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CO2 is plant food? If only it were so simple

Posted on 27 April 2011 by Dawei

In the climate change debate, it appears to be agreed by everyone that excess CO2 will at least have the direct benefit of increasing photosynthesis, and subsequently growth rate and yield, in virtually any plant species: A common remark is that industrial greenhouse owners will raise CO2 levels far higher than normal in order to increase the yield of their crops, so therefore increasing atmospheric levels should show similar benefits. Unfortunately, a review of the literature shows that this belief is a drastic oversimplification of a topic of study that has rapidly evolved in recent years.

Climate control vs. climate change

The first and most obvious retort to this argument is that plants require more than just CO2 to live. Owners of industrial greenhouses who purchase excess CO2 also invest considerable effort in keeping their plants at optimum growing conditions, particularly with respect to temperature and moisture. As CO2 continues to change the global climate, both of these variables are subject to change in an unfavorable way for a certain species in a certain region (Lobell et al. 2008, Luo 2009, Zhao and Running 2010, Challinor et al. 2010, Lobell et al. 2011). More and more it is becoming clear that in many cases, the negatives of drought and heat stress may cancel out any benefits of increased CO2 predicted by even the most optimistic study. 

But there is a more subtle point to be made here. The majority of scientific studies on enhanced CO2 to date have been performed in just these types of enclosed greenhouses, or even worse, individual growth chambers. Only recently have researchers begun to pull away from these controlled settings and turn their attention to outdoor experiments. Known as Free-Air CO2 Enrichment or “FACE”, these studies observe natural or agricultural plants in a typical outdoor setting while exposing them to a controlled release of CO2, which is continuously monitored in order to maintain whichever ambient concentration is of interest for the study (see Figure 1).

Figure 1 - Example FACE study in Wisconsin, USA with multiple CO2 injection plots; courtesy of David F Karnosky, obtained from Los Alamos National Laboratory.

FACE studies are therefore superior to greenhouse studies in their ability to predict how natural plants should respond to enhanced CO2 in the real world; unfortunately, the results of these studies are not nearly as promising as those of greenhouse studies, with final yield values averaging around 50% less in the free-air studies compared to greenhouse studies (Leaky et al. 2009, Long et al. 2006, Ainsworth 2005, Morgan et al. 2005). Reasons for this are numerous, but it is suspected that in a greenhouse, the isolation of individual plants, constrained root growth, restricted pest access, lack of buffer zones, and unrealistic atmospheric interactions all contribute to artificially boost growth and yield under enhanced CO2.

C3 & C4

Photosynthesis comes in a few different flavors, two of which are C3 and C4. Together C3 and C4 photosynthesis make up almost all of modern agriculture, with wheat and rice being examples of C3 crops while corn and sugarcane are C4. The distinction deals mainly with the specific enzyme that is used to collect CO2 for the process of photosynthesis, with C3 directly relying on the enzyme RuBisCO. C4 plants also use RuBisCO, but unlike C3 plants, they first collect CO2 with the enzyme PEP-carboxylase in the mesophyll cell prior to pumping it to RuBisCO (see Figure 2).

Figure 2 - A simplified diagram contrasting C3 vs. C4 plant photosynthesis. From Nature Magazine.

The relevance of this distinction to excess CO2 is that PEP-carboxylase has no natural affinity for oxygen, whereas RuBisCO does. RuBisCO will just as readily collect oxygen (which is useless) as it will CO2, and so increasing the ratio of CO2/O2 in the atmosphere increases the efficiency of C3 plants; the extra step in the C4 process eliminates this effect, since the mesophyll cell already serves to concentrate pure CO2 near RuBisCO. Therefore excess CO2 shows some benefit to C3 plants, but no significant benefit to C4 plants. Cure and Acock 1986 (a greenhouse study) showed excess CO2 gave a 35% photosynthesis boost to rice and a 32% boost to soybeans (both C3 plants), but only a 4% boost to C4 crops. More recently, Leaky et al. 2006 (a FACE study) did not find any statistically significant boost in photosynthesis or yield for corn (a C4 crop) under excess CO2.

Going a bit deeper, it has recently been found that in some C3 plants—such as cotton and many bean species—a further enzyme known as RuBisCO activase is required to convert RuBisCO into its “active” state, the only state in which it can be used for photosynthesis. The downside of this is that the activase enzyme is much more sensitive to high temperatures compared to RuBisCO itself, and also responds poorly to excess CO2: Heat can destroy the structure of the activase enzyme at temperatures as low as 89.6 F, while excess CO2 reduces the abundance of the cellular energy molecule ATP that is critical for RuBisCO activase to function properly (Crafts-Brandner & Salvucci, 2000, Salvucci et al. 2001). This effect may potentially nullify some of the gains expected from excess CO2 in these plants. 

Chemical Responses & Nutrition

Even within a specific type of photosynthesis—indeed, even within a specific species—the positive responses to enhanced CO2 can vary widely. Nutrient availability in particular can greatly affect a plant’s response to excess CO2, with phosphorous and nitrogen being the most critical (Stöcklin and Körner 2002, Norby et al. 2010, Larson et al. 2010). The ability of plants to maintain sufficient nitrogen under excess CO2 conditions is also reduced for reasons not fully understood (Bloom et al. 2010, Taub and Wang 2008).

It has also been found that excess CO2 can make certain agricultural plants less nutritious for human and animal consumption. Zhu 2005, a three-year FACE study, concluded that a 10% decrease in the protein content of rice is expected at 550 ppm, with decreases in iron and zinc contents also found. Similarly, Högy et al. 2009, also a FACE study at 550 ppm, found a 7% drop in protein content for wheat, along with decreased amino acid and iron content. Somewhat ironically, this reduction in nutrient content is partially caused by the very increase in growth rates that CO2 encourages in C3 plants, since rapid growth leaves less time for nutrient accumulation.

Increased CO2 has been shown to lead to lower production of certain chemical defense mechanisms in soybeans, making them more vulnerable to pest attack and diseases (Zavala et al. 2008 and Eastburn et al. 2010). Other studies (e.g. Peñuelas and Estiarte 1999) have shown production of phenolics and tannins to increase under enhanced CO2 in some species, as well as many alkaloids (Ziska et al. 2005), all of which may have potential consequences on the health of primary consumers. The decreased nutritional value in combination with increased tannin and phenolic production has been linked to decreased growth rate and conversion efficiency of some herbivores, as well as an increase in their relative demand and consumption of plants (Stiling and Cornelissen 2007).

Furthermore, many “cyanogenic” species—plants which naturally produce cyanide, and which include 60% of all known plant species—have been found to increase their cyanide production in an enhanced CO2 world. This may have a benefit to the plants who use cyanide to inhibit overconsumption by pests and animals, but it may in turn reduce their safety as a food supply for both humans and animals (Gleadow et al., 2009a and Gleadow et al. 2009b).

Interactions with other species

Competing plant species have also been shown to drastically alter expected benefits from excess CO2: even in the best FACE studies, most research still involves artificial experimental plots consisting of fewer than five plant species, and often only one species is present. It has long been understood that due to increased growth of competitor species, benefits from isolated experiments cannot be scaled up to explain how a plant might respond in a monoculture plot (Navas et al. 1999). The distinction is even greater when comparing the behavior of isolated species to those of mixed plots (Poorter and Navas 2003). The lack of correlation (r2 = 0.00) between biomass enhancement (BER) of isolated plants and that of plants in mixed plots is presented in Figure 3.

Figure 3 – Isolated vs. mixed biomass enhancement ratios under excess CO2; From Figure 8 of Poorter and Navas 2003

That some plant species may benefit more fully and/or rapidly from excess CO2 also introduces the possibility that the abundance of certain species in an ecosystem will increase more than that of others, potentially forcing the transformation from one type of ecosystem to another (Poorter and Navas 2003). There is also some evidence suggesting that invasive species and many “weeds” may show relatively higher responses to elevated CO2 (Ziska and George 2004), and become more resistant to conventional herbicides (Ziska et al. 2004, Ziska and Teasdale 2000).

There is some evidence that interacting bacterial communities, particularly in the roots, will be affected through elevated CO2, leading to mixed results on overall plant health. Mutualistic fungal  root communities (known as ‘mycorrhizae') are typically shown to increase under excess CO2, which facilitate nutrient transport to the roots (Treseder 2004), although infections of pathogenic species such as Fusarium (the agent of the disease known as ‘crown rot’) have been shown to become more severe under excess CO2 as well (Melloy et al. 2010).

Temperature

It has long been known that stomata (the pores through which plants take in CO2 and exhale oxygen and water) tend to be narrower and stay closed longer under enhanced CO2. This effect is often cited as a benefit in that it increases water efficiency in drought situations.

But there is another key piece to reduced stomatal conductance, considering that 90% of a plant’s water use is actually for cooling of the leaves and nothing more: heat from the sun is absorbed by the water in the leaf, then carried out as vapor in the form of latent heat. So while it is true that the plant may retain water better under enhanced CO2, doing so may cause it to retain more heat. This can potentially carry a plant to less optimal temperature ranges (Ball et al. 1988 and Idso et al. 1993). An image present in Long et al. 2006 (Figure 4) shows this effect quite clearly; while a 1.4 C increase is probably not enough to cause significant damage in most cases, global warming will only serve to exacerbate the effect.  It is also of note that the study above represented a well-watered situation, and so during a drought condition the temperature increase would be even higher. 

Figure 4 - Increase in local temperature under enhanced CO2 due to reduced evapotranspiration. From Long et al. 2006

On the cold end, it has been found that for seedlings of some species of evergreen trees, excess CO2 can increase the ice formation temperature on the leaves, thereby increasing their sensitivity to frost damage (Roden et al. 1998).

Ozone

CO2 is not the only atmospheric gas that is on the rise: concentrations of ground-level ozone (O3) are expected to rise 23% by 2050 due to continuing anthropogenic emissions of precursor gases like methane and nitrous oxides. In addition, Monson et al. 1991 found that natural plant emissions of volatile organic compounds (another group of O3 precursors) increase under excess CO2 in many plant species, thereby introducing the potential that local O3 concentrations around plant communities may rise even higher than the baseline atmospheric level.

O3 has long been known to be toxic to plants: Morgan et al. 2006 found a 20% reduction of soybean yield in a FACE study of 23% excess O3. Similarly, Ainsworth 2008 showed a 14% decrease in rice yield at 62 ppb O3, and Feng et al. 2008 (a meta-analysis of 53 peer-reviewed studies) found on average a 18% decrease in wheat yield at 43 ppb O3. Ozone also appears to reduce the structural integrity of plants as well as make them more vulnerable to certain insect pest varieties such as aphids (Warrington 1988).

Figure 5 - Yield reduction for several crop species under excess ozone. From Wang and Mauzeral 2004

With respect to this effect, excess CO2 may actually prove beneficial in that it causes a narrowing of leaf stomata, thereby reducing the quantity of ozone that can enter the more sensitive internal tissues. Needless to say, the combined effect of excess CO2 and excess O3 is complex, and as it has only recently been given attention it is an area that requires much further research.

Conclusion

A specific plant’s response to excess CO2 is sensitive to a variety of factors, including but not limited to: age, genetic variations, functional types, time of year, atmospheric composition, competing plants, disease and pest opportunities, moisture content, nutrient availability, temperature, and sunlight availability. The continued increase of CO2 will represent a powerful forcing agent for a wide variety of changes critical to the success of many plants, affecting natural ecosystems and with large implications for global food production. The global increase of CO2 is thus a grand biological experiment, with countless complications that make the net effect of this increase very difficult to predict with any appreciable level of detail.

NOTE: This post is also the Advanced rebuttal to "CO2 is plant food".  And a hearty welcome to Dawei to the ranks of Skeptical Science authors

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Comments 101 to 106 out of 106:

  1. layzej - "Of the three positions, I'm inclined to trust......" Not a very scientific approach eh? How about reading those studies I linked to? Full copies of all are freely available online - at least they used to be, that's how I got hold of them. The crux of the issue is that the CO2 fertilization effect, i.e a net increase of global land-based vegetation biomass, doesn't seem to be panning out in the real world. So the climate model simulations could be drastically underestimating the amount of warming we're likely to get. Scarily so, because the CO2 fertilization effect is a biiig negative feedback in the simulations. That John Nielsen-Gammon suggests that net land biosphere CO2 uptake should be prima facie evidence of the CO2 fertilization effect underscores his lack of research on this topic. He's failed his own litmus test. I think you'll be surprised about where a lot of that CO2 we emit is ending up. You shouldn't really be, if you think deeply about the amount of new buildings we humans erect.
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  2. Greening of the Sahara-Sahel region: Effects on Africa Study of rice yields in response to higher CO2 Rice yields, varity selection results The forest is young, which would confirm other FACE findings: Forest growth at the Yatir forest To bad we can't keep the higher CO2 without the other projected negatives.
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  3. Rob Painting @ 101, not scientific no, but I'm no scientist - just a mere mortal. There are about 27,000 hits on Google scholar for CO2 fertilization - more than I could possibly review. Even if I were to trust that the four you selected are representative, I am not qualified to judge their merit. For us mortals it really is a matter of trying to assess the reliability of our sources, and relying on them to honestly portray the literature. I look forward to reading the IPCC AR5 and finding out which of you came closest to their assessment. A source does earn credibility to the layman when he goes out of his way to point out when his own case is being overstated. Real Climate does a good job of this. JNG perhaps goes too far in this case. I recommended this SkS article to JNG's site as a source that gives an honest assessment of the effect of rising CO2 on plant growth. I was surprised when he included this post among the ignorant and/or [-preemptive snip-]. JNG may well have failed his own litmus test. I'll wait for AR5 before I judge.
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  4. layzej- "Even if I were to trust that the four you selected are representative, I am not qualified to judge their merit." No need to rely on trust. You can search for yourself, just limit your search from 2010 onwards because that is representative of recent research (obviously). If you can find any paper that observes a net global benefit from increased CO2 I'd be interested. I haven't found one. "A source does earn credibility to the layman when he goes out of his way to point out when his own case is being overstated" There is no need for SkS to overstate things, the facts are what they are. The carbon cycle models assume a big CO2 fertilization effect this century, you can check the 2007 IPCC report for yourself. That this isn't being observed should be cause for concern, because we are talking about many decades worth of fossil fuel emissions (at today's burn-rate) staying up in the atmosphere. That there is a lot of extra warming if the CO2 fertilization effect doesn't pan out. I was surprised when he included this post among the ignorant.... As was I. Dawei's post is very well-balanced in my opinion - although it deals strictly with the effects on crops, rather than global vegetation. Nevertheless what John Nielsen-Gammon has written there is wrong. The man just hasn't done his homework. The tropics, and the Amazon in particular are in a precarious situation that could very rapidly turn the tropical forest carbon sink, the largest forest sink, into a source. Of course it may not happen, we are talking about projections after all, but the current trends are not encouraging.
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  5. Rob Painting: No need to rely on trust. You can search for yourself, just limit your search from 2010 onwards ... If you can find any paper that observes a net global benefit from increased CO2 I'd be interested. I haven't found one. Limiting the search to 2010 onward reduces the results to about 14,000. Still more than I can possibly review. I took a look at the first few dozen. The literature appears to be as inconclusive as it is encyclopaedic. Most studies conclude that the future is uncertain. One found considerable increase with mechanistic process-based models but reduced habitat suitability from niche-based models. Some found a net gain for certain local environments. Some find a net gain for specific species. One found that approximately 20 percent of sites globally exhibit increasing trends in growth that cannot be attributed to climatic causes, nitrogen deposition, elevation, or latitude, which we attribute to a direct CO2 fertilization effect. This of course does not mean that 80% are stable or declining, just that the growth could not be attributed to CO2. None of the first few dozen were global in scope and were able to conclude that a net global increase or decrease should be expected. One that was interesting to me (because I don't eat tree's but I do drink beer) showed a large increase in wheat yields for a doubling of CO2: http://www.sciencedirect.com/science/article/pii/S0378377409001991 I think I'm going to have to rely on the IPCC to distill this all down. Regarding SkS overstating things - I never implied that they do. I'm just pointing out that a great deal of credibility and trust can be gained by calling out cases where ones own position is being overstated by others. The source is no longer seen as an advocate for his "side" but is seen as an advocate for the truth.
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  6. layzej- "Limiting the search to 2010 onward reduces the results to about 14,000.' Once you start going through them you'll find there are far less than that. I normally go through the first 100 pages of hits. Gives one a very good idea of what the scientific literature says. Time consuming but necessary, if facts are what you are interested in. "The literature appears to be as inconclusive" Nope. All the recent literature finds limited evidence for the CO2 fertilization effect in the 20 the century, and forests that did once benefit during that century, have stopped doing so. Suggesting some sort of threshold, or acclimation, to elevated CO2. Probably what you are doing is looking at the literature without regard to its chronology. "Some find a net gain for specific species" Undoubtedly. Liana for instance, which are woody tropical vines, are growing like crazy. But that's not John Nielsen-Gammon's contention is it? He claims a net global benefit. "One found that approximately 20 percent of sites globally exhibit increasing trends in growth that cannot be attributed to climatic causes" That's undisputed. It has been the subject of considerable scientific debate over the years - whether this might be the consequence of elevated CO2. Trouble is, the latest global forest inventory finds the CO2 is currently going into forest re-growth and is not due to CO2 fertilization. This tallies with recent (last 2 years) studies which find no fertilization effect at a global scale. "showed a large increase in wheat yields for a doubling of CO2" Yes, Dawei's post is well-balanced. The title of it says it all really.
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