Photosynthesis and CO2 Enrichment

The benefits of increased atmospheric CO2 on crops are so extensive that a long article or book needs to be written to do justice to the subject and to the results of thousands of research trials. The improvement in photosynthesis efficiency at higher CO2 levels does not tell the whole story, but it is a good place to start, since all plant growth relies on this process.

Photosynthesis is the process by which plants utilize visible light energy (e.g. sunlight) to convert aerial CO2 and water (from roots) into plant matter. This process also requires phosphorus and nitrogen.

There are three photosynthesis ‘pathways’, known as C3, C4 and CAM. CAM is unimportant for food crops, being the method used by cacti, succulents and agaves. Pineapple is the only food crop of any importance to use CAM, so CAM can be neglected for the present purposes. World food security depends on C3 and C4 photosynthesis.

Less than 1% of all plant species in the world use the C4 photosynthesis pathway. Of the 86 plant species that supply most of the world’s food, only five use the C4 photosynthetic pathway, of which only four are of much importance (corn [=maize], sorghum, millet, and sugarcane) yet these four constitute some 20% of all the food crops grown. Because of their high photosynthetic efficiency, the C4 crops corn and sugarcane are favoured for ethanol production by those who want to produce liquid biofuels rather than food, thus increasing food prices and poverty.

Those crops using the C3 pathway include nearly all cereals (wheat, rice, barley, oats, rye, triticale etc), all legumes (dry bean, soybean, peanut, mung bean, faba bean, cowpea, common pea, chickpea, pigeon pea, lentil etc), nearly all fruits (including banana, coconut etc), roots and tubers (potato, taro, yams, sweet potato, cassava etc). C3 is also the pathway for sugar beet, for fibre crops (cotton, jute, sisal etc) and oil crops (sesame, sunflower, rapeseed, safflower etc), and for trees.

At present atmospheric levels of CO2, C4 plants are more efficient at photosynthesis than C3: in absolute conversion efficiency of light energy to stored chemical energy they are around 7% efficient, compared to 4% for C3. C4 plants typically use less water per weight of biomass produced, and can tolerate greater water and temperature stress than C3 plants. Accordingly, C4 crops are most often grown in tropical and equatorial regions.

The advantage that C4 plants have in terms of photosynthesis does not always translate into higher harvest yields, however, as only parts of the plant are edible. In terms of ground use, C3 crops can produce some of the highest amounts of edible calories and protein per acre: for example, potatoes and soybeans respectively.

C4 plants show a relatively small improvement in photosynthesis rate with increasing atmospheric CO2 above present levels; however, at increased levels of CO2 the leaf pores (stomata) of both C4 and C3 plants increasingly close up, which also reduces the amount of water lost by the plant (transpiration). Thus C3 and C4 plants significantly improve their water efficiency as CO2 levels increase. This is shown below for C4 (corn) and C3 (soybean).


C3 photosynthesis is less efficient than C4 partly because of an effect known as photo-respiration, which results in the loss (to the atmosphere or soil) of a substantial proportion of the carbon that has been extracted from the atmosphere by photosynthesis. C3 photo-respiration increases under heat stress and drought, which is a major factor behind the choice of C4 crops for hot dry climates. However, as CO2 levels increase, photo-respiration is suppressed, such that at double today’s levels of atmospheric CO2 the efficiencies of C3 plants (in photosynthesis rate and water use) are as good as or better than C4 plants. Moreover, at higher levels of CO2, C3 plants can maintain efficient photosynthesis rates at considerably higher temperatures than today’s conditions – their optimal temperatures for photosynthesis increase.

The effect of CO2 concentration on photosynthetic rate at constant temperature is shown below for C3 and C4 crops.


As CO2 concentrations increase, the photosynthetic efficiency gap between C3 and C4 plants rapidly closes, and at double today’s CO2 concentration (i.e. at 780 ppm instead of today’s 390 ppm), the photosynthesis rates are the same. Incidentally, the majority of the world’s most troublesome weeds use the C4 pathway, and so have a competitive advantage over C3 crops at current CO2 concentrations. At higher CO2 concentrations, competing for the same resources on the same patch (light, water, CO2, nutrients etc), C3 crops increasing out-compete the weeds.

The photosynthesis rate with temperature is shown below for C3 plants at today’s CO2 levels (Low CO2), and at double CO2 level (High CO2).


The upper curve is the same for C4. From this it is clear that at double CO2 concentration, not only has the efficiency of C3 crops improved tremendously, but the temperature at which optimal photosynthesis occurs in C3 increases up to that of C4. Thus the vast majority of food crops will benefit hugely by increased CO2, and even more so by increased CO2 coupled with warming.

A dangerous combination would be increased warming without increased CO2. Since there is no proof whatsoever that increasing CO2 is having any significant effect on climate (any climate changes might be taking place by completely natural means over which we have no control), but there is incontrovertible evidence that increasing CO2 is positively beneficial with or without warming, then on the basis of risk mitigation and precaution it is utterly foolish to be reducing carbon emissions. As S.A. Cowling put it in Plants and temperature – CO2 uncoupling (Science, 1999, 285, 1500-1501)

We should be less concerned about rising CO2 and rising temperatures and more worried about the possibility that future atmospheric CO2 will suddenly stop increasing

The article Global Temperature Change and Terrestrial Ecology in the Encyclopedia of Water Science (CRC Press, 2007) has the matter stated correctly:

[It is a] well-established fact that CO2 is a powerful aerial fertilizer, which when added to the air can substantially increase the vegetative productivity of nearly all plants…numerous studies have demonstrated that the percent increase in growth produced by an increase in the air’s CO2 content typically rises with an increase in air temperature. In addition, at the species-specific upper-limiting air temperature at which plants typically die from thermal stress under current atmospheric CO2 concentrations, higher CO2 concentrations have been shown to protect plants and help them stave off thermal death…[and] increase the species-specific temperature at which plants grow best. Indeed it has been experimentally demonstrated that the typical CO2-induced increase in plant optimum temperature is as great as, if not greater than, the CO2-induced global warming typically predicted…Hence, [with] an increase in the air’s CO2 concentration – even if it did have a tendency to warm the earth (which is hotly debated) – …[plants] …would grow equally well, if not better, in a warmer and CO2-enriched environment.

We will set out the full range of benefits of increasing CO2 in future posts, but as an initial summary the following is helpful by Vaclav Smil from China’s environmental crisis: an enquiry into the limits of national development (M.E. Sharpe, 1993)

There could also be important beneficial effects, above all a roughly 30 percent higher crop productivity brought by 600ppm of atmospheric CO2 and higher tropospheric temperatures. And the benefits may not end with higher productivity. As photosynthesis is predicated on a very uneven CO2—H2O exchange, higher atmospheric CO2 levels would significantly boost the water use efficiency of all plants. This reduction [in water use] would also average about 30 percent.

Other notable benefits or a higher CO2 level include lower photorespiration (which would increase both the optimum as well as the upper temperature range for photosynthesis), substantially improved symbiotic fixation of nitrogen in leguminous plants, increased resistance to lower temperatures, and air pollution, and a better tolerance of soil and water salinity. A combination of these responses would mean that all major crops would yield more in their current environments while using less water and, when rotated with leguminous species, less fertilizer—or they could be grown in areas considered today too arid for continuous field farming, or that they may be able to outperform the current yields in those regions where precipitation may decline…

Policies such as reducing carbon dioxide emissions, carbon capture and storage, taking land out of food use for biofuels (or onshore wind farms), increasing energy costs, grossly inefficient and poisonous ‘organic’ farming methods etc all serve to destroy the capability of this planet to support an increasing population. Behind this surely is the stated neo-Malthusian and Green policy to wipe out billions of lives by the sheer force of economics. As hunger increases, it will be blamed on man-made climate change, and the screw will be turned ever more tightly to introduce policies that will accelerate the destruction of mankind.

12 Responses to “Photosynthesis and CO2 Enrichment”

  1. 1 manacker July 9, 2009 at 10:17 am

    Very interesting information. Too bad that this sort of information is all but drowned out by all the negative predictions on AGW.

  2. 2 Ron Huber July 7, 2010 at 12:04 am

    BP oil spill is a quantifiable hydro-carbon enrichment of a large ocean area. What sort of changes in affected area photosynthesis are reasonable to predict?

    Does this give anaerobes the opportunity to occupy more ocean water column habitat space?

  3. 3 David August 30, 2010 at 2:19 pm

    I would like to see this addressed in advance by credible economist. I strongly believe that CO2 has been a hugely net positive for all countries through its well established beneficial effect on all plant growth. To often skeptics are placed in a defensive position trying to refute literally hundreds of poorly done studies predicting the next CO2 induced disaster. A well written and researched report on the realized and future benefits of CO2 would be a powerful ally giving ammunition to the politicians trying to resist the large scale social changes advocated by the political left advancing CAGW as the reason necessitating statism on a worldwide basis. Such a report would need the following…

    An estimate of the current worldwide food, clothing, and wood production including food for livestock with monetary value included.

    An estimate, based on the many available CO2 studies, of what these same production goods would currently be if CO2 was still 280 PPM.

    An estimate of how much it would cost to bring the lower crop production of an 280 PPM CO2 world to current production levels of a 390 PPM CO2 world. This would include a straight line cost analysis based on current production costs in all areas, as well as an estimated inflation cost based on additional demand for more land, more water, more fertilizer, more labor, more machinery costs etc; all at higher costs due to greater demand for said resources. This would add several percentage to the direct line cost estimates.

    A further estimate of the future, based on additional CO2 up to whatever level is reasonable, keeping in mind that any adverse warming from additional CO2 effect decreases exponentially, while the benefits continue to rise in a more linear fashion.

    Ideally this would be broken down by country before totaled.

    And finally, some comments on the social pressures that a lower CO2 world would have, potentially resulting in social revolutions and international military conflicts.

    I could see this as an excellent masters thesis for some young PHD candidates also.

  4. 4 Hans Zimermann September 8, 2010 at 4:26 am

    Well good reading, it is an interessant point of view about benefits/demage about CO2 increasing.
    Good Job, good research.

  5. 5 SAWAN October 2, 2010 at 12:50 pm

    good information it helps people who are unknown from these thing good guide line and helpfull for insanity

  6. 6 Can D February 9, 2011 at 1:29 am

    Since you argue that higher CO2 and heat is good for food crops and Global Warming is BS then continuing your logic;

    As CO2 increases with other gases mirroring out available sun rays on the surface temperature will fall reducing photosynthesis.

    your short term win buries us in the long run…

    ScientistForTruth replies

    No, you are completely wrong. Levels of CO2 have practically no effect on the incoming radiation from the sun. It does not ‘mirror out’ or absorb incoming radiation. Most of the available energy from the sun is at ultraviolet, visible and near infrared, as would be expected from its colour temperature, for which CO2 is transparent. Photosynthesis occurs due to visible light, and CO2 is transparent to these frequencies. Black body radiation from objects at terrestrial temperatures (much cooler than the sun!) is in the far infrared where CO2 has some absorption bands.

  7. 7 Peter Ewing February 20, 2011 at 6:33 pm

    Lost in the media created hysteria re AGW is the fact that atomospheric CO2 levels have been falling for hundreds of millions of years from levels orders of magnitude higher than the current level (370ppm). A drop of only 100 ppm from here would trigger a collapse in plant life worldwide triggering a planetary mass extinction event. We are far closer to the that possibility than we are to run away climate change yet you don’t hear a word about it. Thanks for a great piece of research.

    ScientistForTruth replies

    Thanks for your encouragement. A drop of 100 ppm (to the so-called ‘pre-industrial’ level) would indeed have a dramatic effect on C3 photosynthesizers, which the majority of crops for human consumption are. Since the industrial age, C3 photosynthesizers have been emerging from ‘suffocation levels’ of CO2; it has been absolutely essential to have higher levels of CO2 to support the current world population, and it will be necessary to increase the levels further to support the expected 3 billion extra mouths to feed by 2050. To support cutting back on CO2 emissions is to vote for genocide of the poor.

  8. 8 kit March 29, 2012 at 8:59 am

    kindly site your insights. .asap

    Why are the fruits of the C4 and CAM plants sweeter than that of the C3 plants?

    ScientistForTruth replies

    I don’t think I can agree with the premise. Whilst sugar cane is C4, sugar beet is C3. C3 can be the pathway for plants that produce sugar. If you mean strictly fruits, then many that we consider the sweetest such as plums, strawberries, bananas, apples, peaches, dates etc use the C3 photosynthetic pathway.

    The main CAM plant that is eaten is pineapple, which can be sweet or acid depending on the variety and cultivar.

    Photosynthetic pathway doesn’t determine sweetness – humans select from the vast array of fruits those that are sweetest, and they are of all three photosynthetic pathways.

    Of the important sweet fruits sold worldwide, I would think most are actually C3.

  9. 9 Debunked August 20, 2012 at 7:37 pm

    Ask the farmers in the Mw US how their crop yields are doing this summer!
    Co2 levels are largely irrelevant if climate change vastly reduces the amount of available WATER! You also failed to mention that many crop pests (fungal & insects) have already extended their range northward in a warning world – this will further reduce yields.
    You also showed your true ideological colors in the last couple paragraphs which a legitimate scientist would never do.
    “Poisonous organic farming” and the Green movement wants to purposely kill millions? REALLY!? For those of you intent on finding a conspiracy re: climate change – I suggest you follow the REAL money! Fossil fuel & agrichemical companies have pumped far more money into this debate than the other side could ever DREAM of!
    In the interest of full disclosure, I’d like this author to declare any money/support that he may have received from these industries?

    ScientistForTruth responds

    “You also showed your true ideological colors in the last couple paragraphs which a legitimate scientist would never do.”

    This is a blog, chum, not an article in a learned journal.

    Poisonous organic farming? Yes, organic farming is terribly hypocritical. It won’t use something synthesized as a specific agent against fungus and parasites that breaks down into harmless compounds within weeks. It uses broad spectrum toxins that never decay and simply accumulate in the soil, poisoning it.

    And, yes, the Green movement wants the earth to be de-populated.

    “In the interest of full disclosure, I’d like this author to declare any money/support that he may have received from these industries?”

    Happy to. I have never received any money or support from fossil fuel or agrichemical industries.

    Not everyone who can see the folly of the Green movement is bankrolled by industry. That’s just a silly conspiracy theory.

  10. 10 Shane September 26, 2012 at 11:25 pm

    So, are you suggesting that because increased CO2 is good for C3 crops, then we should just put environmental regulation of CO2 in the back seat without any thought to consequence? If the planet truly does experience climate change from reflected energy being trapped by CO2, (a phenomenon that also has scientific backing), then would not that eventually make the food crops useless because of heat more extreme than the graph depicts above. Regardless of adaptations to water limitations, i feel that a month long 100 degree drought would kill a corn or wheat crop regardless of their CO2 surplus. Would it not be wise to improve food production methods while monitoring CO2 at the same time?

    ScientistForTruth replies

    The proven benefits of increasing CO2 far outweigh the small hypothetical attendant temperature increase.

  11. 11 Gary Waayers September 2, 2013 at 11:48 pm

    If CO2 where the limiting reagent in plant fruit production, why do the plants in gardens produce more fruit as fertilizers are applied?

    ScientistForTruth replies

    Plants can be limited by a number of factors: light, water, available nitrogen, potassium etc. Eliminating the constraints will help a plant flourish, so if soil is poor then fertilizer will help. If a plant has sufficient nutrients, water, light etc then, for C3 plants at least, atmospheric CO2 will be the limiting factor, which cannot be improved unless one grows in an artificial CO2-enriched atmosphere.

    Another point to note is that where CO2 is limited, the plant is less robust to the effects of poor soil, restricted water etc. All in all, a higher level of atmospheric CO2 (to the levels projected over the next couple of centuries) has good effects for plants, especially with the C3 pathway, without any downside, and will be an important factor in being able to sustain a growing population. Parts of the world where it is very difficult to grow crops (because the soil is poor, or there is insufficient rainfall) and where there is currently malnutrition, can be expected to be able to support crops better than they do now.

  12. 12 Andrew S Hansen June 10, 2018 at 1:52 am

    The most important aspect of this blog is its reliance on actual scientific data. The photosynthetic efficiencies of C3 and C4 plants are not based on hypotheses or correlations. They are based on experiments that are repeatable and verifiable. Even the best papers (e.g., from the Berkeley Earth Project or National Academy of Sciences) rely on correlations between the recent rises in CO2 with minuscule rises in temperature over very short periods of time. Correlation is not causation. PERIOD. I find it absolutely atrocious how the foundational principles of science are disregarded in “climate science”. Modern science is suppose to be based on data, not consensus. Politics and religion are founded on consensus. Science is suppose to be based on data. I’ve been searching for years and still haven’t found any data to support the hypothesis that the rise in temperature is a bad thing or that it is caused by CO2. Can someone point me to a paper that doesn’t rely at its foundation on pure correlation?

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