Some big-name science papers ignore CO2 fertilization when assessing agriculture

Guest essay by Alberto Zaragoza Comendador

CO2 is food for plants: a simplistic statement, but also a correct one. The factoid is so well-known, so basic, bringing it up in a discussion may only elicit a shrug or an eyeroll. Nevertheless, I have found that some very prominent papers on the topic of climate change impacts ignore it – just one reason the results they get are dead wrong.

Warning: this is a serious i.e. boring article. If you want to have a laugh check this out.

Yes, CO2 also causes other stuff but the net impact on plants is positive

Let’s get this out of the way. There is some hand-waving about how the CO2 fertilization effect will ‘peter out’ (at some unspecified point in, uhm, the future – it won’t peter out in the past); pessimists also argue that CO2 fertilization is ‘countered’ by other factors, such as the warming induced by CO2 itself (heat stress), or possible weather changes caused by this warming. But such statements are meaningless without quantification: every force in this world is ‘countered’ or ‘offset’ by other forces. If you jump off a tenth floor the force of gravity will be ‘countered’ by the air drag, which will make you fall slower than you would have if affected by gravity alone; and yet the force of this air drag is utterly irrelevant when assessing the impact of jumping off a cliff. In other words, some effects and forces matter more than others.

The bottom line is more CO2 helps plants grow more, the effect will not ‘peter out’ until we reach CO2 concentrations several times higher than now (at least for C3 plants, which is to say 95% of the world’s plants), and there really is no reason to think commercially-grown plants (agriculture) behave differently than wild ones. The last point is critical. When discussing the increase in agricultural yields, there is of course a confounding factor in that agricultural technology and technique change over time (usually for the better). Thus CO2 fertilization cannot be thanked for the entirety of this increase. But forests don’t rely on technological contraptions such as genetically-modified seeds: the fact that they, too, are growing almost everywhere is a strong indication that the effect of CO2, net of any change in technology and technique, is helping plant growth. And yes, a bigger plant will usually produce more – hence it should also be helping to increase yields.

In other words, the increase in global tree cover, green area, leaf area or whatever you call it is evidence that CO2 fertilization is more than offsetting the alleged effects of increased drought and so on. If it works this way with wild olive trees, why wouldn’t it do the same with commercially-grown ones?

The papers in question

I’ve found this problem in:

· Global non-linear effects of temperature on economic production, by Burke, Hsiang and Miguel. I will refer to it as BHM15. Cited 66 times according to Google Scholar.

· Nonlinear temperature effects indicate severe damages to US crop yields under climate change, by Schlenker and Roberts (SR09). Cited 883 times.

· Rising temperatures reduce global wheat production, by Asseng et al (AEA14). Cited 101 times.

BHM15 describe the methodology as: ‘We estimate how economic production changes relative to the previous year—that is, annual economic growth—to purge the data of secular factors in each economy that evolve gradually’. There follows a paragraph providing more detail, but the key is that the comparisons in temperature and production are made between 1970 and 1971, between 1995 and 1996, etc.

If you only compare the year-on-year change in economic activity you’ll eliminate the long-term trend. In principle this sounds good, as we don’t want the effects of an increase in temperatures to be conflated with, for instance, a secular slow-down in economic growth (or the aforementioned improved technology, in the case of agriculture). But if you remove the long-term trend you also remove the CO2 effect. There is no mention of adjusting the data to account for this – in fact the words CO2, carbon, and fertilization appear in the text 0 times.

Let’s use a simplified example. Yearly temperatures tend to zig-zag: rather than every year setting a new record high by 0.01ºC or so, the long-term positive trend is marked by many ups and downs – and when a new record is set, it is usually by more than the average yearly increase. A way to make more sense of what the current temperature level ‘is’ is to use temperatures averaged over more than a year (the longer the better as we reduce year-to-year noise). Let’s assume a decadal average removes all the year-to-year noise. The question is, when will decadal temperatures increase 1ºC from the 2011-2020 average?


(HadCRUT4, monthly values; courtesy of Wood for trees. The yearly changes are smaller than those seen here).

Imagine the data shows that, in years that were 1ºC hotter than the previous, yields fell on average 10%. Yes, I know that in the real world single-year changes are never that big, but it serves to illustrate the example; the authors somehow extrapolate from yearly changes of 0.1 or 0.2ºC to century-scale changes about 10 times bigger and I’m not qualified to comment on how valid their methods are.

The key is that technology in both years should be almost the same so we’re pretty sure that the change in production was due to the weather. Therefore when decadal temperatures have risen 1ºC, you would also expect production to decline 10%.

What the authors forget is that by the time decadal temperatures have risen 1ºC, CO2 levels will be much higher. Thus, the effects of a 1ºC increase from 2015 to 2016 will be radically different from the effects of the same temperature increase from 2011-2020 to, 2091-2100, for instance.

How different? Imagine we start with 400 parts per million (ppm), which is about right for 2015. If transient climate response is 1.5ºC, which is lower than in most climate models but higher than in recent studies of the thermometer record (see slide 51), then a temperature change of 1ºC is equivalent to 2/3 of a doubling in CO2 concentrations. There are other greenhouse gases, and going by the recent ratio of CO2-to-other-GHGs radiative forcing (historically 65%, but about 80% in recent years – see table 2), we can assume that instead of 2/3 of a doubling, this would require 0.67 x 0.8 = 0.536 doublings. There is also a warming effect from the aerosol forcing, which is becoming less negative over time, plus possibly additional warming from a decline in the radiative imbalance (heat released from the ocean).

So it’s unclear whether the additional 1ºC would require increases in CO2 of 30, 40 or 50%. But any of these increases is huge. In the lowest case, we’re talking about an additional 120ppm – about the same increase as took place from the ‘preindustrial’ era, circa 1750, to 2015. The study I linked to at the beginning found a strong greening trend between 1982 and 2009, when CO2 levels rose only by 30ppm. It’s safe to say the greening from a CO2 increase 4, 5 or 6 times bigger would be much greater.

In short: the authors looked at the effect of warming on plants, but failed to factor in the fertilization effect of the very CO2 that will cause the warming. This is similar to the mistake in a paper by McLean and two other authors: they detrended temperatures and saw that the main factor explaining the remaining variance was El Niño, thus concluded that a change in El Niño patterns could have caused the warming seen in recent decades. But of course, if you eliminate the trend you eliminate the CO2 effect, whether we’re talking about agricultural yields or temperatures.

Now it’s time to see SR09. This paper is focused on the growth rates of specific crops grown in the US under different temperatures. It uses daily temperature data rather than year-on-year changes. Having arrived at it after reading BHM15, which cited it, one of the first things I did was search for the word ‘fertilization’ and boom:

‘An important caveat concerns our inability to account for CO2 concentration. Plants use CO2 as an input in the photosynthesis process, so increasing CO2 levels might spur plant growth and yields. Yield declines stemming from warmer temperatures therefore may be offset by CO2-fertilization… We cannot account for CO2 effects in regression analysis of observed yields because CO2 concentrations quickly dissipate throughout the atmosphere, leaving only a gently increasing time trend, which is impossible to statistically disentangle from technological change.’

Emphasis added. On the one hand, it’s admirable the authors themselves note this caveat. On the other, one wonders how many of the people citing the paper are aware of this.

Finally, AEA14 is a study that tries to simulate the past (responses of agricultural yields to temperature increases) to forecast the future. Its headline claim was:

‘Global wheat production is estimated to fall by 6% for each ◦C of further temperature increase’

And how do they know that?

‘We systematically tested multiple models against field and artificial heating experiments, focusing only on temperature responses.’

Emphasis added. There is no deception on the side of the authors, but the decision to run models that only vary temperature is bizarre: it invalidates their results completely, for the same reason as the other two papers. Even more bizarre is the fact that their references show four papers that do take CO2 into account (two of which included Asseng, the lead author of this paper).

Studies refuted by 2015-16 temperatures, not just by CO2 fertilization

I’m just going to comment on what Andrew Bolt pointed out a few days ago with a bit more precision: temperatures over the last two years have surged into what the papers above classify as decline territory, and yet all major crops are at or near all-time highs. In fact, the rapid increase in temperatures is as close to a perfect experiment as the real world could get, because there has been almost no time for adaptation and technological change. One can even dismiss CO2 as a growth factor for the period: concentrations of this gas have risen in the past two years by only 5ppm.

Again checking HadCRUT4 we see that temperatures in the first half of 2016 were about 0.4ºC higher than the average for 2010-2014.


Even this is an underestimate, though. While BHM15 referenced global temperatures, SR09 and AEA14 discuss the impact of field temperatures, i.e. right where the wheat, corn, etc. grow; they also specify that these are the temperatures during the growing season. So even though the last quarter of 2016 will be colder than the first nine months, reducing the gap with average 2015 values, we can ignore that; spring and summer temperatures are what matter most for plant growth.

If the question is ‘what were the temperatures where wheat (for example) was grown’, well, I don’t have an answer. But the fact that a perfect answer is not available shouldn’t keep us from trying to get one that is closer to the truth. At a minimum, we can check temperatures in the Northern Hemisphere, which is where 90% of us live and where most food is grown.


Well, there you have it: the first half of 2016 was about 0.6ºC warmer than the 2010-2014 average. And sorry to repeat myself, but this is probably an underestimate as well – the increase over land was surely greater than over water. But I believe you get the point.

AEA14 expected wheat yields to decline 6% for each additional degree centigrade. So how much did wheat production decline from 2014 to 2016? 2%? 4%?

It’s actually setting a new record…

on top of last year’s record.

‘The outlook for wheat also improved (by 1.2 percent), putting this year’s world production forecast above the 2015 record, at 741 million tonnes’

I could go on and on with more crops showing blockbuster results, but I’d be beating a dead horse. We’ve seen the effects of a temperature surge, and they directly contradict the predictions of many papers and reports.

Including the IPCC’s latest report, which is looking more ridiculous by the day.

More research is needed (on why these forecasts fail so hard)

I’m an amateur. I lack the time to pore over the literature and the skill to analyze these papers in detail. But this seems to be an issue that could affect a lot more papers. There seems to be a consistent tendency for agricultural forecasts in climate science to fail miserably, and although it’s not like the world depends on correcting these studies, if we understand why they are so wrong perhaps they’ll be a bit more useful – and we’ll learn something along the way.


51 thoughts on “Some big-name science papers ignore CO2 fertilization when assessing agriculture

  1. Peer review is failing us miserably, even in the best journals. Unless this is fixed a great amount of the science produced will be totally worthless. Even worse, will confound other researchers. Scientists are the first to know this problem and they learn who they can trust in their field and who is unreliable. However they are usually powerless because a great deal of power is in the hands of journal editors and star scientists that run the show to their convenience.

  2. Why in all the world would you consider a serious article boring? I haven’t got the foggiest idea if you’re right, wrong, indifferent or completely out to lunch (the beer I’m currently drinking has something to do with that… I think) but, I found your article interesting, not boring.

    • Yes, it wasn’t boring – the warning was not necessary. :-)

      I guess it would be boring to those who already “know” that carbon dioxide is “pollution” and therefore have nothing more to learn. (sarc)

  3. What is in the IPCC reports about the agricultural benefits of more CO2? Food security is an important issue for the UN. So it should have a prominent place.

    • I mean it is settled science that CO2 will turn the world into a desert that is flooded and all life will die. Why read anything that refutes that.

      • ShrNfr

        I could shorten your already brief description by paraphrasing out the alarmist claims:

        As CO2 forces temperatures to rise we will all freeze to death in a flooded desert.

        By then wheat production will reach a billion tons per year. The accumulating snow on top of the wheat mountains will avalanche down upon the last breeding pair of unsuspecting polar bears. Death in the midst of plenty – not a pretty picture.

  4. Arrhenius and driving force. The higher the temperature, the greater the rate of reaction. The higher the partial pressure of carbon dioxide the greater the plant mass produced. This is the reason that some glasshouse horticulturists heat their growing environment and/or boost the carbon dioxide concentration to around 1,000 ppm.

  5. I’ve tried to bring this up in some blog comment sections. The almost universal reply is that the extra productivity is an illusion because the nutritional value of the plant per unit decreases.

    When they offer links it’s usually to a site where this is stated as if it were an established fact with no documentation.

    You can’t argue with people for whom this has become a religion.

    And the really annoying thing, is to find myself compared to creationists by these same people.

    • Peter, like in the article, :quantification” is important. In some studies bio-mass increased say 30 percent, and protein concentration declined a much smaller percent, say 5% or 10%. So a great deal more food and nutrition overall, slightly less dense protein concentration per unit of bio-mass. Net result, more food, more nutrition. (Very few eat vegetables for protein as well.)

      In addition this increased production is done on the same amount of land, on the same amount of water! This is a very large real world benefit. What is not real-world, is the harms CAGW scientists predict from a warming world. They are simply failing to manifest, while the benefits are much greater then admitted.

      • Peter, also consider that CO2 makes plants more heat and drought resistance, if their is an actual increase in global drought, which is not happening.

    • Yeah, that’s why the dinosaurs died: all that vegetation they ate (and there was a lot of it) had such low nutritional value because it was so CO2 enriched.

    • peter,

      “And the really annoying thing, is to find myself compared to creationists by these same people.”

      And it does not dawn on you that we may have been indoctrinated to dismiss the possibility of a Creator, just a youngins are being indoctrinated to dismiss the possibility of beneficial CO2 increases today? I suggest you slow down that reactive mind a bit . . and wonder.

      If, I say if, I were a planner/strategist for a bid to enthrone a technocratic dictatorial global government, I’d see indoctrinating children into reflexively seeing Creation as foolishness as a very important aspect of that attempt. Beware your reactive mind’s reactions to the whole realm, I therefor suggest . .

      I believe we have been lied to, in many scientific realms, for this cause . .

  6. CO2 fertilization is not of importance factor in crop production until and otherwise the three basics are met. The three basics in order are energy component, water component and fertilizer component.

    Dr. S. Jeevananda Reddy

    • So you are saying that CO2 is not important at the bottom of a dark cave made out of rocks in a place where rain does not fall. However, we have noticed that most places where plants are relevant, they do have sunshine, some soil and some water.

      • In an open atmosphere, the crop growth is primarily related to firstly energy, secondly water and thirdly fertilizer. In open atmosphere, CO2 is not limiting. This means, with zero energy or low energy level the crop growth under water and fertiler is insignificant.

        The crop growth under complete cloud cover throughout the growing season the yield is quite different from the fully sunshine condition but this vary with water availability level. Please look in to crop-weather models to understand this scenario.

        Inside the cave, the first energy component is not met and thus no growth.

        Dr. S. Jeevananda Reddy

    • So, in the places where most of the world’s food is currently grown, CO2 fertilization is an important factor because the soil is good, there is plenty of sun during the growing season, and water is generally available in sufficient quantities. Thanks Dr. Reddy!

      • Leo Smith — there are several issues relating to greenhouses — no direct impact of rainfall on the crop, controls pests/insects/diseases, controls the humidity & temperature, etc. It is like growing a crop under irrigation and rainfed condition with supplemental irrigation, wherein yields will be high under irrigation without rainfall.

        Dr. S. Jeevananda Reddy

    • Hmm, but on average, particularly plants growing in native locations and on well managed farms, these elements are not limited and thus CO2 becomes the growth limiting factor. In addition, it can be expected that through reducing transpiration loss, higher CO2 may make a difference albeit not as great even when other factors ARE LIMITED (All other things being equal).

      It’s quite simple to me…
      Around 200PPM C3 plants are down to around replacement productivity, they live but don’t grow, like corn that has drawn down CO2 to very low concentration. At 400PPM we have 100% growth (By definition) then we can say that on average C3 plants increase growth by around 1% for each 2PPM (Give or take).

      Even 5PPM can be expected to increase growth by 1-2%

  7. Why not just do a public experiment . Oh.. forgot they are called GREENHOUSES .
    Unbelievable that the overhyped scary global warming scam could even get this far
    without the obvious benefits of CO2 plant food being celebrated .
    The scientific community needs to step up or it will become a little to obvious how bought they are .

  8. When I read “The Martian”, which is much about a desperate fight to grow enough food quickly enough to survive an unexpectedly long stay on Mars, I noticed that the botanist, with complete control over the atmosphere the plants were in, never played with the CO2 level in the air.

    I can’t believe that in a well-researched book, that trick got missed by the author. More likely, he didn’t want to use the fact that CO2 isn’t a big nasty evil poison, but is plant food.

  9. The poor plants can’t tolerate this much longer-

    “plants from nineteen sites including the arctic, tropical and desert regions were collected and exposed to steadily increasing temperatures within controlled tests.

    Researchers closely monitored their leaf metabolisms to identify the ‘tipping point’ where vital processes — such as photosynthesis and leaf respiration — began to be damaged.

    Unsurprisingly, the study demonstrated higher heat tolerance in plants from closer to the equator than in the arctic.

    However, the researchers found the potential for damage is actually most severe in hot, inland, mid-latitude regions — such as the vast bulk of Australia.”

    • However, the researchers found the potential for damage is actually most severe in hot, inland, mid-latitude regions — such as the vast bulk of Australia.”

      You just have to love them don’t you

      Let me see, plants don’t survive well in Hot, inland, mid latitude regions – hmm, Inland, Mid latitude – Aha, The Simpson Desert

      So our revered researchers found out that astonishingly plants don’t grow well in deserts…

      Of course the Corollary, plants are tolerant in Moderate, coastal and tropical/temperate, like Hmmm, everywhere we grow food in Australia

      Just about spilled my coffee over that one

  10. The notion that a world with warmer weather, more rain, longer growing seasons, more arable land and more CO2 to augment and fortify the basic photosynthetic process of agriculture should be a very hard sell but it seems to have been done!

    The steady drum beat of propaganda has been amazingly effective.

    That, and regards to the temperature rise, it can be shown that the rise in average temperature is mostly due to the daily Minimums seeing the warm-up. The Maximums, not so much. I wonder if the papers take that into account, I couldn’t tell from the article. It looked like they just concerned themselves with the average, not with the actual heat of the afternoon where a lot of the action takes place.

  11. I could just as easily write a post titled “some big name science papers treat CO2 fertilisation when assessing agriculture”. After all I only need to find three papers and google scholar lists 17500 published since 2012.
    Looking at the first page of results there is:
    “Regional disparities in the CO2 fertilization effect and implications for crop yields” in Environmental Research Letters, Volume 8, Number 1 (2013). Table 1 in it shows estimates for increased yields across the
    globe of the order of between 10 and 25%.

    Going further back there is
    “The effects of increasing CO2 on crop photosynthesis andproductivity: a review of field studies” from
    1991 in Plant, Cell and Environment {1991) 14,807-818

    And so on and so on.

    • Surprise, surprise. An industry gets blamed for *bad* things, and since they can’t prove that they aren’t bad, they shift the blame. Now the finger pointing has turned back to “big sugar”. This disgusting blame game will just continue on draining money from research into the real causes of these illnesses.

  12. USDA September crop report was out earlier today. The update, includes the estimate for this years record corn and soybean crops:

    “Corn production is forecast at 15.1 billion bushels, up 11 percent from last
    year” “If realized, this will be the highest yield and production on record
    for the United States.”

    “Soybean production is forecast at a record 4.20 billion bushels”

    The last 4 decades of weather/climate have been the best in at least 1,000 years(since the Medieval Warm Period that got the name for a reason)

    Dial in the increase in CO2, now at a more beneficial 400ppm, but till low and we have the best environment for growing plants/food and much of life since humans have walked the earth.

    • I’m amused to see that the same kind of linguistic crap that happens in Federal budgeting is starting to creep in to Federal crop yield estimates. In this bizarre world, it’s a “decrease” if actual production increases, but not quite enough to match forecasts by infallible bureaucratic “experts.”

  13. 25°C is optimum for plant growth. The only places where that is achieved on a constant basis is in the tropics. Much of the globe is temperature limited. (Too bloody cold). Enhanced CO2 means that plants require less water. So, its the arid zones that are greening directly increasing the carrying capacity of the ‘Farm’.

    It’s all good.

    By the way, there is no greenhouse effect and all climate change is a response to external influences in an open system. Demonstration here:

    • By the way, there is no greenhouse effect
      I agree that if there is one, it is hardly significant.

      all climate change is a response to external influences in an open system
      One wrong word – “external”. Here is the correct version:

      All climate change arises from the internal (not external) oscillatory dynamics of a dissipative nonlinear open system, either unforced, or entrained by external periodic forcing in a manner that is either strong (e.g. tidal effects) or weak (e.g. Milankovich forcing.)

      The climate is not passive. This is the error of the CAGW meme which insists fanatically that only CO2 can change climate.

      Climate changes by itself.

      This is mainly from the oceans that are not a passive puddle but which have their own complex chaotic dynamics which linked to their vast heat capacity and the century-millenial timescale of its circulation patters, serves up natural climate change continually, and would continue to do so even if – hypothetically – all external forcings were to become fixed and unchanging for many centuries.

      The oceans are the climate gatekeeper. Nothing changes climate except via the oceans.

  14. Author cites current wheat crop “… actually setting a new record ….” This does not mean CO2 levels are responsible; although I do acknowledge CO2 has some involvement in gross yield.

    For the last 56 years (1960/1961 wheat crop) world wheat production rose fairly regularly from ~225 million metric tons to ~725 million metric tons. There were ~500 million acres planted in wheat worldwide 56 years ago & >540 million acres of wheat 2015/2016.

    The total worldwide wheat planting was less in the previous year crop of 2014/2015 than the ” new record” production. Since the 56 year annual increase in production works out to 2.1% /year (planting acreage in that time was once as high as 598 million acres & as low as 489 million acres).

    The author’s cited 30 ppm increase in CO2 from 1982 to 2009 as important is, to me, a poor correlation as a significant contributing cause of the “record” wheat harvest. In 1960/1961 worldwide wheat crops averaged 1.15 metric tons per acre & one projection (USDA)
    is worldwide wheat crop will average 3.36 metric tons per acre; this amounts to 2.92 better yield/area planted.

    Now, D. Keeling’s original Scripps data for 1959 was 315 ppm CO2 & 09 Sept. 2016 MaunaLoa pegged it at 401.31ppm CO2, which is an 86 ppm increase in a bit longer that my worldwide wheat examples. For the sake of time span alignment I’ll approximate this as a 1.54ppm CO2/year increase over 56 years of worldwide wheat harvest data & venture to say correlating an extra 1.54 ppm CO2 as a cause of better yields over the previous year’s yield is confusing the issues related to field grown crops.

    Above, Dr. Reddy commented how energy, water & fertilization are primary for field (“open atmosphere”) crops, rather than CO2. A commentator suggested that “… water…generally …sufficient …” in major food farming regions means CO2 can be important. Water does not restore field CO2 to primacy as “plant food”. Original poster’s link (to “new record” wheat) specifically recognizes the recent “…significant cutback …wet weather …” caused to French wheat production. Farm production is different than greenhouse growing where CO2 can be manipulated within controlled conditions & original poster seems (to my reading) to have blurred the difference in trying to develop his concept.

    • I’ll add that original poster realizes he doesn’t “…have an answer …” for temperature wheat grows well & is convinced “…4,5 or 6 times …” more CO2 would result in greater greening. Am typing on Tablet so am re-composing these subjects.

      C3 leaf photosynthetic energy conversion is generally considered at 4.6% in the best case. Unlike in a greenhouse, field crops deal with sun angle changes; dawn light spectrum & dusk light spectrum are generally not recreated in greenhouses (although some do manipulate far red/red in a kind of pulse).

      Field crops, unlike greenhouse plantings, can experience variable cloud cover & even if this is not all day long the episode can reduce CO2 assimilation 6.5-17%. Once the cloud has stopped shading a leaf & in order to get photosynthesis going back up to the optimal level as irradiance (photon flux) rise requires the enzyme called “rubisco activase.”

      Rubisco activase enzyme takes away “sugar” phosphate(s) that have bound to a key activity site of the carbon fixing enzyme abreviated RuBisCo. Among different plants & even different varieties of the same plant there are variations in individual rubisco activase of the same type, plus more than just one type of rubisco activase.

      Field grown C3 plant leaves are estimated to achieve a bit more than 1.5% photosynthetic energy conversion. When wheat stomata are open the regeneration of the substrate (abreviated RuBP) of RuBisCo limits the dynamic of carbon fixing. A feature of semi-dwarf wheat is their “flag” leaves have greater stomatal conductance than tall wheat & this helps with yield (mass from carbon assimilated).

      When wheat stomata are closed in the field the amount of RuBisCo in a leaf limits the CO2 processing; different plants & differently fertilized plants vary in RuBisCo content. Wheat exposed to nitrogen will have at least 21% RuBisCo/total soluble leaf protein. One researcher found greenhouse wheat had 21-35% RuBisCo/total soluble leaf protein & one researcher working on crop improvement tweaked field wheat to have 40-58% RuBisCo/total soluble protein.

      Temperature going up changes RuBisCo affinity for CO2; heat affects CO2 less than it makes O2 more soluble. Rising temperature elicits photo-respiration, which also involves O2 being more attuned (than CO2) to RuBisCo. Heat on earth’s surface does not damage RuBisCo.

      However, rubisco activase enzyme is sensitive to heat stress & by it’s role in getting RuBisCo fully functional it (rubisco activase) can impair photosynthesis when temperature is excessive. The dynamic here seems to be that rubisco activase has to be knocked down about 60% in function to significantly lower RuBisCo driven photosynthesis.

      Rubisco activase enzyme activity really gets going downward above 30°C. Of course, native hot climate plants’ rubisco activase are more stable in heat – though wheat is not naturally like those plants. Still, if a plant is gradually acclimated to high heat then rubisco activase can exhibit more stability in heat. One commentator linked to Australian region’s problem with wheat crop & I suggest rubisco activase is integral to that scenario.

      High levels of CO2 assimilation are also impacted by phosphate & sugar making/export. Wheat upper “flag” leaves make (source) more sugar than other leaves. If leaves can get sugar out to somewhere (sink) then there isn’t interference by sugar (& phosphorus availability) on photosynthesis. I have discussed sugar/phosphorus in another comment, elsewhere in WUWT.

      The wheat “spike” is an important sink for leaf sugar made from CO2 derived carbon. However, raising CO2 is characterized by more closed stomata & when it comes time to fill the individual grains this requires increased transpiration to get water up there. Once late-grain filling stage occurs wheat benefits from more water & ever increasing CO2 may become rate limiting if it stymies that stomatal transpiration segue.

      Modern wheat has been bred for better light capture by altering the leaf angle of the plant’s arquitecture. Still, high light (photon flux) instigates a leaf photo-system 2’s “antenna” to disipate some of that energy (as heat). When performing photo-protection photo-system 2 is not capturing energy & then there is less CO2 fixed; unlike in the field greenhouses control the level of irradiance to be appropriate for the growth stages of different plants.

      Since original poster used olive trees as a proxy for CO2 benefit let me add here that olive (& deciduous trees, but not many field crops) produce isoprene; isoprene is adaptative to both high temperature & high light stress. Isoprene levels rise & fall in a seasonal pattern, plus a diurnal day & night pattern; basically isoprene output from olive trees is higher as the warm season builds in temperature & as the daytime temperature builds up. Although the up or down pattern of isoprene levels are not radically different in relation to light levels, all conditions being fine (weather), there is a greater response in total isoprene put out to high light (typically when sun directly over head).

      It is not accurate to say an ever progressively linear increase in CO2 is going to mean more carbon mass in every crop. Recent assay puts field grown wheat leaves in Europe as having 210 micro-bar of intra-cellular CO2. Other commentator(s) have brought up partial pressure of CO2 inside the leaf as if it were a linear function of available CO2 & so will close on this.

      Transgenic wheat experiments found that once there is ~600 micro-bar CO2 among the leaf cells the amount of CO2 assimilated generally plateaus at 40 micro-mole CO2/square meter/second. Yet in actually viable wheat (can reproduce from seed) it is a confluence of the level of intra-cellular CO2 in a leaf (micro-bar pressure) in combination with the maximum capacity of that leaf to sustain a consistant speed of electron transport.

      For wheat these 2 factors work out to mean that a maximum of 30 micro-mole/square meter/second of CO2 can be assimilated when there is 300 micro-bar CO2 among the leaf cells – when the electron transport chain is functioning at maximum effectiveness . Thus there is a ways to go from 210 micro-bar to 300 micro-bar intra-cellular CO2, but it will require genetic alteration of wheat to make any CO2 level that results in more than internal 300micro-bars CO2 to allow really high CO2 to be assimilated. Greenhouse growing tactics are more consistantly able to sustain electron transport at an ideal rate for crops than field conditions deal out & this makes extra CO2 more useable. Am posting this unedited, pardon any errors.

      • “Thus there is a ways to go from 210 micro-bar to 300 micro-bar intra-cellular CO2, but it will require genetic alteration of wheat to make any CO2 level that results in more than internal 300micro-bars CO2 to allow really high CO2 to be assimilated.”

        Since wheat is an annual plant, wouldn’t the hundreds of generations involved in any plausible scenario wherein such “really high” CO2 levels could be reached globally, provide the opportunity for gradual “genetic alteration” of the sort you mention? I mean, humans are pretty good at this agriculture stuff, ya know?

      • Hi JohnKnight, – The issue with wheat electron transport chain is sort of a kinetic feature. This does not seem to be something where plant breeders have any hope of creating from new iso-from alternative gene splicings.

        The most promising approach, as far as I know, for getting past it’s inherent limitations is to try & introduce genes into wheat from a different photosynthesizing organism with electron transport chain features which would allow more activity before electron transport trips up. This more robust capability would then mean a lot more CO2 inside the leaf (categorized as micro-bar pressure) can be assimilated as carbon without wheat’s electron transport chain stumbling to keep up with what is involved.

        Wheat yields on, up to a point , elevated experimental CO2 are demonstrating higher mass (carbon product) since field CO2 still isn’t creating internal leaf pressure of 300 micro-bars CO2, & the electron transport chain (in good parameters) can cope kinetically with what’s involved.
        To be clear, once experimental CO2 creates greater than 300 micro-bars intra-cellular CO2 conditions in a wheat leaf the electron transport chain does not simply cease functioning as if a switch shuts it off. I think it’s kinetic inability to function optimally is part of why some research shows wheat doing better initially on elevated CO2 & then the promising trend doesn’t seem to hold up.

        Your idea for improved wheat breeding is probably better suited for rubisco activase enzymes, since alternative gene splicing iso-forms wouldn’t need to integrate perfectly into a complex chain ( like electron transport involves). Local plant varieties, considered “land-races”, often grow in a specific micro-climate & some of why a descendant from one land-race can thrive in the heat (as opposed to another of the same kind of plant) is thanks to the form of it’s rubisco activase enzymes.

        With regard to RuBisCo there are attempts to engineer some sub-units. Again, the focus is to introduce genes from a different photosynthesizing organism that has a more adept sub-unit.
        When researchers point out that elevated CO2 increased growth is tied to there being nitrogen fertilization this is, in part, to synthesize the RuBisCo amount which is involved.

        I am not worried over temperature going up or CO2 levels rising. My inclination is to think our food crops will eventually be altered to accomodate those factors. The reason I comment is to try & clarify, as I see it, the dynamics which related research is showing.

      • Well, when CO2 levels are three times higher than now, perhaps the genetic manipulators will have seen fit to test their products long enough to have some credible claims regarding their actual safety . .

      • JohnKnight

        I have not yet seen any credible mechanism for CO2 to triple. Where would the CO2 come from? Trying to get the oceans to expel it would take millennia. By then we may eat peas the size of large pumpkins, not wheat.

        My point is serious – there is simply not enough carbon in anything we could burn to double the concentration in a realistic energy use scenario, let alone triple it. Only models can reach that heady height.

  15. There’s a silly mistake re forcings – I calculated the required CO2 increase in ‘proportional’ or linear rather than logarithmic terms. You can replace that section with this paragraph (from the verion of the article on Medium).

    ‘The percentage increase in ppm required for an increase in radiative forcing of 50%, for example, is less than 50% due to the logarithmic effect of CO2 forcing. In the case of a 50% increase in radiative forcing, concentrations would have to rise 41.4% — because 1.414^2 = 2. In the case of a 40% rise in forcing, the required concentration increase would be 32% (since 1.32^2.5 = 2). These percentages are given for reference only; as stated before, what CO2 increase would be needed is unclear, due to uncertainties in TCR, aerosol forcing, and ocean heat release.’

  16. Another possible mistake: ‘While BHM15 referenced global temperatures’

    I’m going out right now and cannot check in detail, but BHM15 used NATIONAL temperatures. Meaning the temperature over Antarctica, or the Pacific, etc. was not computed. So the situation is the same as with the two other papers: using the global temperature average underestimates the true rise over the past two years. It should be at least 0.6ºC, not 0.4.

    Considering that we were already above the ‘most productive’ temperature, which they determined was 13ºC, the effects on the global economy must have been catastrophic. Pity that nobody noticed.

  17. On page 13 of this publication are some contradictory statements:

    Firstly there are these paragraphs extolling the virtues of re-using CO2 emmisions:
    “Hot water and CO2 are by-products of the power plant at the Wissington factory but both are put to good and environmentally sound use. Hot water from the power plant is used to heat the glasshouse of British Sugar’s subsidiary company, Cornerways Nursery. The nursery is the largest glasshouse in the UK dedicated to growing tomatoes and each year produces 70 million tomatoes on 26 acres.”

    “CO2 produced by the power plant also has a beneficial use. It is blown in to the glasshouse where it boosts photosynthesis of the tomato plants and increases the yield of tomatoes.”

    Then there is this paragraph which implies that CO2 is bad:
    “Love them or loathe them, wind turbines provide another source of renewable energy. There are more than 80 wind turbines in the Fens, many sited on farmland. Altogether the wind turbines in the Fens generate enough energy annually to power 87,000 homes and prevent the release of more than 300,000 tonnes of CO2 into the atmosphere.”

    • Griff, good job, you found an article written by a “carbon pollution” zealot. Everything he notes in that article about negatives on plant production from future climate is highly speculative opinion. Everything he writes about impending failure of more Southerly agriculture presumes that forecasts of climate Armageddon are accurate and true and that drought, floods, wildfires are all true, all solely caused by human CO2 emissions and will devastate agriculture. The main premise of his article is a straw man. He writes “One argument [climate deniers] make is that output from agriculture will increase globally, with especially far northern areas benefiting from increased arability.” Then he attacks his own exaggeration of the second part of that sentence. Of course the idea that if we warm up a lot, increased arability in Canada and Siberia will happen is a given. Because growing seasons will slightly expand, and most particularly, the incidences of killing frosts should then decrease enough to plant more warm crops at the Northern-most margins. This does not mean that “deniers” think that Arctic tundra will be converted into Iowa type corn fields, as your author so idiotically presumes. The image of the ice road trucker fallen through the ice was a particularly nice bit of silly propaganda.

      It’s you and your ilk pushing the “hottest year evah” crap in which humans are enjoying record crop production. Maybe you should stop knee jerking long enough to challenge your own fundamental premises.

      • An expert on Canadian farming (advisor to farmers’ advisors) said that the growing season in Canada has been getting longer by about two days per decade since 1960.

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