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Arthur T Knackerbracket has found the following story:
The area of agricultural land that will require irrigation in future could be up to four times larger than currently estimated, a new study has revealed.
Research by the University of Reading, University of Bergen and Princeton University shows the amount of land that will require human intervention to water crops by 2050 has been severely underestimated due to computer models not taking into account many uncertainties, such as population changes and availability of water.
The authors of the study, published in Geophysical Research Letters, argue forecasters and policy-makers need to acknowledge multiple future scenarios in order to be prepared for potential water shortages that would have huge environmental costs.
[...] "If the amount of water needed to grow our food is much larger than calculated, this could put severe pressure on water supplies for agriculture as well as homes. These findings show we need strategies to suit a range of possible scenarios and have plans in place to cope with unexpected water shortages."
[...] The new research suggests that projections of irrigated areas made by the Food and Agriculture Organization of the United Nation and others have always underestimated the amount of irrigation required in future by basing them on other assumptions.
The study highlights that the potential global extension of irrigation might be twice, or in the most extreme scenario, even four times larger than what has been suggested by previous models.
[...] Agricultural land where crops cannot be supported by rainwater alone is often irrigated by channelling water from rivers or springs, sprinkler systems, or by controlled flooding. Increased irrigation in future would mean more water consumption, machinery, energy consumption and fertilisers, and therefore more greenhouse gas emissions.
Journal Reference
A. Puy, S. Lo Piano, A. Saltelli. Current Models Underestimate Future Irrigated Areas, Geophysical Research Letters (DOI: 10.1029/2020GL087360)
(Score: 2, Interesting) by khallow on Wednesday May 06 2020, @03:49AM (26 children)
Global human population has never grown superexponentially. There's an upper bound to how many kids a woman can naturally have over a given period of time. The exponential rate has changed, and one could say that an increasing exponential rate is mildly superexponential, but there has always been an upper bounded to that rate - namely that no matter how many children, women give birth to, they can't physically exceed this rate - which is just a steeper exponential curve. So when your curve is bounded from above by an exponential curve, then it is at best exponential growth as well.
Thus, the high estimates aren't supported by the formulas because the real world relationships can't follow the formulas.
(Score: 4, Informative) by Anonymous Coward on Wednesday May 06 2020, @04:31AM (12 children)
Research does suggest it has grown super-exponentially, but "super-exponentially" is referring to something other than what you must think. It means anything beyond an exponential function. To be specific in this case, they are talking about what are called "double exponentials" but there are other super-exponential functions. The most used are tetration (which is also sometimes referred to as superexponentiation) and other higher-order hyperoperations.
(Score: 1) by khallow on Wednesday May 06 2020, @04:44AM (9 children)
(Score: 2, Informative) by Anonymous Coward on Wednesday May 06 2020, @05:35AM (8 children)
I still don't get your point then. Double exponential functions grow faster than exponential functions or even factorial functions. You can trivially construct an exponential function that will grow faster in the short term to a given double exponential function, but the growth curve of the double exponential function will win eventually (same as can be done with a linear function for a given exponential function). Nor does that doesn't change the fact that the human growth curve has been super-exponential in nature for quite some time until near-modernity.
(Score: 1) by khallow on Wednesday May 06 2020, @12:09PM (7 children)
Assertions don't make it so. It's physically impossible to have superexponential growth in the first place. Even if women bore as many children as possible, it'd probably cap out around one doubling of population every decade or so.
The point is that you can't construct a superexponential function that successfully models global human population growth over long periods of time because it is bounded from above by an exponential function. Further, they model population growth rate as a power of the present population. Any superexponential growth of that particular model (which corresponds to an exponent greater than one) naturally has an infinite singularity in finite time. In other words, it predicts an impossible result in finite time - so their model doesn't make sense in a regime that they claim existed for much of human history.
(Score: 2) by HiThere on Wednesday May 06 2020, @03:13PM (6 children)
The maximal growth rate for humans is faster than you are supposing, but it *is* bounded by an exponential function. I suspect that the maximal growth rate would be one child per woman per 18 months between the ages of, say, 15 and 45. And all children surviving. That's not very realistic, but it is an reasonable upper bound.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 1) by khallow on Wednesday May 06 2020, @03:26PM (3 children)
That time lag is a large part of the reason I asserted doubling was on the order of 10 years. Having children sooner is more important for fast population growth than having more children over a long time span. Ok, I'll download Libre Office to calculate the true exponential rate. Then you will all pay!
(Score: 1) by khallow on Wednesday May 06 2020, @03:47PM (2 children)
(Score: 1) by khallow on Wednesday May 06 2020, @03:56PM
Here's the polynomial I'm presently trying to factor:
x^90-x^60-x^57-x^54-x^51-x^48-x^45-x^42-x^39-x^36-x^33-x^30-x^27-x^24-x^21-x^18-x^15-x^12-x^9-x^6-x^3-1=0
Multiplication by x is a time step of half a year. This corresponds to saying the number of births 45 years in the future is equal to the number of women who were born in the years 0 through 30 in the future, every 3/2 of a year (which is 18 months). Anyway, once I factor the polynomial, I look for the largest root and that's my exponential factor.
I also noticed that this works only if I assume women only give birth to more women.
(Score: 1) by khallow on Wednesday May 06 2020, @04:00PM
2x^90-x^60-x^57-x^54-x^51-x^48-x^45-x^42-x^39-x^36-x^33-x^30-x^27-x^24-x^21-x^18-x^15-x^12-x^9-x^6-x^3-1=0
(Score: 0) by Anonymous Coward on Wednesday May 06 2020, @07:52PM (1 child)
You are both forgetting many important in human populations. First is that humans don't have distinct generations relative to their lifetime. They overlap to the point where a person can be alive and have grandchildren older than their children. Second is that humans have lived longer over time. Each month of life expectancy adds to the total population of that month due to generational overlap and to the mean number of children per generation by extending reproductive age. Third is that child mortality rate is dropping, meaning that more humans as a proportion of their generation are reaching reproductive age.
Those are just the big three factors and their most obvious ways of affecting human population. There are tons of papers, including the two that TFA cites, on this subject and plenty of free resources online at a simple web or literature search. Feel free to look it up yourself. Many go into great detail as to why an exponential function won't work for human population data.
(Score: 1) by khallow on Wednesday May 06 2020, @10:04PM
None of these items are relevant to the population growth extremes we were discussing.
(Score: 2) by FatPhil on Wednesday May 06 2020, @10:51PM (1 child)
Anyway, attempting to curve fit human population growth onto a simple curve is a fool's errand. There are different phases caused by different environmental factors such as mastery of fire, mastery of communication, mastery of energy sources, and eventually resource shortages, that drive the population on quite unrelated trajectories. It makes sense to curve fit piecewise, with each section having a simpler curve that has parameters which have real world analogues (such that a multiplier would correlate directly to a children-per-household figure, say).
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 0) by Anonymous Coward on Thursday May 07 2020, @01:06AM
No, an exponential function with a big enough exponent wouldn't be super-exponential for the same reason that exponential functions aren't linear. They not only are in different forms, but the resulting functions have different properties.
(Score: 5, Funny) by driverless on Wednesday May 06 2020, @04:58AM (4 children)
You didn't read the title carefully enough, they're discussing irrigatation, for which their assumptions are accurate. If it was irrigation then that's another story.
For those who don't know, irrigatation is the feeling you get when your boss comes up to you and begins a sentence with "Mmmmm, I'm going to have to ask you to ...".
(Score: 2) by RS3 on Wednesday May 06 2020, @01:11PM (3 children)
That was supposed to be funny, right? If so, I got it, but many here often don't grasp the subtle humor (which seems obvious to me).
(Score: 2) by driverless on Wednesday May 06 2020, @01:24PM (2 children)
I was wondering how it could possibly be rated "Informative" when it's a bunch of tongue-in-cheek nonsense...
(Score: 1) by khallow on Wednesday May 06 2020, @01:45PM
(Score: 2) by RS3 on Wednesday May 06 2020, @04:26PM
Seemed obvious. I've been modded "troll" for similar humor. (Personally I think the mod system is badly broken.) I was going to mod it "funny" but I wanted to be sure. Someone did, so all is well in Soylentry.
And thanks for the laugh!
(Score: 2) by DeathMonkey on Wednesday May 06 2020, @04:42PM (7 children)
Goddamn, before you start your armchair anti-sciencing could you at least learn what the terms you are using MEAN first?
Superexponential Growth (J-curves) [foresightguide.com]
(Score: 1) by khallow on Wednesday May 06 2020, @09:45PM (1 child)
(Score: 3, Insightful) by FatPhil on Wednesday May 06 2020, @11:10PM
However, with my novelty "Maths Professor" hat on, yup, you're right for a couple of reasons. It's possible to create a J-curve with superexponential features (e.g. abstract/imaginary things like equities/bitcoin value), but (a) it's not asymptotically superexponential (insert huge duhhhh!!!! here); and (b) it's just as possible, in particular when dealing with models of real world material things (e.g. virons, babbies) to create J-curves that never have anything growing faster than normal exponential features. That "4U" institute looks like little more than a diploma mill, I'd be willing to bet it uses things like stock markets for its examples of such curves. If they've chosen curves best analysed as piecewise functions (split at the catastrophe), and then not analysed historical human population as a piecewise function, that would be disingenuous.
Human need not apply. Sorry, I meant superexponential curves do not apply to humans.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2, Informative) by Anonymous Coward on Thursday May 07 2020, @12:52AM (4 children)
Here [rpi.edu] is a link that better explanation. But basically, the growth is super exponential because the exponential growth is also exponential. But don't expect them to change their mind or admit it if they do.
(Score: 2) by RS3 on Thursday May 07 2020, @02:42AM (2 children)
Are you an RPI student?
(Score: 0) by Anonymous Coward on Thursday May 07 2020, @03:26AM (1 child)
No, but their different colleges have put up a number of courses online over the years that I have used for other things. They are well-regarded, so I figured something from there would be taken more seriously than DeathMonkey's link.
(Score: 2) by RS3 on Thursday May 07 2020, @06:21PM
Thank you for that. My dad went to RPI and I've always found them to produce amazing people and research.
(Score: 1) by khallow on Thursday May 07 2020, @01:17PM
From the link
Nope. And of course, we're in a period of slowing doubling rates, presently at 60+ years. Finally, there's a slide where they fit linear curves to different parts of a log population graph over time. Piecewise-exponential (particularly when the exponential declines at the end!) is not superexponential unless the exponential growth is unbounded.