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Plants cannot live on CO2 alone

What the science says...

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More Carbon Dioxide in the atmosphere is not necessarily good for plants.

Climate Myth...

CO2 is plant food

Earth's current atmospheric CO2 concentration is almost 390 parts per million (ppm).  Adding another 300 ppm of CO2 to the air has been shown by literally thousands of experiments to greatly increase the growth or biomass production of nearly all plants.  This growth stimulation occurs because CO2 is one of the two raw materials (the other being water) that are required for photosynthesis.  Hence, CO2 is actually the "food" that sustains essentially all plants on the face of the earth, as well as those in the sea.  And the more CO2 they "eat" (absorb from the air or water), the bigger and better they grow. (source: Plants Need CO2)

An argument made by those who prefer to see a bright side to climate change is that carbon dioxide (CO2) being released by the burning of fossil fuels is actually good for the environment. This conjecture is based on simple and appealing logic: if plants need CO2 for their growth, then more of it should be better. We should expect our crops to become more abundant and our flowers to grow taller and bloom brighter.

However, this "more is better" philosophy is not the way things work in the real world. There is an old saying, "Too much of a good thing can be a bad thing." For example, if a doctor tells you to take one pill of a certain medicine, it does not follow that taking four is likely to heal you four times faster or make you four times better. It's more likely to make you sick.

It is possible to boost growth of some plants with extra CO2, under controlled conditions inside of greenhouses. Based on this,  'skeptics' make their claims of benefical botanical effects in the world at large. Such claims fail to take into account that increasing the availability of one substance that plants need requires other supply changes for benefits to accrue.  It also fails to take into account that a warmer earth will see an increase in deserts and other arid lands, reducing the area available for crops. 

Plants cannot live on CO2 alone; a complete plant metabolism depends on a number of elements. It is a simple task to increase water and fertilizer and protect against insects in an enclosed greenhouse but what about doing it in the open air, throughout the entire Earth? Just as increasing the amount of starch alone in a person's diet won't lead to a more robust and healthier person, for plants additional CO2 by itself cannot make up for deficiencies of other compounds and elements.

What would be the effects of an increase of CO2 on agriculture and plant growth in general? 

1. CO2 enhanced plants will need extra water both to maintain their larger growth as well as to compensate for greater moisture evaporation as the heat increases. Where will it come from? In many places rainwater is not sufficient for current agriculture and the aquifers they rely on are running dry throughout the Earth (1, 2).

On the other hand, as predicted by climate research, we are experiencing more intense storms with increased rainfall rates throughout much of the world. One would think that this should be good for agriculture. Unfortunately when rain falls in short, intense bursts it does not have time to soak into the ground. Instead, it  quickly floods into creeks, then rivers, and finally out into the ocean, often carrying away large amounts of soil and fertilizer.

2. Unlike Nature, our way of agriculture does not self-fertilize by recycling all dead plants, animals and their waste. Instead we have to constantly add artificial fertilizers produced by energy-intensive processes mostly fed by hydrocarbons, particularly from natural gas which will eventually be depleted. Increasing the need for such fertilizer competes for supplies of natural gas and oil, creating competition between other needs and the manufacture of fertilizer. This ultimately drives up the price of food.

3. Too high a concentration of CO2 causes a reduction of photosynthesis in certain of plants. There is also evidence from the past of major damage to a wide variety of plants species from a sudden rise in CO2 (See illustrations below). Higher concentrations of CO2 also reduce the nutritional quality of some staples, such as wheat.

4. As is confirmed by long-term  experiments, plants with exhorbitant supplies of CO2 run up against  limited availability of other nutrients. These long term projects show that while some plants exhibit a brief and promising burst of growth upon initial exposure to C02, effects such as the  "nitrogen plateau" soon truncate this benefit

5. Plants raised with enhanced CO2 supplies and strictly isolated from insects behave differently than if the same approach is tried in an otherwise natural setting. For example, when the growth of soybeans is boosted out in the open this creates changes in plant chemistry that makes these specimens more vulnerable to insects, as the illustration below shows.

Figure 1: Plant defenses go down as carbon dioxide levels go up, the researchers found. Soybeans grown at elevated CO2 levels attract many more adult Japanese beetles than plants grown at current atmospheric carbon dioxide levels. Science Daily; March 25, 2008. (Credit: Photo courtesy of Evan Delucia)

Figure 2: More than 55 million years ago, the Earth experienced a rapid jump in global Carbon Dioxide levels that raised temperatures across the planet. Now, researchers studying plants from that time have found that the rising temperatures may have boosted the foraging of insects. As modern temperatures continue to rise, the researchers believe the planet could see increasing crop damage and forest devastation. Science Daily; Feb. 15, 2008.

Figure 3: Global Warming reduces plant productivity. As Carbon Dioxide increases, vegetation in Northern Latitudes also increases. However, this does not compensate for decreases of vegetation in Southern Latitudes. The overall amount of vegetation worldwide declines 

6. Likely the worst problem is that increasing CO2 will increase temperatures throughout the Earth. This will make deserts and other types of dry land grow. While deserts increase in size, other eco-zones, whether tropical, forest or grassland will try to migrate towards the poles. Unfortunately it does not follow that soil conditions will necessarily favor their growth even at optimum temperatures.

In conclusion, it would be reckless to keep adding CO2 to the atmosphere. Assuming there are any positive impacts on agriculture in the short term, they will be overwhelmed by the negative impacts of climate change.

Added CO2 will likely shrink the range available to plants while increasing the size of deserts. It will also increase the requirements for water and soil fertility as well as plant damage from insects.

Increasing CO2 levels would only be beneficial inside of highly controlled, enclosed spaces like greenhouses.

Basic rebuttal written by doug_bostrom


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

 

Last updated on 8 July 2015 by MichaelK. View Archives

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Comments

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Comments 26 to 46 out of 46:

  1. james baggett @25, fundamental to the ecology of plants is that they are preyed on by a great variety of insects and animals. To remain healthy in the face of this predation, they must produce far more organic matter (sugars and cellolose) than they themselves directly use.  Further, if they are to grow in mass, they must draw down the carbon in that mass from the atmosphere by photsynthesizing carbon.  Because of that, in any healthy plant  over the course of a year, it draws down far more CO2 through photosynthesis then it produces by oxidizing sugars.  If plants did not do this, the presence of plants would not have substantially increased the O2 content of the atmosphere today relative to values prior to the carboniferous.  Ergo, it is simply wrong to assume that any excess draw down of CO2 due to the CO2 fertilization effect will not only be matched, but exceeded by an increased production of CO2 by cellular respiration.

  2. I would like to know if there are measurements done showing the output of CO2 at night above seas, forests and grassy lands. If someone knows how many tons of CO2 is released at night and how much is absorbed by day.

    Also when plants die they release a lot of CO2 in the atmosphere, almost as much as they took in while they were growing and alive. One of the reasons I bring this up is to see what is the natural cycle and volume of CO2 going out and back into the atmosphere as we need some standard as to how nature processes CO2 .

  3. Hans @27, there are a large number of studies of carbon exchange in a variety of ecosystems and seas, many of which will also analyse the day/night (diurnal) cycle.  As examples, Leinweiber et al (2009) analyze the diurnal cycle in Santa Monica Harbour.  Friend et al (2007) compare model and observaltional values net ecosystem exchange of CO2 (NEE) across a variety of land based ecosystems, including the diurnal variation in NEE (Figure 5, bottom panel).  However, it is easier to just look at the gross fluxes from the IPCC AR5 Figure 6.1:

    The gross fluxes are are indicated by the large arrows.  Where the gross flux is two way, the net flux is indicated above the brackets.  Values inside the boxes are the total amount in the reservoir.  Black figures indicate preindustrial values, while red values indicate the change to the preindustrial value due to anthropogenic influence.

    Although the gross flux is what you appear to be interested in, it is the net flux that is the relevant comparison for anthropogenic emissions, given that the gross flux largely represents churning which does not alter atmospheric concentrations, except locally on a diurnal basis, and regionally on a seasonal basis.

  4. Recommended supplemental reading:

    Global warming threatens nutrition levels in staple crops by Adam Wernick, Public Radio International (PRI), Oct 29, 2017

  5. After 24 years, the pattern reverses: C3 plants grow worse as CO2 continues to increase, and C4 plants grow better. New article.

  6. According to NASA, the earth is greening, and it is 70% due to anthropogenic CO2.

    https://www.nasa.gov/feature/goddard/2016/carbon-dioxide-fertilization-greening-earth

  7. @ remmons 31: 

    The NASA article that you reference was published on April 26, 2016. It has been susequently updated by the following article:

    Human Activity in China and India Dominates the Greening of Earth, NASA Study Shows, NASA, Feb 11, 2019

    The concluding/summary paragraphs of the update article:

    The researchers point out that the gain in greenness seen around the world and dominated by India and China does not offset the damage from loss of natural vegetation in tropical regions, such as Brazil and Indonesia. The consequences for sustainability and biodiversity in those ecosystems remain.

    Overall, Nemani sees a positive message in the new findings. “Once people realize there’s a problem, they tend to fix it,” he said. “In the 70s and 80s in India and China, the situation around vegetation loss wasn’t good; in the 90s, people realized it; and today things have improved. Humans are incredibly resilient. That’s what we see in the satellite data.”

    This research was published online, Feb. 11, 2019, in the journal Nature Sustainability.

     The moral of the story: Science is a continuous process. 

  8. During the Carboniferous period the level of CO2 in the atmosphere was three times that of 1750, and double that of 2019. The Carboniferous period had lush forests, and a great deal of the coal, methane, and crude oil that was trapped in the Earth came from that period. Plants seem to thrive in high CO2 concentrations.

    It can be projected that had humans not come along and extracted and burned fossil fuels, CO2 levels would in the future go so low for this critical plant nutrient that plants would be unable to grow. In the long-term, I believe humans' greatest contribution to life on Earth is our re-introduction of CO2 into the atmosphere.

  9. Wowzee , you are being an alarmist about "dangerously low CO2 levels".

    Without the recent human intervention, it would still have taken millions of years for CO2 to have fallen to a level dangerous to the "C3 metabolism" types of plants (which are so important for our food production).

    And taken even longer to be dangerous to the "C4 metabolism" plants [maizes , grasses , etcetera] . . . assuming that they did not make further evolutionary adaptation to low CO2.   

    As for the very distant future (as atmospheric CO2 gets gradually absorbed into rock-carbonate form) . . . then sure, the humans of that time will know the lessons of the distant past [= 20th Century AD] and will know that all they need do is dig up and burn a few gigatons of coal/oil every 100,000 years or so.

    Makes sense for us to stop burning coal/oil . . . and leave it in the ground in case our ultra-distant descendants should ever need it as an easy way to raise CO2 and warm the climate !   That's the sensible and responsible thing for us to do at present.

  10. Also plants get more nutritious at higher temperatures which would counteract any drop in nutrition due to higher CO2 at the FIXED temps the experiments were done at.

    This of course makes sense as the world did not fall to pieces when CO2 levels were higher in the past.

     

    It is kind of like "intelligent design" if you wish, or if you are an evolutionist, plants evolving to cope with whatever life could throw at them. Or maybe the Gaia hypothesis?

    Response:

    [PS] Sloganeering. You must justify your comment with links to backing research. You continue to behave as if comments policy does not apply to you.

    That planet is greening is not disputed - you dont seem to be following the detail of the problem.

     

    Final Warning

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Moderating this site is a tiresome chore, particularly when commentators repeatedly submit offensive, off-topic posts or intentionally misleading comments and graphics or simply make things up. We really appreciate people's cooperation in abiding by the Comments Policy, which is largely responsible for the quality of this site.
     
    Finally, please understand that moderation policies are not open for discussion.  If you find yourself incapable of abiding by these common set of rules that everyone else observes, then a change of venues is in the offing.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter, as no further warnings shall be given.

  11. In the moderation, the following statement, "That planet is greening is not disputed - you dont seem to be following the detail of the problem." has me a little unsure of how to view this.

    I've followed the claim that the Earth has been greening due to the increase of CO2 for at least the last decade.  The NASA article I read last spring, as I remember, said that it was mostly due to the planting of trees in China and India.  In addition the greening of the Arctic was also in play.  It went on to say, as I remember, that it had become greener in the areas that were the greenest due to the increase of moisture.  But it said that the browner areas were growing.

    Then  I recently ran upon this article in Scientific American in which they conclude that the greening stopped 20 years ago.  They site two other sources, one of which is 9 years old.  I blog quite a bit and feel unsure about engaging on this subject.  I've been considering asking you guys about this for a couple of weeks.

    https://www.scientificamerican.com/article/earth-stopped-getting-greener-20-years-ago/

  12. Estoma,

    I read this recently: https://www.nasa.gov/feature/ames/human-activity-in-china-and-india-dominates-the-greening-of-earth-nasa-study-shows

    The SciAm article you linked refers to this recent study, which I believe I had also referenced recently after seeing in a French newspaper:

    https://advances.sciencemag.org/content/5/8/eaax1396

    However, as the SciAm article rightfully reminds us, the greening is largely a red herring.

    Whatever benefit may be had from increased CO2 for plant life, it is dwarfed by the most important factor that always has been and always will be water availability. That is water in sufficient but not excessive quantity and at a the appropriate time. Go ask the midwest farmers if the extra CO2 in the atmosphere helped their flooded fields last spring.

  13. Philippe Chantreau, thanks for your reply.  It's one of those topics that get a lot of work by skeptics. 

    I'm Indiana farmer.  I read recently that here in the south central part of our state we have increased our yearly rainfall amount by nearly 6" since  the 1950's, as I remember.  They reported that the lion's share of that 6" occurs in the spring.  They said that would be followed by dry conditions and that's what's occured. 

    Over the past couple decades I've had more than a few springs where the opportuniy to plant was low.  You didn't wait.  This year we are having what the models predicted back in the 90's.  

    I'm not sure how the harvest is going to shake out this year.  I don't do corn or soybeans.  If they do have decent crops the one good thing is they won't spending a lot drying their grain.

  14. Recommended supplemental reading:

    CO2 can increase plant growth in greenhouses while also negatively affecting ecosystems and human societies by Nikki Forrester, Climate Feedback, Mar 19, 2020

  15. The climate deniers are getting out of hand here in the US.

    I have a Canidain denier trying to covince me that CO2 is plant food and good for the earth.  He proceeds to back his beleif by posting this on LinkedIn.  

    …CO2 is Greening Planet Earth…

    ¯\_(ツ)_/¯

  16. Ops I meant Canadian denier. 

  17. TVC15  @#40 ,

    undeniably, CO2 is a plant food, and the increased planetary greening is of benefit to insects and goats.  Whether the general increased greening is to be beneficial to humans, is an arguable point (see the Lead Article, above).

    Also note that some of the greening in India has come from increased irrigation, and some of the greening in China has come from extensive governmental tree-planting programs.

    And the greening of past decades may be plateauing out ( I do not have a convenient citation for that).

    What is amusing, is how so many prominent denialists will vehemently [and correctly] assert that nearly all life on Earth is supported by the tiny amount of CO2 in the atmosphere . . . yet, minutes later, they will assert that the atmospheric level of CO2 is simply too tiny to affect Earth's climate.

  18. TVC15 @#40 ,

    the Canadian denialist has linked to an "amusing" website article showing :- 

    (A) "Plants have been starved for CO2 at the low levels existing before the industrial revolution".    (Which would be why the South American Amerindians have dwelt in semi-arid wastelands for thousands of years rather than discovering the lush vibrant Amazonian Jungle ? )

    and (B) "Mankind has been living on the edge of extinction with low levels of atmospheric CO2"  . . . since all (terrestrial?) plants die if atmospheric CO2 falls below 150ppm.

    I gather that under experimental conditions, plants can survive with less than 150ppm CO2  ~  but this is confounded by many other associated factors that would apply in a real world situation (including very extensive glaciation).

    See my comments in #34 above.

    But the very low CO2 scenario is moot because of recent anthropogenic CO2 emission.   Without current human emissions, the planet would have continued for millions of years before reaching below the 150ppm mark.  During that time, "assisted" or natural evolution would likely produce plants adapted to low CO2.  Assuming that the humans of the future would decide not to burn a few gigatons of coal occasionally . . . or chose not to use heat to decompose calcium carbonate rock.

    Humans "on the edge of extinction"  is alarmist hogwash.

  19. Perhaps rather than considering that CO2 as being plant food it is more exact to describe it as the basic building block of a carbon based life form, given all life as we know it is just that.

    Typically, 99% of the human body consists of six elements, oxygen 65%, carbon 18.5%, hydrogen 9.5%, nitrogen 3.2%, calcium 1.5%, and phosphorus 1.0%. which explains why when the human body combusts, so too with most other materials, all that is left is carbon.

  20. Eclectic @42, @43

    I truly enjoy your responses.  I've learned so much from posting the links and comments from the occasional denialist I run across.

    I told the Canadian to go ask the Midwest farmers if the extra CO2 in the atmosphere helped their flooded fields in 2019 and the econimic impact it cost.   I suggested that while he's at it, please ask the CA farmers if the extra CO2 in the atmosphere helped their drought stricken crops last year and to explain how climate change impacted these events.

  21. johnd@44

    Hi johnd,

    CO2 is not the basic buidling block for a carbon based life.

    CHNOPS, which stands for carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, represents the six most important chemical elements whose covalent combinations make up most biological molecules on Earth.

    Carbons ability to hybridize orbitals is the key factors here, not the CO2 molecule.

    It's also Carbon's capability of forming a vast number of compounds, more than any other element, with almost ten million compounds described to date. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. Carbon is important because of its ability to form long chain-like molecules (carbon chains form the backbone of organic molecules.

    Finally, consider the abundance of carbon in the universe ( 4,600 ppm in the universe) versus the next group 14 element silicon (650 ppm in the universe) and you see that carbon has a  unique abilty  to hybridize its S and P electron shells, thus making 4 electrons of equal energy level, this proptery of carbon really was the only way to achieve a carbon based life here on earth. Why? Because carbon can form stable bonds with itself, but also with a variety of other types of elements. This is why life is "carbon-based" or predominantly carbon.




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