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Maize harvest to shrink under Global Warming

Posted on 16 March 2011 by Rob Painting

With global food prices at an all-time high, out comes a new study demonstrating once again, that global warming will generally have a very negative impact on food production. The study shows that warming of 1°C will lead to smaller maize (corn) harvests throughout Africa.

David Lobell (Lobell 2011) and his co-authors analysed a dataset of over 20,000 maize crop trials which had been carried out in Africa between 1999 and 2007. These trials had originally been designed for the purpose of testing new varieties of maize, and were especially useful for climate research because the trial locations covered a broad range of environmental conditions dotted throughout the continent. By comparing the trial data with that obtained from local weather stations, the authors found a relationship between warming, rainfall and falling crop yields.

Surprisingly, for a crop previously thought to be tolerant of warming, the study revealed that maize yield fell in crops exposed to temperatures above 30°C. At sites in ideally managed rain-fed conditions, for every degree day (degree day = a tally of hours over 30°C adding up to a 24 hour period) over 30°C yields fell 1% And for plots in managed, but drought conditions, the yield fell further, a 1.7% decline for every degree day over 30°C.

Next the authors calculated the likely effects on crop yields if average growing season temperatures at each site, were raised an additional 1°C, as shown below:

Figure 1 - Model estimate of yield impact of 1 °C warming for trials at different average growing-season temperatures. Shaded areas show an estimate of the 95% confidence interval.

The last step in the study was to compare what effect an increase of 1°C might have on future maize cropping in Africa. They discovered some cooler regions benefit from the warming, but the majority fare poorly. Under ideally managed rain-fed sites, projections show yield declined in 65% of areas where maize is currently grown. Under drought conditions this decline impacts all areas, with 75% of areas seeing yields fall by over 20%.

Due to the large scale and geographical extent of the trials, the study analysis represents the best understanding, to date, of how maize/corn harvests will fare in a warmer world. Regions such as Central and South America, where maize is a diet staple, are also going to be badly affected by rising temperatures and falling yields.

That hammering sound you may hear is another nail being driven into the coffin of the argument It's not bad: it is that bad.

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Comments 1 to 50 out of 56:

  1. What were actual conditions the crops were growing under- average temperature is useless descriptive of conditions. So you fried it under 40C and then froze under 0C so it didn't do well. Average 20C.
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  2. cloa513 @1: Your comment comes across as if you're just making stuff up, to be honest, to try and de-value this study. The bar to get a study into Nature is quite high, and if the authors had ignored such factors, it's unlikely the review process would have let the paper through.

    I think the point to take away is that this was a study of real-world conditions - it just so happens that the variety of climates where the crops were trialled allowed the researchers to estimate what would happen in the event of warming.

    I.e. how does the yield vary between an area with mean growing season temperature of X ºC, compared to one with mean temp of (X+1) ºC.
    I also assume that variances of rainfall etc were controlled as far as possible when making the comparison. Having said that, a 1ºC increase in global temps may significantly change rainfall patterns, so there are still plenty of caveats, I imagine!
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  3. The 'dotted' link took me on a very convoluted trip to nowhere.
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  4. Until the 'dotted link' is fixed here it is.
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  5. Adelady - Thanks for pointing that out. Some glitch that inserts extra characters into the link, so I can't fix it. I've deleted the link, and MichaelM has kindly provided the map I was attempting to highlight.

    cloa513 - Not sure what your point is.

    Bern - Clearly the picture will be different with changing rainfall patterns, however allowing only for temperature, yield declines and this is amplified by drought. Heat seems to affect the reproductive (flowering) process.

    The authors also point out the trials used artificial (nitrogen) fertilizers, whereas most African farmers don't. Under nitrogen stress (the typical African farm) the response to heat & drought is reduced. The end result being the trials slightly exaggerate the stress response.
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  6. I'm glad to see SkS highlighting this information, because I think it's one of those "little details" that has immense implications.

    If you start with large, sustained (for now) CO2 emissions, you see a range of effects, some of which interact with each other, and most wind up in some way affecting humanity's ability to feed itself. Given the role that drought, rising sea levels (and the resulting salt water incursion into coastal farm land and wells), and population growth will have our situation, even a small decrease in our ability to grow a major food is a very big deal.
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  7. “... a new study demonstrating once again, that global warming will generally have a very negative impact on food production.”

    That conclusion is nothing “illegitimate” - optionally may apply only to maize and part of Africa.

    “Projected impacts relative to current production levels range from −100% to +168% in econometric, from −84% to +62% in process-based, and from −57% to +30% in statistical assessments.” (Climate change risks for African agriculture, Müller et al., 2011.)
    “Despite large uncertainty, there are several robust conclusions from published literature for policy makers and research agendas: agriculture everywhere in Africa runs some risk to be negatively affected by climate change; existing cropping systems and infrastructure will have to change to meet future demand.”
    Only this request has a strong scientific basis - but only for Africa.

    Interannual variability and climate sensitivity of Net Primary Productivity: a process-based multilayer-canopy vegetation model compared against observed tree-ring width, Bodin, Alton, and Krakauer, 2010.:
    “We estimate a global increase in NPP of 0.32% yr - 1 over the period 1951-2000. This is in
    approximate agreement with previous studies. Most of this increase can be attributed to CO2 fertilization rather than climate change.”

    Generally ... - this is the 0.32% NPP increase per year in the Positive.
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  8. Does this tell us anything that is not already known?
    Commercial croppers know the optimum growing conditions required for the crops they grow, and based on how seasonal conditions are progressing, and forecast, will adjust their sowing time accordingly.
    Grow out of season and the results will disappoint. Nor is the season ruled by the calender.
    This is why accurate forecasts are so important to croppers, and why many are prepared to pay private forecasters handsomely for a premium product.

    It is really no surprise that if the conditions change after sowing, or if the crop is sown late, that the yield will be effected.

    As for how the trials were conducted, I can't understand why they had to use data from local weather stations remote from the trial sites if these were properly controlled trials on research stations.
    The weather data should have been collected at the trial plots by those conducting the trial, especially if irrigation was being used, as it was.
    It has happened here in Australia where trial results have been invalidated because the rainfall data was collected from nearby stations, not from the actual trial location.

    I also don't understand this rationale behind this artificial adjustment if the trials were producing yields that could be physically measured-
    "The net effect of warming on yields was computed for each trial by artificially raising observed temperatures on each day by 1 °C, recomputing temperature indices such as GDD8,30, and using the regression equations to predict the new yield."
    Can someone explain the reason for having to do this?
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  9. johnd: I would say this study does tell us something we didn't know -- it quantifies the effects you mentioned. It's one thing to say "condition X is sub-optimal compared to condition Y" and quite another to know just what the impact is in change from Y to X. Surely various studies will arrive at different values -- look at the assessments of the life cycle carbon savings from ethanol, which are all over the map -- but given the implications for humanity and public policy, I would rather see us start soon with a possibly flawed set of studies and work (quickly) to improve them than to guess.
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  10. Maybe the results wouldn't be quite so surprising had the researchers factored two undisputed and well-established (although generally ignored) tenets into their calculations:

    1. more heat = higher levels of tropospheric ozone and;

    2. tropospheric ozone is toxic to vegetation - multiple international studies have conclusively demonstrated that at ambient levels ozone is already significantly (5 - 80%) reducing both crop yield and quality and that amount is expected to increase as climate change continues.

    http://witsendnj.blogspot.com/p/basic-premise.html
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  11. While studies like this give strong indications, I would be very careful when it comes to actual estimation. Implying that we may have too little knowledge as of yet about how large the net effects will be. There are too many parameters which I think are not properly controlled here. Adaptations in genetics, growing techniques etc may to some extent mitigate.

    I would also be careful about generalizations. While there may be very good support for a claim that warming, all other factors held constant, will reduce crops, all other factors will not be held constant in practice.

    But extrapolating from past improvements is even worse, I think..
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  12. This is why I keep telling people in various online forums: bad as nuclear power is, even a thousand Fukushimas will not cause as many deaths in misery and agony as our continued use of fossil fuels, especially coal, causes due to rising famine, war and pestilence.

    I try to phrase it better than that, though;)

    Of course, it is still quite an uphill battle, because the same "green instincts" that usually accompany taking AGW seriously, also find nuclear power repellent. Then there is the widespread fear of radioactivity simply because it is invisible and poorly understood. TEPCO's incompetence both in handling the disaster itself and in handling the public relations around it don't help, either.
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  13. MattJ : I thought that the continuous of fossil fuels had increased our food production by several dozens, but again, we may not live in the same world ....
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  14. I literally just finished reading Lester R Brown's 'World on the edge' a few hours ago - this problem is a main theme and I recommend you check his book out. Food security is intertwined with so many issues attached from climate change, both its causes and results.
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  15. SNRatio
    There are too many parameters which I think are not properly controlled here. Adaptations in genetics, growing techniques etc may to some extent mitigate

    The whole purpose of the trials was to identify varieties of maize that were robust i.e.selecting those genes which would better suit the harsh African conditions. Despite this a consistent response was found; maize don't like temperatures over 30°C.

    I don't think it's rational to expect the "technology fairy" to fix everything.
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  16. Rob @15,

    It is worrying that a C4 crop (i.e., maize) does not fare well under high temperatures, b/c C4 vegetation has (up until now at least) tended to cope quite well with high temperatures.

    It also struck me in their Figure 3, that the Sahel region (an important crop producing region) is also expected to suffer yield losses under higher temperatures. Also, AFAIK, maize is also a staple crop in Africa.

    People also tend to forget that much of the farming in Africa is subsistence-type farming, and thus much more at the whim of heat, drought and flood.

    As an aside, apparently Canola also does not like temperatures above 30 C. From a paper by Kutcher et al. (2010, Agricultural and Forest Meteorology):

    "Iterative Chi square analysis and iterative principal components analysis both showed that the beginning of July, which coincides with the early part of the flowering period of the crop in SK [Saskatchewan, Canada], was the critical time in which high temperatures (>30 C) and low precipitation led to yield loss."
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  17. Rob Painting at 09:00 AM, whilst I understand the purpose of the trials which is reflected in the majority of trial sites being in areas where maize is less commonly grown, it seems that the negative response to increased temperatures was not evident or very low in those areas where maize is more commonly grown.
    The two main areas seem to be are the region in the vicinity of Johannesburg and southern Nigeria which interestingly have different average growing temperatures with southern Nigeria at about 25°C being about 5°C higher than the main area near Johannesburg.

    If the findings of the study were applicable to those areas, then the yields of those crops in southern Nigeria must be only a small % of those around Johannesburg

    If it is not rational for technology to fix everything, is it rational to expect that maize should like temperatures over 30°C when, whilst it is grown in a wide range of conditions, the area where the greatest % of land is utilised for maize in Africa is in a more temperate zone?

    Also, if maize was previously thought to be tolerant of warming, was there any real foundation for such opinion, and if so on what was it based?
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  18. I'm a bit disheartened by the overall quality of comments, but hey just in case anyone is genuinely interested, here is a link to a similar study done recently for wheat in the conditions of NW Turkey.
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  19. So, what does this article tell us that we did not already know? To those who are concerned with the broader topic of food security in a warming world with a population expected to exceed 9 billion within 40 years – not much. To those who wonder what impact SLR and salt water flooding of river delta’s and low-lying fertile coastal plains will have on food production – even less. To those worried about the already failing adequacy of freshwater for agriculture in the face of diminishing mountain snow and glaciers – nothing useful.

    Academic research, publication of papers and debate thereon is undeniably important and is to be encouraged. However sometimes those efforts do seem rather specialised and esoteric in a world facing some very serious, relatively short-term problems, arising from the broader effects of AGW on food production. A dissertation on the effects on maize yield grown where temperature exceeds 30C by 1degree-day is important but does it deal with future real world conditions where average global temperature may rise by 2C by mid-century?
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  20. The parimiters of this study do not provide the required details to do a credible analysis.

    I will leave it at that.
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  21. Albatross - yes, I agree it is a concern. As you say much of Africa is subsistence, but the findings, which basically confirm some earlier research, indicate there are global implications.

    John D - it seems that the negative response to increased temperatures was not evident or very low in those areas where maize is more commonly grown.

    Don't know where you get that idea.

    if maize was previously thought to be tolerant of warming was there any real foundation for such opinion

    See Albatross' comment @ 16. Maize is a C4 plant and therefore more heat tolerant than it's C3 cousins. There's a large amount of literature on the topic.

    Agnostic - the study provides some hard data to back up the obvious implications of a warming world, and drought, on crop productivity. IIRC much of Africa will experiencing conditions 1 degree warmer than now in the 2030's.
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  22. For those who are critiquing the paper without apparently having read it (and it is linked to), the authors outline the rationale for their research:

    "New approaches are needed to accelerate understanding of climate impacts on crop yields, particularly in tropical regions. Past studies have relied mainly on crop-simulation models or statistical analyses based on reported harvest data, each with considerable uncertainties and limited applicability to tropical systems. However, a wealth of historical crop-trial data exists in the tropics that has been previously untapped for climate research. Using a data set of more than 20,000 historical maize trials in Africa, combined with daily weather data, we show a nonlinear relationship between warming and yields."
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    Moderator Response: [DB] Fixed html tag.
  23. Rob Painting at 12:20 PM, re "Don't know where you get that idea."

    Firstly from this comment in the introduction of the paper,
    "Roughly 65% of present maize-growing areas in Africa would experience yield losses for 1°C of warming under optimal rain-fed management, with 100% of areas harmed by warming under drought conditions."

    More specifically from this -
    "For optimal management, at present, maize growing below ~23°C in average growing-season temperature tends to gain from warming, owing to positive effects of GDD8,30, whereas yields of maize grown in areas above this baseline temperature tend to decline with warming."

    The maps provided in the study identify such areas as well as the % of land utilised for growing maize, it's a simple matter to then correlate the two.

    With regards to the matter of tolerance to warming. Warming is a very subjective term, do they mean, plus 5°C, 10°C, 20°C, 30°C ?
    What data is there that identifies the exact degree of warming that it was thought to be able to tolerate, or was it all based on anecdotal stories?
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  24. John D - The maps provided in the study identify such areas as well as the % of land utilised for growing maize, it's a simple matter to then correlate the two.

    Still no idea what you're referring to.

    Here are the trial and weather station sites:



    And the area currently growing maize.



    John D - With regards to the matter of tolerance to warming. Warming is a very subjective term, do they mean, plus 5°C, 10°C, 20°C, 30°C ?

    It appears you haven't even read the study. Please do so, and then maybe some rational discussion can ensue.
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  25. Rob Painting at 15:29 PM, I take it that you have by now digested the two quotes from the study that I drew your attention to.
    Would you like to confirm that and perhaps comment on how you interpret them, perhaps after you have studied the relevant maps.

    It would help tremendously if you selected the appropriate maps that are relevant to the points made.
    The map showing the trial sites is irrelevant to the findings of the study, if you understand the objectives involved for the selection of the trial sites, then you will understand why that is so.

    Please study the maps I have provided below, then perhaps their relevance will become apparent.

    Map (a) shows the average growing season temperatures allowing the areas above and below 23°C to be identified, and hence those areas that tend to gain from additional warming and those that tend to decline with warming.

    Map (d) shows the current maize growing areas as various shades of green according to the % of land utilised for maize. Note the heavily shaded area in the vicinity of Johannesburg and the next most heavily shaded area in southern Nigeria.

    Map (e), apparently you have overlooked it, as it shows the present growing areas where yields will vary, up and down, shaded according to the degree of variation. You should note the positive or zero shading in relation to the two areas I mentioned.

    Map (b) with it's prominent shadings of deep red, shadings that are absent from map (e) show that the greatest impact on maize yields will occur where maize is not currently grown, though that hardly needs to be said does it?

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  26. John D - Map e) shows what 1° C will do to current areas under a managed rain-fed regime:



    That is, taking the results from the trial data analysis and applying it to the areas presently under maize cultivation, 65% of areas see a decline. And under drought management (map f) all areas decline, with 75% of areas seeing declines of over 20%.



    Those are the take-home points. The fact that presently uncultivated areas will be negatively impacted too (seemingly your gripe) simply means that there doesn't seem to be any upside.
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  27. Rob Painting at 18:27 PM, whilst you might see some take-home points, I see only bring-along points which is what my very first comment on this thread indicated.

    Regarding your comment of uncultivated areas, perhaps further study of the information able to be extracted from the maps might lead to such areas being better described as areas beyond those already considered marginal.

    We must see this study for what it is. The authors are not those people who conducted the original trials. They have been able to utilise some data yielded by the trials but I expect that other data would have been considered propriety by the trial managers, and as such their, the trial operators, take-home points likely to be somewhat different to what this study could allow.

    The reason this is likely comes with understanding what trials such as these set out to achieve. The development of new varieties is an ongoing process as various traits are sought to be refined or developed. Developing varieties that allow cultivation to be extended into areas now considered unsuitable or marginal is as much an objective as increasing yields under existing conditions, and it appears that the former was one of the primary objectives of the original trials.

    Not all varieties live up to expectations when field trialled, some show negative responses, others none, others positive responses to varying degrees. Whilst we don't know how many different varieties were trialled or how each performed, what is indicated in this study is that the data is all in, in other words, the data includes the failures and the successes.

    This is where those conducting the original trials will most likely be taking home something different to what this study allows. They will have identified, and quantified, those varieties that responded positively to the trial conditions. It will be those such varieties that extend the ongoing development in plant genetics that allow productivity to keep pace, or even out-pace change as it occurs.
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  28. The problem with simplistic extrapolations such as "Roughly 65% of present maize-growing areas in Africa would xperience yield losses for 1 °C of warming under optimal rain-fed management," is that it assumes farmers are dumb.

    Farmers in different regions and different climates plant and harvest at different times of the year.

    It is unreasonable to assume that planting and harvesting times will not change.
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  29. Johnd and Charlie,
    Perhaps the two of you are not familiar with the peer review process. This paper was published in Nature, one of the most respected journals in science. The peer reviewers have concluded that simple issues like you have raised are not germane to this discussion. If you have a problem with that you need to produce actual analysis of data. Idle hand waving and saying "I doubt it" for no particular reason is not an argument against a paper published in Nature. Provide links to data supporting your extraordinary claims or put in the effort to analyze the data yourself.
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  30. michael sweet at 06:08 AM, the most disappointing form of hand waving is that when the only knowledge that can be offered to a discussion is "it's been peer reviewed".
    That carries the implication that the peer review process is perfect, and that each peer reviewed paper encapsulates all the knowledge there is to know on the subject.

    If you carefully read back through this thread, you will find that most of the comments questioning the paper are not on the integrity of the study process, but rather whether the findings provide anything new or are even relevant to the real world.

    Even if you have little appreciation of how or why field trials such as those that provided the base data for the study are conducted, you surely must appreciate the fact that not all new varieties being trialled perform up to expectations.

    The difference between what this study did by including the results of all the varieties trialled, and what the plant breeders would have done, is that the plant breeders would have selected the results from those varieties that met or exceeded their expectations and having done so, worked to refine or further develop the desired traits before releasing them for commercial use.
    You can be fairly certain that when such new varieties are being released and promoted, that it will be the data for each specific variety that will be used to illustrate how each variety will perform under different conditions,and certainly not what this study projects.
    It is largely irrelevant to the real world and provides virtually nothing new to those whose purpose is to actually produce the food required.
    Given the rate at which new varieties are produced, the study is likely many steps back from where plant breeders would currently be, certainly well off the pace.

    The one point I do question about how the study was conducted is about why they used rainfall data other than what normally would be collected at the immediate trial site. It will be unknown if any bias is present unless rainfall had been measured at the trial site, in which case, that is the data that should have been used.

    If you can provide some insight as to whether my concerns are valid or not, then I would certainly be interested to listen.

    The use of rainfall data collected at other than the immediate trial sites has on occasion been sufficient reason for the findings of such trials to fail passing the peer review process here in Australia.
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  31. John D - Whilst we don't know how many different varieties were trialled or how each performed.

    The idea that the trials would incorporate any old rubbish maize is nonsense. Simple invention on your part. And quite frankly odd, why would an organization carrying out such trials, use varieties that were ill-suited to the prevailing conditions?.

    The CYMMT was set up in the 1970's, they've been breeding for maize drought tolerance ever since. Selective breeding is an ongoing process. I expect the private seed companies to have agricultural expertise too.

    Charlie A - planting and harvesting times are not optional. These are determined by local conditions.
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  32. Rob Painting at 09:24 AM, the purpose of the field trials is to identify how each variety performs under field conditions. They have no way of knowing this unless they conduct the trials.
    What happens in a laboratory enclosure is often quite distant from what occurs in the field.
    Of all the people that have posted on this thread, surely it was incumbent upon yourself to bring such understanding to the discussion.
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  33. Rob @31,

    I agree with your position on this. This is, IMHO, an interesting example of a "skeptics" who is in denial. Johnd is continuing to make unsubstantiated assertions. These findings are no doubt inconvenient to him--I know he'll deny that, but that much is clear from his posts and efforts to obfuscate. What he fails to realise is that hand waiving and talking through his hat does not further his case. If someone elects to dismiss the findings of a paper published in Nature (or any reputable journal for that matter), it is incumbent on them to bring substantial evidence and science to the table in order to be taken seriously.

    From the start of this thread he has been making comments clearly without having bothered to read the paper, and he was called on that.

    Being a true "skeptic" does not mean that one has to contort in order to dismiss the findings of papers which do not support your position. "Skeptics" are permitted to agree with some of the literature on AGW without compromising their position-- in fact, doing so from time-to-time would work towards improving their credibility.

    To stay on topic, these findings are also consistent with those of Samson et al. (2011), who found that Africa is going to be one of the countries which is most negatively affected by AGW. We are doing them no favours if we decide to bury our heads in the sand and ignore these troubling findings. We have all been warned of the possible perils ahead.
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  34. John D -What happens in a laboratory enclosure

    So now an African field is a laboratory?. Please get serious. The point of my last comment (which you clearly do not understand) is that these trials are simply a continuation of earlier ones, where hardier breeds have been identified. The study analysis is therefore of the hardiest known breeds at the time.

    I expected you would actually understand what selective breeding is, before commenting here. My mistake.
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  35. Rob Painting at 11:09 AM, if you are unable to differentiate how the development of new varieties of plants involves trials that range from those in laboratory enclosures, through trial plots to full blown field trials, then you need to research more.
    But if you want to be pedantic, then a trial plot, or a paddock used for a full blown trial meets the criteria of a laboratory being a place where scientific research and experiments are conducted.

    This study provided results that are the averages of about 20,000 separate trials conducted over the period 1999-2007.
    The yields provided from individual trials ranged from about zero to 15mg/Ha under the Optimal conditions, and from about zero to about 8mg/Ha under Drought conditions. That is a considerably wide range.

    I would suggest that any projections about how maize, or any plant variety, may perform in the future should be based on those varieties that out perform the average.
    This study merely benchmarks the average response of the average variety averaged out over the entire period studied, it tells us nothing about how plant productivity is advancing, or not.
    If it had divided the period in half, we might have been able to see how plant technology is advancing through later development of new varieties, but it doesn't, so the study offers nothing to indicate whether optimism or pessimism is the most appropriate response.
    The whole premise of the study is based on the status-quo remaining so.

    If anyone wants to remain up to date as to how any plants are likely to respond to changing conditions, then they should focus on the varieties that progress through trials to commercial release and analyse what data is available from the commercial breeders. Then perhaps one has a yardstick that is more relevant, and more up to date than some academic study based on averages.
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  36. John D @32 - What happens in a laboratory enclosure is often quite distant from what occurs in the field.

    John D @ 35 - But if you want to be pedantic, then a trial plot, or a paddock used for a full blown trial meets the criteria of a laboratory being a place where scientific research and experiments are conducted.

    Seems you can't decide exactly what you mean.
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  37. John D This study merely benchmarks the average response of the average variety averaged out over the entire period studied, it tells us nothing about how plant productivity is advancing, or not

    Indeed, for these trials, but given that these programs have been going on for many decades, and the 30°C mark has been identified as a limiting factor. It doesn't bode well for the future.
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  38. Johnd,
    I note that in this entire thread you have yet to cite a single paper, peer reviewed or otherwise, that supports your position. The paper cited in the original post reports data from about 20,000 trials. These are the best performing varieties during the period in questions and not "average varieties". Please suporting papers or data for your positions and stop hand waving.
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  39. Paper Lobell et al., 2011, this work is a fundamental flaw. It has a very poor literature. For example, I do not understand why there is not such a fundamental work as: Temperature dependence of growth, development, and photosynthesis in maize under elevated CO2, Kim et al., 2007.:

    “All measured parameters responded to growth temperatures. Leaf appearance rate and leaf photosynthesis showed curvilinear response with optimal temperatures near 32 and 34◦C, respectively. Total above ground biomass and leaf area were negatively correlated with growth temperature. The dependence of leaf appearance rate, biomass, leaf area, leaf and canopy photosynthesis, and C4 enzyme activities on growth temperatures was comparable between current and elevated Ca [carbon dioxide concentrations]. The results of this study suggest that the temperature effects on growth, development, and photosynthesis may remain unchanged in elevated Ca compared with current Ca in maize.”

    Conclusion in this paper:

    “Temperature optima for leaf Am and leaf appearance rate were near 34 and 31 ◦C, respectively.”
    “These results indicate that while many of the growth and photosynthesis parameters examined here were minimally responsive to elevated Ca, an acclimation process might occur in the C4 cycle by way of reducing the activities of C4 enzymes.”
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  40. Michael Sweet @ 38 - John D is simply acting the obstinate contrarian, but there is one point he raises which interests me; what improvement (yield %) would there be by comparing the most robust varieties against the trial average. The best of the best so to speak. He seems to believe that this would be significant, but that seems unlikely, given that years of selective breeding only provide incremental improvements. I'd be curious to know nevertheless.

    Arkadiusz Semczyszak - It has a very poor literature.

    Simple handwaving brutha. Some analysis?. As for Kim et al 2007, they don't look at yield, and there are so many differences between their experiments and the African field trials, it prevents a direct comparison. But I do note that even in conditions where they are well watered (mitigating heat stress) maize growth declines when temperatures exceed the low 30's. The point of the study of course was to see what effect, if any, CO2 fertilization had on maize. The answer was a resounding zilch.
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  41. michael sweet at 23:06 PM, the trials produced yields between zero and 15Mg/Ha under optimal conditions.
    Please show me where in the study it is noted that they only used the results from the best performing varieties, and if that is indicated, just how many specific varieties were used out of all that were trialled to arrive at the conclusions reached.
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  42. Rob Painting at 09:13 AM, Finally!!
    Firstly, it is both fortunate and unfortunate that it takes an obstinate contrarian to ignite the spark that seemed so readily apparent.
    Whilst it may be comforting to be rubbed or patted by others to maintain a pleasant soft glow, such treatment is unlikely to result in the spontaneous combustion that a contrarian can invoke.

    In Australia, we are now producing 3 times as much cereals from the same acreage as we did 50 years ago. Whilst there are many factors that contribute to that, improved plant genetics have played a significant role. Importantly, the opinion is that that same rate can be maintained over the next 50 years over the world as access to better fertilisers, pesticides and new varieties extend into areas not yet benefiting as we are.

    I think that many seed companies might disagree with you that significant improvements are not possible.
    Most such companies claim significant improvements in the yields of their latest varieties over their competitors, and all companies are regularly releasing all types of new varieties, almost annually, with details of trials conducted to support their claims.
    The real peer review process is conducted by the growers who with commercial, rather than academic, interest driving them will quickly determine whether a new variety offers improvement of not.
    Perhaps an annual inventory of the seed suppliers shelves would yield the necessary data that would allow an academic study to be undertaken, but by the time it went through the normal convoluted academic peer review process, who would find it of benefit, or even of interest.
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  43. Before my post is taken out of context, my comments above were about cereals and other crops in general, with a perspective from within Australia.
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  44. JOhnd:
    Why would you trial a variety that was not a top performer? To make it to the trial stage a variety must have shown promise.

    You still have cited only your personal opinion in opposition to peer reviewed data. Not very convincing.
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  45. michael sweet at 11:35 AM, stop assuming, just show us where in the paper it is stated that all the trials were only of the top performers, which is what you are claiming.
    Top performers is relative, top performers as compared to what? Top in what aspect?
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  46. #25 JohnD says "Map (d) shows the current maize growing areas Map (e), shows the present growing areas where yields will vary, up and down, shaded according to the degree of variation. You should note the positive or zero shading in relation to the two areas I mentioned."

    It is no coincidence that the areas that are now have the highest percentages of corn acreage are the ones where 1C of warming results in an increase in yield.

    The effects of global warming at any one location are small compared to the natural year-to-year variations. That's why global warming is evident only upon analyzing hundreds of locations rather than just one or two.

    For a crop to have good yields for a reasonable percentage of years, the climate has to be such that normal year to year variations don't often dramatically reduce yields. A 1C global warming is small compared to normal year to year variations and doesn't present a major challenge, on the average, to the crops in the areas that are currently optimal for that crop.

    On the other hand, in areas with marginal climate, there will be many years with crop failure. If the failures are mostly due to high temps, then 1C of warming will significantly worsen the situation. But farmers already "know" that .... if not consciously, then as part of the overall local body of knowledge of what crops are suitable for that area.

    The above is a long winded way of saying the the above report confirms what is already known by farmers.

    Except that farmers are also smart enough to change the planting time in response to changing climate, while the paper assumes such a trivial adaptation will not take place. If the climate changes are more extreme, then farmers will move to other varieties of corn or even to other crops.

    This sort of low cost, low tech adaptation is why there is such a wide variation in percentage of different types of crops in different areas.
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  47. Charlie A at 13:06 PM, Charlie, it's so obvious isn't it?
    One expects that anyone wanting to discuss this subject would at least have some basic understanding of how, this of all subjects, plays out in the working world.

    I found map(b) the most revealing with it's indications that the greatest impact on maize yields will generally occur in those areas which are already beyond where maize can be grown under present. conditions.
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  48. Charlie A - A 1C global warming is small compared to normal year to year variations and doesn't present a major challenge, on the average, to the crops in the areas that are currently optimal for that crop.

    Not what the study & particularly the rather obvious graph in the middle of the post reveal. I also pointed out the trials were conducted over an 8 year period.


    John D - In Australia, we are now producing 3 times as much cereals from the same acreage as we did 50 years ago

    Citation?. All due to selective breeding?.


    John D -I think that many seed companies might disagree with you that significant improvements are not possible.

    If you had read the supporting literature you would realize that the maize from private seed companies (Monsanto, Pioneer etc) were significantly outperformed by the CYMMT varieties. So they've got some catching up to do.
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  49. John D I found map(b) the most revealing with it's indications that the greatest impact on maize yields will generally occur in those areas which are already beyond where maize can be grown under present. conditions.

    Now you're repeating yourself. I responded to that @26.
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  50. Rob Painting at 17:55 PM, If you read the comment it was responding to Charlie A at 13:06 PM!!!!!!
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