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Will Humanity Face a Carbohydrate Shortage?

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farmland-from-spacePhotosynthesis is the single most important transformation on Earth. Using the energy in sunlight, all plants—from single-celled algae to towering redwoods—knit carbon dioxide and water into food and release oxygen as a byproduct. Every year, humanity uses up roughly 40 percent of the planet’s photosynthesis for our own purposes—from feeding a growing population to biofuels. Given that growing human population, is there a limit to how much of the world’s photosynthesis we can appropriate?

Satellite measurements now allow precise measurements of the amount of photosynthesis taking place on the planet’s seven continents and assorted islands—or what scientists call “net primary productivity.” Such measurements are based on the amount of ground covered by plants, the density of that growth, and observations of temperature, sunlight and available water. Using these measurements, ecological modeler Steven Running of the University of Montana concludes that plants produce nearly 54 billion metric tons of carbohydrates a year—the bulk of it the complex organic chains of cellulose and lignin.

Running has also looked back over the past 30 years and discovered that the total amount of photosynthesis is surprisingly stable. Despite local weather that ranged from droughts to floods, plants soldier on producing roughly the same amount of food year in and year out, varying by less than 2 percent annually. This may be because the inputs of photosynthesis also vary so little—sunlight strength fluctuates only mildly, as does precipitation on a global basis. This finding suggests to Running that the plants’ “net primary productivity” might be usefully thought of as a planetary boundary, a threshold or safe limit for human impacts on natural systems, or so he argued in Science on September 20.

A suite of 10 such planetary boundaries had already been proposed in 2009, ranging from climate change to chemical pollution. But Running notes that this measure of photosynthesis involves at least five of those proposed boundaries—land-use change, freshwater use, biodiversity loss, nitrogen and phosphorus cycles—as well as being impacted by at least one more: climate change. And there is no question that photosynthesis on land does have a planetary limit—there is only so much land on which plants can grow.

Moreover, our population is estimated to swell to 9 billion by 2050. Will the plants be able to keep up?

Already, agriculture covers 38 percent of the globe and there’s little room to grow further, although once-productive lands in Eastern Europe could be brought back and better management practices could boost output elsewhere. And as the other planetary boundaries suggest, we may be approaching or have already passed geophysical limits for fertilizer application to fields in places like the U.S. or China, as well as the potential to increase the amount of irrigated land to boost crop growth. Finally, we’re already diverting more and more agricultural production away from stomachs and into fuel tanks, as exemplified by the U.S. practice of making ethanol from corn.

The vast bulk of the remaining productivity not co-opted by humans is presently inaccessible to us, whether by being part of root systems or protected national parks. That’s not to say humanity won’t keep trying to expand those boundaries, either by colonizing parks, breaking down formerly inaccessible cellulose to make biofuels, or extending agriculture to the seas in the form of algae farms.

But Running suggests that only roughly 5 billion more metric tons of carbohydrates can be diverted to human uses, meaning a “net primary productivity” boundary of roughly 25.6 billion metric tons. We’re closing in on that fast. “The question is thus not whether humans will reach the global [photosynthesis] boundary but when we will do so,” he writes. “The obvious policy question must be whether the biosphere can support the 40 percent increase in global population projected for 2050 and beyond.”

Image: Courtesy of NASA

David Biello About the Author: David Biello is the associate editor for environment and energy at Scientific American. Follow on Twitter @dbiello.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. G. Karst 1:58 pm 09/26/2012

    Exactly why enhanced CO2 in the atmosphere is important. It is how all carbohydrates are created, in plants. This should be, a loud warning, about the dangers, of the deliberate decreasing of CO2 plant food. Warmer temps, with additional rainfall, and enhanced CO2, may save us all. GK

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  2. 2. Ralf123 3:51 pm 09/26/2012

    Food production will be the limit to humans on Earth. Food is carbohydrates first and foremost. So, the answer is “obviously.”
    Oh and grain productivity drops by about 10% per degree C of warming and the heat waves and droughts we’ve been seeing don’t help either.
    And re G. Karst: It has long been shown that plants are not usually limited by CO2 availability but by water and minerals. However the additional rainfall doesn’t help if it usually comes in deluges.

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  3. 3. G. Karst 5:19 pm 09/26/2012

    Ralf123 says: “productivity drops by about 10% per degree C”

    No! The rate of increasing yield in the productivity rate decreases with increasing CO2. Plant growth continues, but with a slower rate of increasing yields. It’S ALL good.

    for further education on CO2 effects and temperature response on plant growth.

    “However the additional rainfall doesn’t help if it usually comes in deluges.”

    So according to your logic, less rainfall and drought is what is needed. There is no valid reason to think rainfall will not fall where it is most needed. In fact, some most certainly will.


    Instead of accepting everything negative… why not look for some of the silver linings involved. You may actually find the future of the biosphere is bright.

    That is, of course if the politicians and ideologues don’t kill us all first. GK

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  4. 4. Mythusmage 8:27 pm 09/26/2012

    If we ever domesticated East African grains we could open up millions of hectares of farmland.

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  5. 5. dwbd 9:23 pm 09/26/2012

    Yeah, so let’s continue expanding food to fuel programs as the USA is doing with reckless abandon – 45% of Corn crop goes to fuel automobiles, when you could use Methanol from Coal to replace that Ethanol at < 20% the cost and same if not less emissions. Agro-fuels are a criminal assault on the Earth's environment and the World's poor and the perpetrators of this crime of genocide belong in a prison cell. Get all the mickey-mouse drug users out of prison and put the real criminals in their place.

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  6. 6. G. Karst 12:15 am 09/27/2012

    A very interesting finding has been released from the Vienna University of Technology that addresses the question of rainfall location. They announced, what appears to be, a negative drought feedback mechanism, as a direct observation.

    Summer rain more likely over drier soils

    ‘Summer rain is more likely over drier soil – this is the conclusion scientists have drawn from a detailed analysis of satellite data. State-of-the-art computer models predict the opposite effect; these models must now be reconsidered, says the study published in the journal “Nature“.’ GK

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  7. 7. gmperkins 2:55 pm 09/27/2012

    There has to be an upper limit, resources are finite. This states we might be approaching it sooner than later, which is good to know. Of course, we all know that we have to halt population growth, even to the point where it is decreasing. Just another huge problem that human nature fights hard against.

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  8. 8. jerryd 3:26 pm 09/27/2012

    Dbwd the biggest waste of food crops is animal feeds, not biofuels. Fact is after making ethanol you have more protein, all the corn oil plus celulose of the corn, cobs and stalks.

    Since it’s better animal or human food, the dried mash, after making ethanol than raw corn little food value is lost.

    Fact is it’s cows, pigs and chickens that consume the most grain. Switching to goats, lamb, bison and grass fed cows would mean we could feed 3 Billion more people. For every pound of meat we could feed 3-10 people on just grain it’ll eat.

    Farms will make their own fertilizers, etc and using living soils mean little inputs are needed and likely produce fuels from RE, crop, animal wastes.

    Next turn the oceans. lakes and rivers into more productive fishing by great management would triple it’s output or more.

    There is no food shortage and likely never will be. It’s a political problem, not a supply one. Same as energy, there is plenty to go around cheaply from solar, wind, tidal/river and biomass if used eff for everyone.

    And lastly I don’t think we’ll ever hit 9 B people on earth as wealth increases, population drops.

    And of course most future transport will be electric from RE, gen 4-5 nukes, etc.

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  9. 9. Diesel67 4:03 pm 09/27/2012

    Life will survive; it survived catastrophes far worse. We might even have another Carboniferous, and humans should be clever enough to use some of that extra photosynthesis to our advantage. And God save us from negative population growth. You can’t have a civilization without people, and Europe is being taken over by Muslims because Europeans aren’t replacing themselves. In China they already have a shortage of females (boys are preferred and female infanticide is common) and a geriatric society is on the way. That means either senilicide or resources diverted to the care of old folks that might have been used to improve society as a whole. If we Americans think we have a problem with too many old folks and not enough workers to support them, wait til we too have negative population growth.

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  10. 10. Diesel67 4:25 pm 09/27/2012

    Oops. I should have specified COMMUNIST China with its one-child policy and forced abortion/sterilization. In Free China (aka Taiwan), you can still have as many or as few children as you can care for.

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  11. 11. Steve D 10:08 pm 09/27/2012

    Diesel67: Continued population growth is the ultimate Ponzi scheme. We have more workers to support our old folks now, which will mean we will need still more workers to support them when they get old, but that’s okay because we’ll have still more ….. Until we can’t grow any more, which is what happens to all Ponzi schemes.

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  12. 12. Dr. Strangelove 6:52 pm 09/30/2012

    If you burn all the agricultural waste we are producing to generate electric power, that would be more than enough to replace all the power plants in the world. If you convert some of the waste into biofuel, that would be enough to power all transportation. If you cultivate all the arable land on earth, that would feed 100 billion people. BTW, the by-product of burning biomass is biochar, an organic fertilizer that sequester carbon in the soil preventing it from escaping to the atmosphere.

    Fantastic? 95% biomass is agricultural waste. Only 5% is food. We are scrambling our food and biofuel from the 5% and ignoring the 95%.

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  13. 13. G. Karst 2:06 pm 10/1/2012

    Dr. Strangelove: You make a good point, regarding which part, of the plant, should be burnt as fuel. Unfortunately, the best use of so called “plant waste” is to be plowed under.

    This releases the nutrients back to the soil and the cellulose keeps the soil organically alive (worm, bacteria, fungi food).

    Dead soil will not support heavy crops, no matter how much one fertilizes. Plants can’t make it alone, but rely on symbiotic relations with other small critters. GK

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  14. 14. IslandGardener 3:17 pm 10/1/2012

    Yes it’s usually water (or warmth or rootroom) that’s the limiting factor to plant growth, not carbon dioxide.

    And yes we have to reduce the human population – not by demonising groups of people – Diesel 67, you should know better! – nor by forcing people, but by making sure women have equality with men, education, health care, and access to contraception and abortion. Most women only want two children, given the chance.

    And yes we need to eat most plants directly, not waste most of them by feeding them to animals.

    But the main thing which nobody has challenged so far is the assumption that human beings have the right to use as much of the Earth’s productivity as we choose. Surely we should aim to use as little as possible, to allow more room for other species and habitats.

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  15. 15. priddseren 12:34 am 10/2/2012

    Human ingenuity will compensate for any potential shortage of food supplies. Each year more and more agriculture is done far more efficiently and with much of todays farmland in countries doing the most inefficent farming methods there is plenty of room to grow here. In the end humans will expand to farm most land, probably spread out to oceans and so on. Humans are also chemically processing foods to be more effective at delivering carbs to the human body, something that is likely contributing to obesity, which is itself proof we eat to much anyway. In the end, the idea is to get energy from the sun into energy for life on the planet and human’s artificial enhancements to life. We will find was to do this ourselves and in the end if we need more square miles for sunlight and we have used every possible square mile of earth, we will likely move on to cultivating the moon and beyond.

    So this is not really a problem as long as we humans keep getting more and more efficient. However, we will never be in a condition of having too many humans and not enough food. The population can not possibly be more than the supply of food to keep that population alive. This has been true throughout history. We never would have gotten to 7 billion today if we did not improve and expand the food supply and that truth will continue. If we reach a cap of food supply, then we will have also reached a population cap.

    This is not really as big a problem as it the article implies but is certainly something to pay attention too.

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