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Impact of irrigation on global warming?

  1. Jun 18, 2008 #1
    I once had a discussion with a Ph.D. at a university regarding global warming. His answer was interesting and something that I do not see addressed commonly in global warming discussion.

    He basically stated that when we change the biomass on a piece of land, such as a large tract of land and turn into a farm which we irrigate and make highly productive, or even our lawns, that what we are doing is increasing the amount of land that will absorb CO2. However, when these plants die, they decompose and then release that CO2. So, in essence, since the population of earth has become so large, over the past few hundred years globally, that we have continuously increased the total area on earth that has become 'managed irrigated lands' and have continuously been locking up more and more CO2 per year in these lands which is released later in the year when the crops are harvested or the plants die off.

    Another point on global warming that I would like to add that seems to be missed:

    Global warming has been going on since the end of the last ice age, after all that's why the ice age ended.
  2. jcsd
  3. Jun 18, 2008 #2
    Have you considered how many CO2 is locked up in the bodies of a few more billion people?

    That was under the assumption that the increase in CO2 concentration ended the cold, however seems to look like the ending of the cold increased the CO2, as it lagged a few hundred years.

    Also, since the onset of the last interglacial, The Holocene, considering the warm earlier period, the Holocene Thermal Optimum roughly from 9000-6000 years ago, it looks like we are in a slow general decline of temperatures, that is if we interpret the geologic records correctly.
  4. Jun 19, 2008 #3


    Staff: Mentor

    Well, according to my Biomedical Engineering Principles textbook (David O. Cooney), a 70 kg man contains about 25.5 kg of fat, protein, and carbohydrate. So let's use an upper limit of 25 kg for the amount of carbon which, bound with O2 is about 90 kg of CO2 per adult man. Let's say that half the population are children and half are adults, let's say men and women have equal mass and that children mass 1/2 of an adult. So that is about 400 M ton CO2.

    A forest (in New England) stores 4 tons/acre/year of CO2. Since life expectancy is 78 years this is equivalent to 1.3 M acres of forest, or 2000 mi^2, or an area smaller than the state of Delaware (but bigger than Rhode Island).

    Frankly, I am a little surprised that the number is that low, but I don't see an error in my arithmetic.
  5. Jun 20, 2008 #4
    I dont understand what you mean by co2 locked up in bodies, are you saying thats a good or bad thing and what does that have to do with crops?
  6. Jun 20, 2008 #5
    Sorry the remark was a bit tongue in cheek. Background is that we have a very complex carbon cycle (not CO2). CO2 is only a temporarily transporter/carrier of carbon between carbonate rocks, oceanic carbonate ions and biota. The amount of carbon dioxide in the atmosphere is the result of many dynamic processes, constantly seeking for balance. The OP only addressed an insignifcant part of those processes, microscopic compared to the annual megatonnes exchange between ocean and atmosphere.

    Point is much more that the historically poorly understood dynamics of CO2 in the atmosphere and the global temperature.
  7. Jun 20, 2008 #6
    Thanks for clearing that up Andre, I have heard that as co2 levels in the atmosphere increase the oceans can take part in the carbon cycle by absorbing excess carbon. How does this work does the co2 from the atmosphere simply dissolve into the salt water or is the above statement false?

    Last edited: Jun 20, 2008
  8. Jun 20, 2008 #7


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    Staff: Mentor

    No idea how you define "average biomass density", so it is hard to comment. What I am sure is that farmed land is much much more productive (ie absorbs much more CO2 in a vegetation phase) than most (if not all) natural biotopes. After all those specialized in farming know their job, and their job is to harvest as much from the hectar as possible.
  9. Jun 20, 2008 #8
    Anyway, linking land use via the carbon cycle to climate encounters two major questions. Would it increase or decrease the contribution of CO2 to the atmosphere and to what extent? Secondly, do we really know the order of magnitude of the sensitivity of climate to greenhouse effect in general and that of CO2 in particular?

    For the second part, perhaps have a look at these threads:

    https://www.physicsforums.com/showthread.php?t=239131 (need to work on the May update..cold!)
  10. Jun 21, 2008 #9
    I would disagree. Most places where irrigation takes place is where there is insufficient rainfall to support crops. Modern farming produces a huge amount of 'biomass' per acre compared to the pre-farmed land in most cases. Desert areas and such. Where a river that flows through an area may produce a small floodplain and support plantlife near it's banks, a river that is drained and pumped onto surrounding lands will greatly increase the biomass surrounding it. Another thing to consider that is different between a farmed section of land versus a native section of land is that most farming is annual crops, meaning that all it's locked up CO2 is released after the season, where in a natural environment there would be some plants that were not annuals such as shrubs and trees, which would not release the CO2 back into the environment on an annual basis.
  11. Jun 21, 2008 #10
    In addition, since water vapor contributes to the greenhouse effect, when you spread a river over a huge area you dramatically increase the water vapor over that region.
  12. Jun 21, 2008 #11


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    Perhaps I am missing the prof's point, but it sounds to me like he's saying that farming has no net effect. The farmed land traps more carbon than it did ebfroe it got farmed, then relesases more when the crops are harbvested. Take the increase in sequestration and subtract the incrase in release, and you get zero, right? Exactly the same amount of carbon in the atmosphere as there would have been without the farming?
  13. Jun 22, 2008 #12
    2 things. First there is a larger annual swing in CO2 because of a larger uptake of CO2 in the spring/summer and a larger release in winter. Second. Since the land that is irrigated is ever increasing, the total amount of CO2 locked up in irrigated lands is ever increasing. Now. Imagine no ppl to irrigate it any more. All the lawns die off and all the farmlands die off. This now makes a huge net CO2 increase globally.
  14. Jun 22, 2008 #13


    Staff: Mentor

    I was thinking about CO2 and this thread while on an airplane busily burning fossil fuels. First, CO2 is known to be a pretty weak greenhouse gas, so I am not convinced that CO2 emissions are the thing to worry about. However, assuming that they are, then the best thing for the environment would be to buy lots of durable wood products, like furniture and log houses and real wood paneling.

    Natural land has a lot of biomass, but since it is stable there is not a lot of new CO2 uptake. Food crops have a lot of CO2 uptake, but it is released right after you digest it. Ditto with biofuels. However lumber is different, a tree farm would have a lot of CO2 uptake but the resulting biomass is not returned to the environment!

    No more need to feel guilty about that beautiful mahogany desk you always wanted. In fact, it is one of the most "green" purchase you could make!
  15. Jun 23, 2008 #14


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    I agree with this. But if we avaerage out the total amount of CO2 through the whole year, it comes out the same, right?
    I don't agree with the logic hear. The irrigated land takes in more CO2 in one season, and releases just that much more in another season. So, if the land stopped being irrigated, it would stop both taking in and releasing more CO2, and the net result would still be zero, wouldn't it?
  16. Jun 23, 2008 #15
    A Volcanic euroption could cause a large supply shock and throw this equilibrium out of balance, what would be the effect of this increase in CO2?
  17. Jun 24, 2008 #16
    I never said "average biomass density" clearly that makes no sense. However, it probably wasn't the best way to word my response. I was merely speculating that perhaps on average when you clear cut a site to produce farming land there might have been more biomass on that site before then after the crops have been planted. Possibly because there were larger trees or fields of weeds and shrubs before they were cut and burned and now there is a field of dirt until the crops reach a decent size again. Not to mention the amount of co2 released while the area is cleared.

    That is a good point considering we are talking about irrigating a strip of land, for example taking a few acres in the southwestern US which is most likely arid land and turning it into rich farming land would defiently increase the annual co2 lock up. I was more thinking of an area where a small forest has been clearcut not a rainforest or anything that substantial but i beleive a few acres of land containing mutliple species of trees, ferns and other plants would be more productive at abosorbing co2 then a corn or wheat field of the same acreage. We still irrigate this land it just doesnt need as much irrigation as arid land. Some forests also contain conifers which absorb co2 year round as farming crops absorb c02 one season of the year. So if you were taking a strip of dessert or prairie land and turning into luscious farm land then your defiently increasing the co2 absorbtion. But in the grand scheme of things when its more then just arid land being transformed to farmland but coniferous and deciduous forests or even jsut strips of bush and trees beeing clearcut i cant see it balancing out.
  18. Jun 24, 2008 #17
    Also to add to this crops are harvested at the end of every summer and turned into food and the leftover biomass from the crops dies or is used to feed animals or whatever. Hay for example, a field where trees used to grow now gros only grass wich is cut at the end of the growing season. Same with corn for cattle. Then it does or is eaten and the co2 is back in the environment. Crops do not provide long term storage of co2 as natural plants do.
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