"...AndAttai begat Nathan, and Nathan begat Zabad, And Zabad begat Ephlal, andEphlal begat Obed, And Obed begat Jehu, and Jehu begat Azariah, And Azariahbegat Helez, and Helez begat Eleasah, And Eleasah begat Sisamai, and Sisamaibegat Shallum, And Shallum begat Jakamiah, and Jakamiah begat Elishama..."- The Bible,Chronicles

Past Human Population Growth

In previous lectures, we have describedhow human cultural development was closely tied to changes in the naturalenvironment. Successive cultural revolutions, such as the agriculturalrevolution, have led to surges in population. Figure 1 summarizes againthe historical record, typical of a "J-shaped" growth, with humans fillingnew niches and (perhaps) not yet reaching a limiting carryingcapacity. One feature to note in this plot is the lack of huge fluctuationsassociated with famines or wars. In fact, the nature of J-shaped (exponential)growth is such that episodic reductions due to such catastrophes usuallydo not affect the inexorable and overpowering upward acceleration in populationsize. An exception is the period of the "blackdeath" in Europe, which produced a noticeable but small downward spikein the curve. The wholesale loss of life due to world wars of the 20thcentury produced only small perturbations to the upward trend.

You are watching: Human population growth most closely resembles the s-shaped growth curve.

Figure 1: Human PopulationGrowth over Time
The human population growth of thelast century has been truly phenomenal. It required only 40 years after1950 for the population to double from 2.5 billion to 5 billion. This doublingtime is less than the average human lifetime. The world population passed6 billion just before the end of the 20th century. Present estimatesare for the population to reach 8-12 billion before the end of the 21stcentury. During each lecture hour, more than 10,000 new people enter theworld, a rate of ~3 per second!Of the 6 billion people, about halflive in poverty and at least one fifth are severely undernourished. Therest live out their lives in comparative comfort and health.The factors affecting global humanpopulation are very simple. They are fertility, mortality, initial population,and time. The currentgrowthrate of ~1.3% per year is smaller than the peak which occurred a fewdecades ago (~2.1% per year in 1965-1970), but since this rate acts ona much larger population base, the absolute number of new people per year(~90 million) is at an all time high. The stabilization of population willrequire a reduction in fertility globally. In the most optimistic view,this will take some time.

Fertility

The current growth of population isdriven by fertility. Figure 2 shows how total fertility rate is astrong function of region. It can be readily seen that the more developedcountries ("the North") have lower fertility rates than the less developedcountries ("the South"). The fertility rates in the developed world areclose to replacement levels (i.e., the population is roughly stable), whilethe rates in the developing world are much higher. Thus, population growthand level of development are clearly linked.Figure 2: Global Total FertilityRates, 1990-1995Fertility is largely controlled byeconomics and by human aspirations. The high fertility of the developingworld can be partially explained by the large number of hands needed toperform low-technology agricultural tasks. In these areas, families withlarge numbers of children realize an enhanced economic status. As technologyimproves, parents realize that having more children decreases rather thanincreases their standard of living. A dramatic example of this effect occurredin Thailand, where, as soon as parents realized that future economic statuswas linked to the secondary schooling (which is expensive in Thailand),the fertility rate dropped from about 6 to 2 in a decade!

Population-Age Pyramids

While fertility rates are obviouslyuseful, the demographics of the existing population are also importantand can provide key information to predict future growth rates. An exampleof two such population-agepyramids is shown in Figure 3.Figure 3: Population/Age Pyramidsof the Developed and Developing WorldThe top figure shows the population-agepyramid for the developed world and the bottom figure is for the developingworld. The figure illustrates the pyramids for the years 1975 (pink) and2000 (blue). The population/age structure of the developed world representsthat of a stable (or nearly stable) population. Here, the pyramid is morerectangular than for a rapidly growing population (bottom) where thereis a much larger number of young people. The bottom figure (typical forcountries like Mexico, Malaysia, India, etc.) is more triangular and showshow a rapidly growing population is dominated by young people. The femaleside of the diagram is particularly important in understanding future growth.This is because fertility is largely controlled by the number of femalesin their reproductive years (roughly ages 15 - 40).In the developing world, not onlyare there many females capable of reproduction, but there are many moreyoung females who are of potential mothers. Thus, the shape of the population-agepyramid for the developing world indicates that the population will continueto grow aggressively for the near future as the cohort of fertile femalesgets larger each year, fed from the lower parts of the pyramid. It takesmany tens (perhaps hundreds) of years to steepen the slopes of the population-agepyramid. Such a steepening is essential before populations can become stable.Intensive efforts to control population have been implemented in variouscountries. In China, aggressive population control via a one-childfamily policy is bringing remarkable change to age structure and populationsize. Click herefor a short case study and dynamic graphic representation of China"s futurepopulation and food security.As can be seen, the aggressive populationplanning policies in India (discussed further in the next lecture) haveserved to steepen the pyramid - but only marginally so. The pyramid for1991 has a long way to go before it resembles the stable structure seenin the developed world.Figure 4: Comparitive Population Pyramids forIndiaClearly, population control is achallenging task for which both persistence and patience will be needed.

Mortality

Mortality, or the death rate per individual,is another determining factor of population growth. In the developing world,the death rate has dropped, more or less continuously, since the startof the industrial revolution. The following figure shows the slow, hardwon, reduction in death rate in various European countries. Personal hygieneand improved methods of sanitation have played a major role and precededthe impact of modern medicine and, in particular, the development of antibioticscapable of reducing death due to infection. The downward trend of the deathrate is common to most countries, although there are some countries (forexample, Russia) where the death rate remains high and refuses to moveappreciably.Figure 5: Death Rates per1000 over Time
The combination of decreasing deathrate due to the march of progress in sanitation and medicine, coupled withthe decrease in birth rate due to changes in the economies, has led toa profound change in the population growth curve in the developed world.This change is called the Demographic Transition.

The Demographic Transition

This is the name given to the processthat has occurred during the past century, leading to a stabilization ofpopulation growth in the more highly developed countries. The DemographicTransition is shown schematically in Figure 6. It is generally characterizedas having four separate phases or stages.Figure 6: The DemographicTransitionStage 1. In this early stageof the demographic transition in Europe, birth rates and death rates areboth high. Modern medicine had not yet developed techniques to lengthenlife substantially and standards of personal hygiene were comparativelylow. Both rates fluctuated depending on circumstances.No demographic transitionhas occured.Stage 2. In this stage, standardsof hygiene and more modern medical techniques began to drive the deathrate down, leading to a significant upward trend in population size. Thebirth rate remained high, as much of the economy was based on agriculture.Mexico is currently between this and the following stage.Stage 2 and 3are indicative of a partial or first demographic transition.Stage 3. Urbanizationdecreases the economic incentives for large families. The cost of supportingan urban family grew and parents were more actively discouraged from havinglarge families. In response to these economic pressures, the birth ratestarted to drop, ultimately coming close to the death rate. In the meantime,however, the increased population in Europe led to tremendous societalpressures that caused large scale migration (e.g., to the USA) and extensiveglobal colonialization.Stage 4. The last stage ofthe demographic transition in Europe was characterized by a higher, butstable, population size. Birth and death rates were both relatively lowand the standard of living became much higher than during the earlier periods.The developed world remains in the fourth stage of its demographic transition.A good example of a country in this stage is Sweden. At stage 4, we speakof countries having completed the second or a full demographic transition.The demographic transition did notoccur overnight in Europe. It is anticipated that a transition like thiswill occur in all countries as they become further developed. However,time (many decades) will be needed for the birth and death rates to equilibrate- during which time the population will continue to grow rapidly.The demographic data from the variouscountries of the world has been analyzed by many separate entities, includingthe United Nations. Figure 7 shows the expected future growth curve. Mostof the future growth will occur in the developing world as each countrystruggles to go through a demographic transition of their own. This particularprojection shows a total population approaching 9 billion by the year 2050.The projected curve more closely resembles a sigmoidal (logistic) or "S-shaped"curve.Click on image to enlargeFigure 7: Projected WorldPopulation GrowthBecause most developed countrieshave undergone a complete demographic transition, and have low populationgrowth rate, their numbers increase little over the present. In contrast,developing countries with their high population growth rate will comprisea larger and larger fraction of the world populations.

Future Global Population Growth

Anyone who examines world populationgrowth over the past two centuries certainly must be astounded, and quitepossibly alarmed. The global population reached one billion in 1804.In 1927, some 123 years later, it passed two billion. Sixty yearslater, in 1987, the world population was five billion, and 12 years later,in October 1999, it is estimated to have passed six billion. Smallwonder that many are concerned about what this bodes for our future.Due to the momentum represented by steeply pyramidal age distributions,population growth surely will continue for one to several generations.Most of that growth will occur in developing nations. An eventualworld population of 8-12 billion is expected by the end of the century.But estimates change frequently.According to a report from the UnitedNations Population Fund, based on 1998 analyses (see TheState of World Population 1999), projections for the future globalpopulation are being revised downward. The projection for 2050 nowis 8.9 billion (medium variant), substantially lower than the 1996 projectionof 9.4 billion.The major reason for the lower projectionis good news: global fertility rates have declined more rapidly than expected,as health care, including reproductive health, has improved faster thananticipated, and men and women have chosen to have smaller families.About one-third of the reduction in long-range population projections,however, is due to increasing mortality rates in sub-Saharan Africa andparts of the Indian subcontinent. The most important factor is HIV/AIDS,which is spreading much faster than previously anticipated.The HIV/AIDS epidemic is having adevastating effect on Africa. Estimates released in 1998 by UNAIDS andthe World Health Organization indicate that global HIV infections increased10 per cent in 1998 to 33.4 million people worldwide. In 1999 alone, anestimated 5.8 million people contracted the virus that causes AIDS. Fig.8 shows the likely impact of AIDS in Sub-Saharan Africa, comparingpopulation projections with and without AIDS in the 29 most-affected countries,1980-2050.
Figure 8.

Changing Distribution of World Population

Populations in certain regions willgrow; elsewhere, human numbers will stabilize or even decline. Within countries,populations will continue to shift from rural to urban areas, while becomingincreasingly older and better educated. Migration between countries willbe an increasingly important factor in international relations and thecomposition of national populations.Fig 7 shows the regional make-upof the world population, 1950-2050. Clearly, the fraction that livesin less-developed regions will dominate, continuing a trend that alreadyis well underway. In 1960, 70 per cent of the globalpopulation lived in less-developed regions. By late 1999, the less-developedregions had grown to comprise 80 per cent. Of the projected growthof the world population by 2025, 98 per cent will occur in these regions.Africa, with an average fertilityrate exceeding five children per woman during the entire period, has grownthe fastest among regions. There are almost three times as many Africansalive today (767 million) as there were in 1960. Asia, by far the mostpopulous region, has more than doubled in size (to over 3.6 billion), ashas Latin America and the Caribbean. In contrast, the population of NorthernAmerica has grown by only 50 per cent, and Europe’s has increased by only20 per cent and is now roughly stable.A global trend towards urbanizationalso is taking place. The world’s urban population is growing by60 million a year, about three times the increase in the rural population(Fig. 9).Themovement of people towards cities has accelerated in the past 40 years,particularly in the less-developed regions, and the share of the globalpopulation living in urban areas has increased from one third in 1960 to47 per cent (2.8 billion people) in 1999. Increasing urbanizationresults about equally from births in urban areas and from the continuedmovement of people from rural regions. By 2030, it is expected thatnearly 5 billion (61 per cent) of the world’s 8.1 billion people will livein cities. (see Lecture on Migrationand Urbanization)Globally, the number of cities with10 million or more inhabitants is increasing rapidly, and most of thesenew "megacities" are in the less-developed regions. In 1960, only New Yorkand Tokyo had more than 10 million people. By 1999, the number of megacitieswas 17, 13 in less-developed regions. It is projected that there will be26 megacities by 2015, 22 in less-developed regions (18 will be in Asia);more than 10 per cent of the world’s population will live in these cities,up from just 1.7 per cent in megacities in 1950.

Is there a Carrying Capacity for Homosapiens?

As we have seen, the human populationgrowth curve is currently following an exponential curve or a "J-shape"(fig. 1). Common sense tells us that such growth cannot continue - otherwisewithin a few hundred years every square foot of the smashville247.net"s surface wouldbe taken up by a human. Furthermore, experience with other species tellsus that, ultimately, resource limitations and/or habitat degradation willforce the human population curves to approach an upper limit or asymptote- the carrying capacity, often symbolized as " K" by ecologists. It isvery natural to ask the linked questions - does humanity have a carryingcapacity and, if so, what is it - and when will we reach or overshoot thislimit?JoelCohen"s recent book on human carrying capacity summarizes the continuinglack of scientific consensus on the subject. Estimates of the number stillvary widely according to the specific assumptions used. In fact, the estimatesare more scattered than before - indicating a quantitative field stillvery much in its infancy. One strand of thought, represented by the authorJulianSimon discards the notion of a human carrying capacity altogether,claiming that the additional people will provide sufficient creativityand innovation to break through any possible natural barriers to humanpopulation growth. Most of the serious estimates of K for humans, however,lie in the range 10 -20 billion people.There are no easy answers to thequestions: “How many people can the smashville247.net support?”, and “At whatlevel of well-being?”. Cohen suggests we think in terms of threepossible (and non-exclusive) solutions:Make a bigger pie: Increase human productivecapacities through technology and innovationPut fewer forks on the table: Reducenumbers and expectations of people through such means as family planningand vegetarian dietsTeach better manners: Change the termsof people’s interactions through improved planning and government to enhancesocial justice.

Summary

Human population exhibits an J-shapedgrowth curve, and is accelerating.Age pyramids are important descriptorsof a population’s recent history and medium-term future. Population growthrates are highly dependent upon level of development.A decline in both death and birth ratesis referred to as a complete demographic transition. Most current and future growth is taking place in developing countries, which have experienced only a partial demographic transition.

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Suggested Readings

Ehrlich, P. and Erhlich, A. 1990. ThePopulation Explosion. New York: Simon and Schuster.Gallant, R. 1990. The Peopling of thePlanet smashville247.net. New York: Macmillian Publishing Company.Gore, A. 1992. smashville247.net in the Balance.New York: Houghton Mifflin Company.

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