This week my Twitter feed directed me to an article by Dana Nuccitelli in the Guardian titled: “New study undercuts favorite climate myth ‘more CO2 is good for plants”. The Guardian article was about a new study out of Stanford, published in the Proceedings of the National Academy of Sciences (PNAS). The intention of this blog post is to explain why this PNAS article does not, as Mr. Nuccitelli claims, “undercut” the “climate myth” that “more carbon dioxide is good for plants”.
As anyone who has studied any horticulture knows, it is an accepted fact in the academic literature that elevated concentrations of carbon dioxide are expected to result in increased yields and improved growth in the majority of green plants. There is a commonly accepted proviso to that axiom, which is that increased carbon dioxide concentrations will have little effect on plant growth in conditions where there is an established alternative limiting factor in that growth. So adding carbon dioxide to a plant that is lacking in essential nutrients will not increase growth rate as carbon dioxide does not represent the rate-limiting step in that growth. That doesn’t mean that the increases in carbon dioxide concentrations are not good for these plants (by decreasing water stress etc…), but simply that increasing concentrations of carbon dioxide will not increase crop yields when some other feature limits plant growth.
The effect of increased carbon dioxide concentrations on plant growth is not contentious. Even the folks at Skeptical Science admit as much (admittedly they only do so briefly before quickly changing the subject to start talking about how more plant growth will use up all the soil nutrients and will be likely to increase pest effects both of which are irrelevant to the science under discussion). So what Mr. Nuccitelli calls a “climate myth” in his Guardian article (that more carbon dioxide is good for plants) actually represents a demonstrated scientific fact confirmed in thousands of academic studies and used to increase crop yields in tens of thousands of greenhouses worldwide. Now at least three scientists have had the expression “extraordinary claims require extraordinary evidence” attributed to them, but regardless of who said it first, this axiom is very much applicable in the context of this report. Given the extraordinary claim made by Mr. Nuccitelli I was really interested in establishing what is so special about this PNAS study that allows it to “undercut” the globally accepted fact that increased carbon dioxide concentrations are generally good for green plants.
The PNAS article in question has a particularly unsexy title: “Nonlinear, interacting responses to climate limit grassland production under global change. It details the results of a “17 year study of California grassland exposed to full-factorial warming, added precipitation, elevated CO2, and nitrogen deposition”. Let’s start with the praise. Any 17-year scientific study deserves an award just for perseverance and this one is no exception. In the study the authors planted 132 plots of local, grassland plants and then varied how the plants were exposed to carbon dioxide, temperature, water and nitrogen. Then over 16 years they measured above-ground and below-ground net primary productivity (ANPP and BNPP, respectively) in the plots. The results of the study indicated that, in this one study, increased carbon dioxide concentrations resulted in no significant increase in net primary productivity (NPP); increased nitrogen resulted in significant increases in NPP while increases in temperature and precipitation had initially a positive, but ultimately a net negative effect on NPP. Now I was a bit surprised by the conclusion until I noticed that the study was undertaken in a Mediterranean climate biome. This cleared up a lot of confusion for me as I recognized that the results, while interesting, were likely not globally applicable as I will explain below.
The Stanford test site used in the study is located at the Jasper Ridge Global Change Experiment (JRGCE) site in the Jasper Ridge Biological Preserve in California. The site is “located on the east side of the Outer Coast Range of central California near the Stanford campus. Jasper Ridge has a Mediterranean climate with cool, wet winters and very dry summers (ref)”. Now this should immediately perk up your ears since the Mediterranean climate biome is a relatively rare one. Put another way, it is hard to overstate how small the area defined as having a Mediterranean climate really is from a global perspective or how minimally this area (in its natural state) contributes to global NPP. As the JRGCE web site points out: areas with Mediterranean climate are known for being wet and cool in winter and very dry and hot in summer. This means that growth is limited to a very short effective growing season and one that occurs primary in the winter time (as the authors point out, between November and May) when the days are shorter and the sunlight intensity is lower. Plants in Mediterranean climate zones are optimized to survive long droughts in summer and wet winters and include all the classic Mediterranean trees and vines like olives, grapes, figs, etc… (i.e. plants that store winter rains in their fruit and ripen in the hot summer sun) as well as grasses and small annual flowers. That is not to say that areas with Mediterranean climate can’t be very good for growing other crops, but this is thanks to complex irrigation systems intended to extend the growing season into the hot, dry springs and summers. The farms in these areas thus take advantage of the plentiful natural spring/summer sunlight and the availability of industrial fertilizers in addition to the availability of water for irrigation.
As described in the PNAS paper, Mediterranean climate areas tend to be nitrogen limited for plant growth. To explain why, recognize that the climate regime does not encourage the growth of nitrogen fixing plants like legumes that struggle to grow/survive in the hot, dry summers. They instead favour grasses, small seasonal annual flowers and drought-resistant trees and vines. Thus the inclusion of nitrogen in this study presents an obvious way to find a limiting nutrient that would be expected to increase NPP. By including nitrogen in the study the authors have essentially included a positive control that demonstrates that, through their action, they can increase NPP.
The major concern with the Mediterranean climate, from a crop-growing perspective, is that a most of the rain comes in winter, often in short bursts. During the wet, growing season water is not a limiting feature in plant growth, rather the availability of strong sunlight and critical nutrients limit growth. This makes the way the study adds water a bit odd. As described in the paper, additional precipitation was not provided at the beginning or tail end of the rainy season (to extend the growing season) but rather the elevated precipitation was mostly added during, or immediately after, rainfall events. As the study put it: “The precipitation treatment was +50% of ambient rainfall, plus two 10-mm additions after the last rainfall event”. Thus, the increase in precipitation was in individual event intensity and not overall number of wet days. So in this study they took the expression “when it rains it pours” quite literally. Unfortunately this creates an obvious weakness: too much of a good thing. Too much rain will result in excess leaching of nutrients and, as the study showed, the wettest years had the worst NPP results. As well wetness would negatively correlate with sunlight intensity since the sun is not shining brightly when the rain is falling.
It is quite sad that such an important piece of research was placed in such an inopportune location. Doing a study of this kind in an area with a Mediterranean climate basically guaranteed the results observed. As described in the UC Davis web page on the region it is a combination of water stress, nutrient availability and winter sunlight intensity that determines growth in Mediterranean climates. Or put in a way that matters to readers of this blog, carbon dioxide availability would not be expected to represent a limiting feature in the growth patterns in this biome. Adding carbon dioxide would not be expected to have a significant effect on growth rate in this biome and not surprisingly, that is precisely what they saw. So the outcome of the study is confirmation of what any horticulturalist would have told you. That being said, confirmation of theories is one of the tasks we expect our universities to undertake and as such this study would be considered a success.
So let’s go back to the Guardian article in question which claims that the conclusions of this study debunk the “climate myth” that additional “carbon dioxide is good for plants”. Does the study do this? Well the answer to that is a categorical no. Rather the PNAS study demonstrates that in a Mediterranean climate water stress and winter sunlight intensity represent the biggest determinants for plant growth and that in grassland biomes nitrogen often represents the limiting nutrient in plant growth. The study also demonstrates that there can be too much of a good thing (i.e. rainfall in the wet season). The study says nothing about whether carbon dioxide is good for plants…don’t even get me started on what “good for plants” really means anyways.
So going further, does the PNAS study counteract the plentiful studies that show the greening of the northern hemisphere attributed to the increases in global carbon dioxide concentrations? No it does not. Does the PNAS study allow one to dismiss the comprehensive reviews that demonstrate increased yields associated with increased atmospheric carbon dioxide concentrations? No the study does not.
Now don’t get me wrong here, I am not claiming that adding carbon dioxide to the atmosphere at the rate we have over the last century is a good thing. What I am saying is that trying to extrapolate a single study from a geographically tiny biome to upend a generation of research on the effects of increased atmospheric carbon dioxide concentrations on plant growth might be a step too far, even for the most alarmist of authors. In this case Mr. Nuccitelli, in his Guardian article, has taken a gigantic and unsupported leap from the conclusions (and limitations) of this report. He has taken a small a study in a biome of limited global significance and extrapolated it to global proportions and come up with global conclusions that are not supportable using the data presented. This study does not undercut any “climate myths”.
A Chemist in Langley 
You started off well and then it all went to shit at the end. Like any climate contrarian, you just cherry picked your studies which show how hypocritical you are. Ok, so what can we agree on? We can agree that every study ever conducted has its limitations. Obviously this study has its limitations as you pointed out, well done, and well said but Dana also said this in the article if you actually read it – I suspect you didn’t. He says, “While this experiment was specific to California grasslands…” He never applies this study globally. Then your bias begins to show and you say stupid things. For example:
>”the plentiful studies that show the greening of the northern hemisphere attributed to the increases in global carbon dioxide concentrations?”
Plentiful studies? You linked one study derived from modelling. That’s far from plentiful. Is greening necessarily good? No (ie regime shifts, allergies, etc), though some surely is. But leave it to you to dismiss the negatives of greening. So what does this have to do with those limits you talked about in regard to the study? Well, as it turns out there are limits to greening as well… glad you mentioned that (hint: you didn’t). I can’t find the study right now but I believe the ‘greening’ effect should stop around ~2050(?).
>”Does the PNAS study allow one to dismiss the comprehensive reviews that demonstrate increased yields associated with increased atmospheric carbon dioxide concentrations?”
Again with the hypocrisy. Didn’t you just argue that this study was limited in scope and shouldn’t be applied globally? And yet here you are doing the exact same thing you just condemned. Increased yields? Really? Sorry but not all crops respond the same way, and some crop yields are down, while others will most certainly decrease in the near future:
1) http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3061.html
2) http://www.nature.com/nclimate/journal/v6/n8/full/nclimate2995.html
3) http://science.sciencemag.org/content/353/6304/aad9837
4) http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3115.html
5) http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2947.html
Not only does CO2 effect crop yields in various ways (including decreasing them), it can also reduce the nutritional value in crops such as rice, wheat, soy, maize, sorghum and field peas (significant reductions of iron, zinc, and protein).
>”He has taken a small a study in a biome of limited global significance and extrapolated it to global proportions and come up with global conclusions that are not supportable using the data presented.”
Again, you literally just did the same thing as I’ve highlighted above. And he never extrapolated it to global proportions. The only people that did that were the same idiots that go around saying ‘CO2 is plant food.’
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The “plentiful studies” comment linked to a single study that included in its references a plethora of references and the other two references represented reviews included over 200 direct references. So yes there is lots of data supporting my position. Similarly the crop yield paper included references to reviews that assessed lots of studies. This is nothing new.
As for the references you provide, they are all modelling exercises while the major references I provide are real-world examples of yield increase. Finally I am not extrapolating anything the research on this topic is copious and robust. Certainly increased growth can reduce crop nutritional value but that is often simply a feature of too much of a good thing that can be addressed with the appropriate use of fertilizer.
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No one’s arguing that CO2 isn’t required for plants, but your position assumes that CO2 is good for plants… period. That’s only half the story. It’s good but only within certain constraints. To little, is a bad thing, too much is a bad thing, and given that these constraints vary not only from species to species but for the age of the plant as well, and other limiting factors you cannot extrapolate from your ‘plethora’ of studies and apply that to every. single. plant. on Earth. Like your critique of this study I suppose you’ve given the plethora of other studies the same due diligence, or did you just assume that they weren’t limited in their scope and no had criticisms (ie. greenhouses and growth chambers)? Where’s your analysis of those studies?
As for my references, they are no different than yours. Based on modelling exercises, AND field experiments. From the (2): “…we combine results from networks of field experiments and global crop models…” From (4): “Here we show that grid-based and point-based simulations and statistical regressions (from historic records)…” Funny enough there are a plethora of other studies that show the same thing, just check the references, and their references, and other studies I didn’t bother linking. This is nothing new.
Since when did you discredit a study or assume it wasn’t robust enough simply because it included modelling? Didn’t you argue somewhere about how robust Kinder Morgan’s modelling was?
>”that can be addressed with the appropriate use of fertilizer.”
Wow. It’s just that simple hey? No dead zones or excess nitrogen in the soils. I say this because.. as a pragmatist, ‘appropriate use’ is a just dream and you know it.
I’d love for you to show me Mr. Nuccitelli extrapolates the study’s findings and applies them globally, really I would because he never did that… though you think he did. In case you missed it, here’s what he said, “While this experiment was specific to California grasslands…”
Seems to me this article simply rubbed you the wrong way, pushed your contrarian button, and sent you off to the blogosphere.
In summary: Is this study limited? Sure is. Are all studies limited? Sure are. Did Dana extrapolate this study’s findings globally? Nope. Did you? Nope, but you sure think Dana did. Is there more to NPP than CO2? Yup (countless studies show this, from field experiments, historical data, and modelling). Is ‘greening’ limited? Just like everything else, yes it is. Did you even raise that aspect of ‘greening’? Nope. Should you have? Yup.
Does this study or any other study undercut ‘climate myths’ by contrarians, and deniers? No, but not a single study supports their misuse and misleading use of ‘CO2 is plant food.’
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“My position assumes that CO2 is good for plants” are you seriously questioning that? There is so much research on the topic that I don’t even know where to start. Tens of thousands of greenhouses around the world are demonstration that CO2 is good for plants. 20 years of horticultural research says CO2 is good for plants. This is not a questionable topic, this is black letter science. The constraints on the improvement of plant health with CO2 have been shown to far exceed our current atmospheric levels. The research suggests 1000-1200 ppm represents the optimum concentration range for photosynthesis (http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm) I didn’t bother to fill my blog post with details because anyone who has a clue about this topic alredy knows it to be correct. Mr. Nuccitelli and his crew have worked to convince the owrld that this research is not true because it is used by contrarians against them. This is only one of the many locations where they have done so. Go to the link at Skeptical science to see what he and his colleagues say on the topic. He repeatedly makes the claim that CO is NOT good for green plants and this is only the latest iteration. So I finally got sick and tired of his mis-representation of the science and wrote something at my blog.
As for your last comment “not a single study supports their misuse and misleading use of CO2 is plant food” that is simply inane all the work in horticulture on the topic says exactly that. This is why greenhouses have CO2 pumped into them, because CO2 is the limiting nutrient in plant growth in good soil with good sunlight. That isn’t something that needs to be studied anymore because it has been studied to death and tens of thousands of greenhouses later it is accepted practice.
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Good post. But you can’t argue climate science with people who treat it like a religion.
By the way, I live in the ultimate Mediterranean micro climate. Around here they’ve taken to building huge greenhouses to allow winter crops. It also helps manage water. Some farmers are experimenting with co2, but it’s a bit expensive to increase concentration.
Anyhow, we are getting huge Murcia nectarines and onions, extremely good Alicante eggplants, and Malaga mangoes.
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Critical Thought – you have showed your pseudonym is a misnomer
Four of the papers abstracts you reference show they are model outputs, not field trials. The fifth appears to just be a thinktank.
If you want to play the paper games, there is:
http://www.nature.com/nclimate/journal/v6/n8/full/nclimate3004.html
Note 70% of greening was CO2
From IPCC AR4 we get:
“Overall, about two-thirds of the experiments show positive response to increased CO2 (Ainsworth and Long, 2005; Luo et al., 2005). ”
That sounds like plentiful studies to me as the quoted papers are summaries of others.
AR5 has:
Field experiments provide a direct evidence of increased photosynthesis rates and water use efficiency (plant carbon gains per unit of water loss from transpiration) in plants growing under elevated CO2. These physiological changes translate into a broad range of higher plant carbon accumulation in more than two-thirds of the experiments and with increased net primary productivity (NPP) of about 20 to 25% at double CO2 from pre-industrial concentrations (Ainsworth and Long, 2004; Luo et al., 2004, 2006; Nowak et al., 2004; Norby et al., 2005;Canadell et al., 2007a; Denman et al., 2007; Ainsworth et al., 2012; Wang et al., 2012a). Since the AR4, new evidence is available from long-term Free-air CO2 Enrichment (FACE) experiments in temperate ecosystems showing the capacity of ecosystems exposed to elevated CO2 to sustain higher rates of carbon accumulation over multiple years (Liberloo et al., 2009; McCarthy et al., 2010; Aranjuelo et al., 2011; Dawes et al., 2011; Lee et al., 2011; Zak et al., 2011). However, FACE experiments also show the diminishing or lack of CO2 fertilisation effect in some ecosystems and for some plant species (Dukes et al., 2005; Adair et al., 2009; Bader et al., 2009; Norby et al., 2010; Newingham et al., 2013). This lack of response occurs despite increased water use efficiency, also confirmed with tree ring evidence (Gedalof and Berg, 2010; Peñuelas et al., 2011).
Nutrient limitation is hypothesized as primary cause for reduced or lack of CO2 fertilisation effect observed on NPP in some experiments (Luo et al., 2004; Dukes et al., 2005; Finzi et al., 2007; Norby et al., 2010). Nitrogen and phosphorus are very likely to play the most important role in this limitation of the CO2 fertilisation effect on NPP, with nitrogen limitation prevalent in temperate and boreal ecosystems, and phosphorus limitation in the tropics (Luo et al., 2004; Vitousek et al., 2010; Wang et al., 2010a; Goll et al., 2012). Micronutrients interact in diverse ways with other nutrients in constraining NPP such as molybdenum and phosphorus in the tropics (Wurzburger et al., 2012). Thus, with high confidence, the CO2 fertilisation effect will lead to enhanced NPP, but significant uncertainties remain on the magnitude of this effect, given the lack of experiments outside of temperate climates.
That looks like it is very similar to what the author wrote.
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Nothing you presented contradicts what I have already stated. My position is that while CO2 is required for plants, just as water is for life, too much of something can be a bad thing (just like too much water). There are limits. The vast majority of studies are in good agreement with this. You can keep increasing CO2, and for a good portion of crops and plants you will see a decrease in nutrition with an increase in biomass; however, you cannot keep increasing CO2 (and therefore temperatures, decreased soil moisture and nutrients) indefinitely and expect to see continued ‘greening’ via CO2 fertilization: https://uanews.arizona.edu/story/study-north-american-forests-arent-saviors-climate-change
There’s more to plants than just CO2. Apparently a difficult concept to wrap your head around.
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The only question in this case is what amount of CO2 is too much and the science says that photosynthesis optimizes at 1000 ppm – 12000 ppm which are concentrations that will not be reached in either of our lifespans. As for your comment about most plants showing a decrease in nutrition, that is simply hogwash. In selected plants under selected growing regimes we have seen minimal decreases in nutritional value and those decreases were more than made up for in the increased biomass produced.
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Yeah. We need lots of fiber anyway. I’ve taken to eating a lot of fiber and my total cholesterol dropped to 89.
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“
” Evidence, please. Oh, and as you complain about Blair reading skills, perhaps you should brush up on your own. Methinks fernandoleanme is right: you can’t argue climate science with people who treat it like a religion; it is Nuccitelli that is cherry-picking with his claim that the PNAS study undermines what observations in horticulture around the world. And you are rapidly moving into the ad homs, already.
Anyway, CO2 has nowt to do with global warming, so why the fuss? (Unless you happen to believe the odd coincidence that the principle control of climate just so happens to be the only element of the atmosphere that we have convinced ourselves that we can have some control over. Hahahahaha – no-one can be that daft, surely!? … Can they?)
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Right, because cherry picked links are sufficient to undercut the “myth” that CO2 is plant food.
Next paper will argue that the high CO2 levels of the Cambrian Period caused massive extinctions and not the “explosion” of life we had always believed.
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>“My position assumes that CO2 is good for plants” are you seriously questioning that?
Perhaps your reading comprehension is a little under the wind. Take all the time you need to let my previous comment sink in regarding CO2. If you can’t seem to get beyond that let me make it clear: Everything has constraints. Nice link you provided, but again… no one, not even me, is arguing that CO2 upwards of 1200ppm IN A GREENHOUSE (ie. controlled environment) doesn’t lead to an increase biomass. What I am arguing is that in the real world, when there are other limiting factors (temperature, water, nutrients, light) you cannot push CO2 up to 1200ppm and expect the same results.
I finally found the study I had mentioned in my first comment which supports this (as does common fucking sense):
“…Using a network of over two million tree-ring observations spanning North America and a space-for-time substitution methodology, we forecast climate impacts on future forest growth. We explored differing scenarios of increased water-use efficiency (WUE) due to CO2-fertilisation, which we simulated as increased effective precipitation. In our forecasts: (1) climate change negatively impacted forest growth rates in the interior west and positively impacted forest growth along the western, southeastern and northeastern coasts; (2) shifting climate sensitivities offset positive effects of warming on high-latitude forests, leaving no evidence for continued ‘boreal greening’…” :http://onlinelibrary.wiley.com/doi/10.1111/ele.12650/abstract
You admit this yourself in your own comment:
>”This is why greenhouses have CO2 pumped into them, because CO2 is the limiting nutrient in plant growth in good soil with good sunlight.”
We’re on the same page here, but for some reason you think the real world is equivalent to a geenhouse with controlled conditions, and that’s just not the case. That isn’t something that needs to be studied anymore because it has been studied to death. Even your first paper (from 1983) you present states this clearly in its abstract:
>”…Most of the studies were performed in greenhouses or growth chambers. Open fields might respond less than greenhouses or growth chambers to increased CO2 because nutrient levels in general world-wide agriculture are lower than those in the indoor studies…”
In Summary: Your position needs a disclosure. You need to state its limitations, the constraints, which you haven’t done. Considering you do this with every other study that opposes your position, the only reason not to do this is to mislead the public, while strongly playing favoritism to your personal bias.
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Thank you very much for sorting this out & presenting the information so clearly.
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CT
One can see that rather than you are doing exactly the thing that you accuse others of in your abusive rant.
In the headline post Blair says:
“There is a commonly accepted proviso to that axiom, which is that increased carbon dioxide concentrations will have little effect on plant growth in conditions where there is an established alternative limiting factor in that growth. So adding carbon dioxide to a plant that is lacking in essential nutrients will not increase growth rate as carbon dioxide does not represent the rate-limiting step in that growth.”
Then at 11:29 post among other things you write
“My position is that while CO2 is required for plants, just as water is for life, too much of something can be a bad thing (just like too much water).”
Then at 11:20 post you write
” Everything has constraints. Nice link you provided, but again… no one, not even me, is arguing that CO2 upwards of 1200ppm IN A GREENHOUSE (ie. controlled environment) doesn’t lead to an increase biomass. What I am arguing is that in the real world, when there are other limiting factors (temperature, water, nutrients, light) you cannot push CO2 up to 1200ppm and expect the same results. ”
You can argue semantics, which no doubt you will, but most would agree the above three say basically the same thing.
And you make unsupported assertions as well:
“and some crop yields are down”
Which ones?
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I note that one of the papers referenced by Nuccitelli to support his argument was a 2012 one by Hawkins on maize production in France. The addition of a few more year’s data is interesting:
http://data.worldbank.org/indicator/AG.YLD.CREL.KG?locations=ES
Looks like it isn’t limiting Mediterranean climates yet. Though no doubt models trump data.
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In other words, “all other things being equal” C02 both warms the planet and promotes plant growth.
which means, one has to take care in finding or cherry picking examples where
increased c02 does not lead to warmer temps or more plant growth.
ahem
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The examples extant indicate no (significant?) CO2 warming in the real world. Models are bunk.
Think clouds.
Watch the atmosphere.
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But if plant growth increases the earth’s albedo; C02 doesn’t really increase warming…it just increases plant growth. Negative feed backs exist.
Trying to solve complex problems while looking at isolated components (like C02), appears increasingly to be foolhardy.
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Critical thought should consider the disastrous consequences of eliminating Crime. All the Police would be out of work! Critical thought makes use of tautology. There is nothing that has ONLY positive consequences. So what?
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“The study says nothing about whether carbon dioxide is good for plants…don’t even get me started on what “good for plants” really means anyways.”
Are we designing civilization for plants, or for people?
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You seem to be saying Mediterranean climate weeds tell us something about our civilization.
The IPCC folks want to design our civilization. Screw them, and you, too, David.
Charlie Skeptic
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If this was my blog, ‘CriticalThoughts’ comments would be sent directly to my spam folder. This person is a cyberbully. He/she calls other people ‘idiots’ and makes comments like ‘Apparently a difficult concept to wrap your head around’ and ‘Perhaps your reading comprehension is a little under the wind’. He/she/it is an anonymous coward and doesn’t deserve the space he/she/it occupies on your site.
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