall, heat, and drought – will create an increased risk of climate-related failure in corn crops.[24]

The broader impacts of climate change

Atmospheric CO2 does not exist in a vacuum. Instead of simply leading to higher growth rates among plants, it affects the entire climate, leading to warming that can exacerbate heatwaves, drought, wildfires and extreme precipitation – all of which can have an impact on plants.

As temperatures warm, some plants are shifting their ranges further North and into higher latitudes. This movement “creates the potential for novel species interactions,” according to a 2020 study in Nature Climate Change.[14] The introduction of new species into an existing habitat can cause problems for the species that had adapted to that habitat: “Both introduced and range-shifting species have been shown to impact recipient communities by consuming, parasitizing or competing with native species that lack the ability or defenses to overcome them.”[14] Research shows that, as the earth continues to warm, plants will continue to experience range losses: With approximately 3.2°C of warming, range losses of greater than 50% are projected in 44% of plants.[15] Meanwhile, if warming is limited to 1.5°C, those loss projections fall to 8% of plants.[15]

Climate change has also already led to extinction in the plant world. One study found that higher temperatures and longer dry periods contributed to the extinction of two plant species in Germany’s Black Forest – and noted that 10 additional species were at risk of extinction from these climate conditions.[16] And research has shown that endemic plants – plants that live in only one part of the world – are even more at risk than native and introduced plants.[17]

Plants need more than just CO2 to survive – water, for instance, is a key ingredient for plant growth. Scientists have found that higher CO2 levels mean plants use less water for photosynthesis. That’s a result of CO2’s impact on plants’ stomata, tiny pores that plants use for taking in CO2 and releasing water. Elevated atmospheric CO2 can cause plants to partially close their stomata – as well as, in some cases, actually reduce the density of stomata, a finding that has implications for the global water cycle.[4]

But the relationship between elevated CO2 and water usage is complicated: As increased greenhouse gases in the atmosphere cause the planet to warm, these higher temperatures will give plants longer growing seasons – and thus more time to absorb water. And as plants grow bigger in warmer, higher CO2 environments, they’ll need more water, according to a 2019 study.[5]

“For vast continental regions with large populations demanding water, the latest generation of models project that total vegetation water demand will increase with warming, regardless of the increased surface resistance to [evapotranspiration] ET from stomatal closure, which has long been painted as a panacea for predicted continental drying from aridity metrics using [potential evapotranspiration] PET.”[5]

As climate change exacerbates extreme drought in some regions, plants are expected to be further stressed. A 2019 study looked at the impact of droughts on gross primary production (GPP) – the “basis of vegetation growth and food production globally,” the study notes – and found that “the magnitude of globally averaged reductions in GPP associated with extreme droughts was projected to be nearly tripled by the last quarter of this century (2075–2099) relative to that of the historical period (1850–1999) under both high and intermediate GHG [greenhouse gas] emission scenarios.”[21]

“Even though plants can, in many cases, benefit from increased levels of carbon dioxide that are predicted for the future atmosphere, the impact of severe drought on destroying these plants will be extreme, especially in the Amazon, South Africa, Mediterranean, Australia, and southwest USA,” lead study author Chonggang Xu of Los Alamos National Laboratory told ScienceDaily in 2019. 

Where are these claims coming from?

Claims linking plants’ need for CO2 to the idea that elevated CO2 in the atmosphere is harmless have shown up in a variety of places. In addition to the American Thinker, outlets including PragerU, which includes the statement that “CO2 is plant food” as part of a longer case against the proposed Green New Deal in the United States, the Heartland Institute, and the Daily Signal have all published pieces that include some variation on one main claim: that although CO2 concentrations in the atmosphere are rising, there is no need for alarm, because CO2’s role in photosynthesis makes it beneficial for plants (and the planet).

These claims are examples of cherry-picking. By focusing solely on an accurate piece of scientific knowledge – that plants need CO2 for photosynthesis – and its associated findings – that increased CO2 in the atmosphere has been linked to overall global greening – these outlets are ignoring the other, important impacts that increased CO2 and the associated warming has had and is predicted to have on plants.

In May, Climate Realism, a blog run by the Heartland Institute, published a piece that included the unsubstantiated claim that climate change “is boosting crop production, not threatening it.” The blog focuses on a recent study from Emory University, published in Environmental Research Letters, which outlines the changes climate change is predicted to have in the United States’ agricultural regions.

The study found that, as climate change progresses, the U.S.’s growing regions will shift further north, with the Corn Belt “becoming unsuitable to the cultivation of corn by 2100.”[10] Climate Realism contested this finding, claiming that the study was based on data from “seriously flawed” models whose “hindcasts fail to accurately reflect past and present temperatures and climate conditions” and whose future projections “are ‘way too hot.’”

Emily Burchfield, assistant professor at Emory University and author of the study, said in an email that the models used for the study were highly effective at predicting past and future temperatures.

“The historical temperature data I use to calibrate the models is from the DayMet project, which is a widely used dataset describing historical climate + weather patterns using interpolated gridded products built from actual daily historical meteorological observations,” she said.

Climate Realism also stated in their blog that “there is no real-world evidence that continued warming will change precipitation patterns or increase droughts, floods, or heatwaves in the Mid-West, in ways that will harm crop production.”

In fact, in 2018 scientists published a study that used satellite observations to show how and where freshwater availability is changing around the world. The study found that freshwater “is rapidly disappearing in many of the world’s irrigated agricultural regions.”[11] In California’s crop-rich Central Valley, the study notes, “annual water demands for agriculture have exceeded renewable water resources since the early 20th century.” Drought has led to groundwater declines in the region, and winter snowpack declines in the Sierra Nevada Mountains are also cause for concern, due to the snowpack’s key role in the valley’s surface water supply.

“If you accept the scientific consensus of thousands of folks working independently around the world to try to support Midwestern farmers and Midwestern food production in the future, then I’d say we have strong consensus that climatic changes in the Midwestern U.S. will render cultivation of the crops that currently dominate the region far more difficult in the future,” Burchfield said. “As I say in the paper, the best evidence that the scientific community can provide suggests that agricultural adaptation to climate change in the Central and Eastern U.S. will be necessary and inevitable.”

References

     

Published on: 23 Jun 2022 | Editor: