Can Plants Overdose on CO2?
Atmospheric concentrations of carbon dioxide (CO2) have been steadily rising, from approximately 316 ppm in 1959 to a current atmospheric average of 407 ppm (Mauna Loa Observatory). Antarctic ice-core records show “at no point during the last 800,000 years have CO2 levels been as high as they are now.” (Climate Central, 2013). And, current projections forecast CO2 levels will nearly double by 2100.
While debates concerning the impact of elevated CO2 levels (eCO2) on climate change are slowly converging on agreement of CO2 induced global warming, public opinion seems to diverge on whether high levels of atmospheric CO2 are beneficial or harmful to plant life.
This stands to reason, since it universally known that plant life depends on atmospheric carbon dioxide, light energy from the sun, water and nutrients to produce oxygen and sugars that builds roots, stems and leaves – cell wall, biomass and seeds – during photosynthesis. Without carbon dioxide, plants cannot get carbon and therefore, cannot live. Common logic then suggests that, as the level of atmospheric CO2 increases, so would plant growth. But is this true?
To find the answer, this paper surveyed thirteen scientific studies, from September 1992 through April 2016, investigating the impact of elevated levels of CO2 on plant growth. Understandably, this review constitutes a micro view of the vast number of research projects on this subject, which possibly goes back as far as 1779 when Jan Ingenhousz, a Dutch scientist, discovered photosynthesis.
This article gives only a brief statement on the results of each study. Methodologies used in these investigations range from a compilation of prior research projects to in-situ and ex-situ experimentation. Studies also involved mathematical modeling, genetic variations, nitrogen availability, temperature and rainfall variations, protein buildup, CO2 levels up to 1000 ppm, and rain-forest ecologies. To draw your own conclusions, the title of each study links to the published paper, abstract or press release.
Whatever the case, for those that have faith in the scientific community’s ability to find the truth, this article should have value. For those that think scientists conspire to deceive the public, you might as well stop reading and turn on Fox News
Selected Studies, from most recent to oldest:
An international team of 32 authors from 24 institutions collaborated on prior research and published in the journal Nature Climate Change Greening of the Earth and Its Drivers, April 2016. The team reported over 25% to 50% of the global vegetated lands became significantly greener in the last 33 years. The researchers concluded that CO2 fertilization effects – an effect by which higher atmospheric CO2 concentrations fertilize, enhance plant growth – explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (4%) (Cayla Dengate, The Huffington Post Australia).
A research team from the Institute of Agricultural Sciences, Spanish National Research Council published, Elevated CO2 impacts bell pepper growth with consequences to Myzus persicae life history, feeding behaviour and virus transmission ability, Scientific Reports, January 2016. The team reported, “Among the observed effects on plants, typical responses to elevated CO2 include increased plant growth and biomass, canopy size, reduction in stomatal conductance and transpiration, improved water-use efficiency and higher photosynthetic rates. At the same time, increasing CO2 alters the chemical composition of plant tissue, with the accumulation of non-structural carbohydrates such as soluble sugars and starch. Elevated CO2 also has an impact on the nitrogen cycle that translates into a decrease in protein content and higher C:N ratio. The reduction in stomatal conductance may lead to a decrease in micronutrients such as calcium, magnesium or phosphorus due to the lower water uptake from the soil.”
A research team from the University of Hong Kong reported at the 2015 Paris Climate Summit the Discovery of a new plant growth technology that may alleviate climate change and food shortage, October 2015. The technology promotes plant growth and seed yield by 38% to 57% in a model plant by increasing CO2 absorption from the atmosphere. The researchers identified a plant-growth promoting gene that imports specific proteins into chloroplasts and mitochondria, thereby, stimulating growth of the engineered plant (University of Hong Kong), see also Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure, February 2001.
Researcher Johan Uddling senior lecturer at the Department of Biological and Environmental Sciences at the University of Gothenburg working with Swedish and international colleagues published Constraints to nitrogen acquisition of terrestrial plants under elevated CO2, Global Change Biology, May 2015. The team investigated “to what extent nitrogen availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2), and whether or not this constraint will become stronger over time.” The study examined various types of ecosystems, including crops, grasslands and forests, and involves large-scale field experiments conducted in eight countries on four continents. They found increased carbon dioxide levels in air restrict plants’ ability to absorb nutrients. For all types of ecosystem, increased carbon dioxide levels impaired plant quality.”
Researches from The Department of Ecology, Evolution and Behavior, University of Minnesota and Department of Biology, University of Wisconsin-Eau Claire reported Plant growth enhancement by elevated CO2 eliminated by joint water and nitrogen limitation, Journal Nature Geoscience, November 2014. The scientists analyzed plant biomass levels during a five-year, open-air experiment in a perennial grassland under two contrasting levels of atmospheric CO2, soil nitrogen and summer rainfall, respectively. They found that the presence of a CO2 fertilization effect depends on the amount of available nitrogen and water. Specifically, elevated CO2 levels led to an increase in plant biomass of more than 33% when summer rainfall, nitrogen supply, or both were at the higher levels (ambient for rainfall and elevated for soil nitrogen). But elevated CO2 concentrations did not increase plant biomass when both rainfall and nitrogen were at their lower level. They concluded that given widespread, simultaneous limitation by water and nutrients, large stimulation of biomass by rising atmospheric CO2 concentrations may not be universal.”
Scientists at the University of California, Davis reported Nitrate assimilation is inhibited by elevated CO2 in field-grown, Journal of Nature Climate Change April 2014. “This first of its kind study demonstrated the inhibition of wheat crops to convert nitrate into a protein, due to increased CO2 levels. Previously, studies suggested this reaction in plants, but this is the first time it is shown in field grown crops” (Twanna Harps, Guardian).
A study by several Australian scientists published Impact of CO2 fertilization on maximum foliage cover across the globe’s warm, arid environments, Geophysical Research Letters, June 2013. They found plant growth surges as CO2 levels rise. “Using a mathematical model to predict the extent of this carbon dioxide fertilization effect, the scientists calculated that in warm, dry conditions, plants would make more leaves if they had the water to do so. They also determined this fertilization effect could account for an 11 percent increase in global foliage since 1982” (Tim Radford, Climate News Network).
Scientists from the Max Planck Institute of Molecular Plant Physiology and the University of Potsdam reported Carbon dioxide could reduce crop yields, AlphaGalileo, November 2012. The researchers discovered that an increase in carbon dioxide levels could cancel out the beneficial effects of dwarf varieties of plants, such as rice, which ensure our basic food supply. These plants are bred for short stalks and high grain yields not for vertical growth. Thus, in the experiment, the dwarf plants gradually lost their advantage and increasingly resembled the control plants” (Philip Bump , Grist).
In a paper, Genetic manipulation of stomatal density influences stomatal size, plant growth and tolerance to restricted water supply across a growth carbon dioxide gradient published by Royal Society B: Biological Sciences, February 2012, a team of scientists from the University of Sheffield, UK and the University of Sydney, Australia investigated the impact of manipulating stomatal density on a collection of mutant plants with a 16-fold range of stomatal densities (approx. 20–325% of that of control plants). The plants were grown at three atmospheric CO2 concentrations (200, 450 and 1000 ppm), and 30 per cent or 70 per cent soil water content. The researchers found under some but not all conditions, mutant plants exhibited abnormal stomatal density responses to CO2 concentration. Plant size negatively correlated with stromal density. For example, at 450 ppm CO2, overexpressing plants, with reduced density, had larger leaves and increased dry weight.
An international team of scientist reported the results of a study, Climate change effects on beneficial plant–microorganism interactions, FEMS Microbiology Ecology, August 2010. The researchers reviewed the results of 135 studies investigating the effects of climate change factors on beneficial microorganisms and their interaction with host plants. “In most cases, plant-associated microorganisms had a beneficial effect on plants under elevated CO2. The effects of increased temperature on beneficial plant-associated microorganisms were more variable, positive and neutral, and negative effects were equally common and varied considerably with the study system and the temperature range investigated. Overall, the review shows that plant-associated microorganisms are an important factor influencing the response of plants to climate change.”
A study on Carbon dioxide and high temperature effects on growth of young orange trees in a humid, subtropical environment by Hartwell and Vu published by Agricultural and Forest Meteorology, January 2009, concluded that at lower temperatures an increased CO2 environment does indeed lead to an increase in plant biomass. However, they also showed that as temperature increases the biomass yield drops as a result of higher temperatures limiting stomatal water flow (Mark Cresswell, UK Science).
The Jasper Ridge Global Change Project, December 2002, an unprecedented three-year experiment, conducted at Stanford University concluded “that elevated atmospheric carbon dioxide actually reduces plant growth when combined with other likely consequences of climate change — namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil. The results revealed while treatments involving increased temperature, nitrogen deposition or precipitation — alone or in combination — promoted plant growth by as much as 84 percent, the addition of elevated carbon dioxide consistently dampened those increases by 40 percent.”
Dr. Christian Korner and Dr. John A. Arnone of the University of Basel, Switzerland reported Responses to Elevated Carbon Dioxide in Artificial Tropical Ecosystems, Journal Science, September 1992. The scientists disclosed that excessive amounts of carbon dioxide may impair plant health. “For their experiments, the scientists built identical sets of greenhouses and recreated complete rain-forest ecologies. In one set of greenhouses, the CO2 content of the air was maintained at 340 parts per million. In the other set of greenhouses, the CO2 content was increased to 610 parts per million. Both sets of plants grew vigorously, but the high-carbon-dioxide group produced no more weight of vegetation than the other group. Moreover, plants exposed to high carbon dioxide formed “massive” amounts of starch grains in their uppermost leaves” (Malcolm W. Browne, The New York Times).
In closing, the answer to the question – is it true that higher levels of atmospheric CO2 increase plant growth is a qualitative Yes when viewed only from the standpoint of CO2 and a resounding No when other factors are considered. While the preponderance of studies endorse the conventional notion of a direct relationship between CO2 concentration and plant growth, a number of well-designed investigations that incorporate secondary factors indirectly associated with the rise in CO2, support the opposing thesis that excessive amounts of ambient CO2 may adversely influence plant growth.
These “secondary” factors – ambient temperature, local precipitation, soil condition, nutrient availability, and microorganism plant interactions – relate to climate change rather than a direct effect of CO2. Plant growth is a complex process not readily explainable through one-dimensional thinking. Future studies need to consider the confounding interactions between these factors as well as the impact of climate and competition on forests, cropland and pastures. Only then, would it be possible to answer the question with a resounding Yes or No.