The average American uses about 9. These data alone suggest the Earth can support at most one-fifth of the present population, 1. Water is vital. Biologically, an adult human needs less than 1 gallon of water daily.
In , the U. Half was used to generate electricity, one-third for irrigation, and roughly one-tenth for household use: flushing toilets, washing clothes and dishes, and watering lawns.
Total world supply — freshwater lakes and rivers — is about 91, cubic kilometers. World Health Organization figures show 2.
Even in industrialized countries, water sources can be contaminated with pathogens, fertilizer and insecticide runoff, heavy metals and fracking effluent. Though the detailed future of the human species is impossible to predict, basic facts are certain. Water and food are immediate human necessities. Doubling food production would defer the problems of present-day birth rates by at most a few decades.
The drive to reproduce is among the strongest desires, both for couples and for societies. What will happen if present-day birth rates continue? Population stays constant when couples have about two children who survive to reproductive age. Practice: Human populations. Human population dynamics. Rule of 70 to approximate population doubling time. Demographic transition model. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Instructor] When we're dealing with population growth rates an interesting question is, how long would it take for a given rate for the population to double?
So we're gonna think about doubling time. Now if you were to actually calculate it precisely mathematically precisely, it gets a little bit mathy. One minute later bottles 1 and 2 are filled. And two minutes later all four bottles are filled. In our example the resource is space—but it could as well be coal, oil, uranium, or any nonrenewable resource.
Picture a pond with a single lily pad. Suppose that each day the number of leaves doubles, until the pond is completely covered by leaves on the thirtieth day.
First question: On what day was the pond half-covered? Second question: One-quarter covered? Third question, this one with no strict answer: On what day did people who love the pond realize there was a growth problem?
Doubling time is intriguingly illustrated by the story of the court mathematician in India who years ago invented the game of chess for his king.
The king was so pleased with the game that he offered to repay the mathematician, whose request seemed modest enough. The mathematician requested a single grain of wheat on the first square of the chessboard, two grains on the second square, four on the third square, and so on Figure 3 , presumably for all 64 squares. At this rate there would be 2 63 grains of wheat on the sixty-fourth square. It is interesting and important to note that each square contains one more grain than all the preceding squares combined.
This is true anywhere on the board. Note that when eight grains are placed on the fourth square, the eight is one more than all previous grains of wheat, the total of seven grains that were already on the board. Or the 32 grains placed on the sixth square is one more than all previous grains of wheat, a total of 31 grains that were already on the board. We see that in one doubling time we add more than all that had been added in all the preceding growth!
To repeat for emphasis: In one doubling time more growth occurs than in all preceding growth combined! The essential characteristic of exponential change is not that it is fast but that it is relentless.
Where the doubling time of virus cases in some cities can be three or four days, continued exponential growth would multiply the cases more than fold in one month! The smaller the doubling time, the sooner other factors come into play to end the exponential phase. What feeds the growth eventually subsides, as with the flu epidemic. Fortunately, unrestrained growth does not usually continue indefinitely.
When personal growth is unrestrained, we have obesity—or worse, cancer. I dedicate this lesson to physics professor Al Bartlett of the University of Colorado Figure 4 , who admonished us with this statement:. The greatest shortcoming of the human race is our inability to understand the exponential function.
See complementary student tutorial screencast on www.
0コメント