Name:________________________________________
Worksheet for Estimating Potential Limits to Population Growth - Science 1102 - Dr. D.
There is no single "right" answer to each of these questions. I expect answers to vary from student to student. As you make these
estimates, you will have to make decisions on what numbers to use and how to calculate your estimates. That is part of the point of
this exercise. Fill in as many empty boxes
as you can:
Column A
Column B
Column C
Statistics you must find from the
Calculation you used to estimate value in
Your estimate. Do not use
library, your text, the internet,
Column C (show equation or describe). Some
previously published estimates in
lecture, etc.
equations or hints are given.
this column, rather derive your
estimates based on stats from
Column A.
Section 1: Estimating Future Population Size
Column A Statistics
A1. Global population size in the year
(Y1)
(any year prior to Y2
below):
A2. Global population size at present
(or recent) year (Y2) ________
(later than Y1):
A3. None: you will use figure derived
in C1
Column B Calculation
B1. Calculation for annual growth rate:
C1 =
[(A2-A1)/A1] * 100
(Y2-Y1)
Column C Your estimate
C1. Annual growth rate (“percent
growth”) based on your A1 and A2
estimates:
show work:
B3. Calculation for doubling time:
Hint: use equation on p. 75
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C3. Doubling time (years):
Section 2: Estimating Available Resources that are Still Available
Column A Statistics
Column B Calculation
A4. Amount of Earth's land that can be B4. Calculation for Ratio in C4:
potentially used for agriculture
(potential arable land):
Column C Your estimate
C4. Ratio of all potential agricultural
land to land currently used by
agriculture:
A5. Amount of Earth's land that is
currently being used for agriculture:
Hint: see ‘Helpful References’ section
A6. None: you will use figures derived
above
A7. Percent of ‘runoff’ used by
humans (Estimate from Figures 10.2
and 10.9, OR find from the attached
article “Anthropogenic Disturbance of
the Terrestrial Water Cycle.”):
B6.
C6. Number of people the Earth can
support based on the percent of
agricultural land used presently (C4):
Hint: Use A2 and C4 to calculate C6. See ‘Math
Reminders’ section
B7.
Hint: A7 is the ratio of runoff used by humans to the
Earth's total runoff multiplied by 100
Section 2 continued:
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C7. Ratio of Earth's available runoff
to runoff used by humans:
Column A Statistics
A8. None: you will use figures derived
above
Column B Calculation
B8.
Column C Your estimate
C8. Number of people the Earth can
support based on the percent of
runoff used presently (C7):
A9. None: you will use figures derived
above
Hint: Use A2 and C7 to calculate C8. See See ‘Math
Reminders’ section
B9. Use your estimate from C6 and the red line on page
90 of your text or the attached graph.
EXTRA CREDIT: Alternatively, make your own graph
using present population size (A7) and your estimate of
annual growth rate (C1).
A10. None: you will use figures
derived above
B10. Use your estimate from C8 and the red line on
page 90 of your text or the attached graph.
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C9. Number of years until Earth's
population is so large that all
potential agricultural is being used:
C10. Number of years until Earth's
population is so large that all availble
runoff water is being used:
Section 3: Assessing Equity
Column A Statistics
A11. Percent of world living at U.S.
standard of living:
Column B Calculation
B11. Don't do a calculation here; make a purely
subjective estimate based on A11, A12, and C10.
i.e. just "guess-timate" (i.e. no calculation)
Column C Your estimate
C11 Total number of people the
Earth could support if everyone lived
at the average U.S. standard of
living:
A12. Percent of resources used by
above (A11):
A13. Number of animal species on
Earth:
B13.
C13. Percent of animal species that
are human
BEFORE YOU TURN THIS WORKSHEET IN, GO TO THE CLASS WEB PAGE AND ENTER YOUR
DATA IN THE "POPULATION WORKSHEET SURVEY FORM"
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Questions to be answered and turned in with this worksheet:
1. What does your estimate of doubling time (C3) assume (i.e. what conditions
might cause this estimate to become wrong, besides assuming that all raw are
correct and that the calculations are done correctly)?
2. What does your estimate of population size the earth can support (C8) assume
(i.e. what conditions might cause this estimate to become wrong, besides
assuming that all raw are correct and that the calculations are done correctly)?
Cite the sources for stats from Column A. If internet source, cite web address
and organization. If book or periodical, cite author(s), title(s), date, etc.
Box Source:
:
A1
A2
A3
A4
A5
A7
A11
A12
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Helpful mathematical reminders:



To estimate one variable from another using a line graph, find the known
variable value on the appropriate axis and sketch a line perpendicular to
that axis from that point. Where your sketched line intersects the line on
the graph, sketch another line perpendicular to the first sketched line.
Where this second line intersects the other axis is the value of your
unknown variable.
A ratio is simply a comparison of two numbers in which one number is
divided by a second. So the ratio of 100 to 50 can be expressed as
100:50 or 100/50 or 2:1 or 2/1 or "the first number is 2 times as large as
the second". If you are assuming the ratio of A to B is the same as the
ratio of C to D, then the if you know A, B, and D and wish to calculate C,
then C=(A/B)*D.
Helpful references for finding the required stats:

Any general environmental science textbook may be helpful for all these
stats.

I have found two web sites that may be helpful in estimating the amount of
agricultural land:
1. http://www.fao.org/ag/agl/agll/terrastat/#terrastatdb The difficulty with this
site is that there are not statistics available for the entire world, so you will
have to sum the arable lands (both for potential and for actual,,
separately) for each of the seven world regions. To use, select the
‘Actual and potential available arable land’ near the bottom of the
Terrastat database list, and then the world region, and hit the display
statistic button. The stats will be listed by country, but don’t panic, they
are summed by region at the bottom of the table.
2. http://apps.fao.org/page/collections?subset=agriculture The difficulty with
this site is that the terminology is confusing. The term ‘Agricultural Area’
appears to be all land that is farmed (permanent and actual arable) and
permanent pasture. The term “Arable and Permanent Crops” appears to
estimate all land actually being farmed but not grazed. This site does not
appear to estimate potential arable land. To use, select "Land use" in table
to go to stat calculator form. Be sure to select an option in all four of the
menu boxes before submitting the form. Use “WORLD+” under country
menu.
Other sites on agriculture that may be helpful:
www.ifpri.cgiar.org/2020/synth/islam.htm
http://www.jhuccp.org/pr/m13/m13chap3_1.stm#top

Other potentially useful web sites:
http://www.aaas.org/international/psd/waterpop/deSherb.htm
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http://www.xist.org/global/pop1.htm
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For the statistic in A7, the exert from an article in Bioscience on the following
pages may be useful.
http://www.findarticles.com/cf_0/m1042/9_50/65576221/p1/article.jhtml?term=
Anthropogenic Disturbance of the Terrestrial Water Cycle.
Author/s: Charles J. Vorosmarty
Issue: Sept, 2000
RECENT ANALYSIS DEMONSTRATES THAT HYDRAULIC ENGINEERING HAS
PRODUCED GLOBAL-SCALE IMPACTS ON THE TERRESTRIAL WATER CYCLE
The terrestrial water cycle plays a central role in the climate, ecology; and
biogeochemistry of the planet. Mounting historical evidence for the influence of
greenhouse warming on recent climate, and modeling projections into the future,
highlight changes to the land based water cycle as a major global change issue
(Houghton et al. 1995, Watson et al. 1996, SGCR 1999). Disturbance of the hydrologic
cycle has received significant attention with respect to land--atmosphere exchanges,
plant physiology, net primary production, and the cycling of major nutrients (Foley et al.
1996, Sellers et al. 1996, McGuire et al. 1997). Changes in land use are also recognized
as critical factors governing the future availability of fresh water (Chase et al. 2000).
Another important but seldom articulated global change issue is direct alteration of
the continental water cycle for irrigation, hydroelectricity, and other human needs.
Although the scope and magnitude of water engineering today are colossal in
comparison with preindustrial times, most of the very same activities--irrigation,
navigation enhancement, reservoir creation--can be traced back several thousand years
in the Middle East and China. Stabilization of water supply has remained a fundamental
preoccupation of human society and is a key security concern for most nations.
Reducing flood hazard, enhancing food security, and redirecting runoff from water-rich to
water-poor areas continue to provide a major challenge to our engineering
infrastructure.
In this article we address three issues. First, we document the nature and magnitude
of direct human alteration of the terrestrial water cycle, specifically through construction
of engineering works for water resource management. We focus on the redistribution of
freshwater among major storage pools and the corresponding changes to continental
runoff. Second, we explore some of the impacts of this disturbance on drainage basins,
river systems, and land-to-ocean linkages. Finally, we review key uncertainties
regarding our current understanding of human-water interactions at the global scale and
make suggestions on potentially useful avenues for future research.
Evidence for global-scale human impacts on the terrestrial water cycle
Although an exact inventory of global water withdrawal has been difficult to assemble,
the general features of anthropogenic water use are more or less known. Reviews of the
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recent literature (Shiklomanov 1996, Gleick 2000) show a range in estimated global
water withdrawals for the year 2000 between approximately 4000 and 5000 km3/yr
Despite reductions in the annual rate of increase in withdrawals from 1970 (Shiklomanov
1996, 2000, Gleick 1998a), global water use has grown more or less exponentially with
human population and economic development over the industrial era. By one account
(L'vovich and White 1990), there was a 15-fold increase in aggregate water use between
1800 and 1980, when the global population increased by a factor of four (Haub 1994).
Aggregate irretrievable water losses (consumption), driven mainly by evaporation from
irrigated land, increased 13-fold during this period. Global consumption for 1995 has
been estimated at approximately 2300 km3/yr, or 60% of total water withdr awal
(Shiklomanov 1996). ntinued from page 1 To place such water use into perspective, it
is necessary to consider the global supply of renewable water. Using recent estimates
of long-term average runoff from the continents totaling approximately 40,000 km 3/yr
(Fekete et al. 1999, Shiklomanov 2000) and an estimated withdrawal of 4000-5000
km3/yr, humans exploit from 10% to 15% of current water supply. It therefore might
appear that water withdrawal over the entire globe is but a small fraction of continental
runoff and that water poses no major limitation to human development. However, of the
31% of global runoff that is spatially and temporally accessible to society, more than half
is withdrawn (35%) or maintained for instream uses (19%; Postel et al. 1996). And, by
the early 1990s, several arid zone countries showed relative use rates much larger than
the global average (e.g., Azerbaijan, Egypt, and Libya, which were already using 55%,
110%, and
770% of their respective sustainable water supplies; WRI 1998).
Contemporary society is thus highly dependent on, and in many places limited by, the
terrestrial water cycle defined by contemporary climate.
This dependency is likely to intensify as a consequence of population growth and
economic development. From 1950 to 1998, water availability had already decreased
from 16,000 to 6700 m3/yr per capita (WRI 1998, Fekete et al. 1999). If we assume no
appreciable change in global runoff over the next several decades, a projected increase
in global population by 2025 to approximately 8 billion people (WRI 1998) means that
per capita supplies will continue to decline to approximately 5000 m3/yr (WRI 1998).
Tabulating these statistics from the standpoint of accessible water, per capita availability
would be reduced to approximately 1500 m3/yr. Given an estimate of mean global water
use of 625 m3/yr per capita for 2025 (Shiklomanov 1996, 1997), withdrawals could
therefore exceed 40% of the accessible global water resource even with presumed
increases in use efficiency. This has obvious implications for human society and natural
ecosystems, both of which are highly dependent on renewable s upplies of water.
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