Human Population: how high will it get

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Human Populationhow high will it get

• CSCI 1210
• Spring 2004

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Population with positive feedback

• When population size increases, there are more
  new births
• More births increases population faster
• Result exponential growth

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Adding negative feedback

• As population approaches the carrying capacity,
  growth rate decreases
• Population continues to increase, but more slowly

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Results of logistic model

• Population makes a soft landing right on the
  carrying capacity
• When population is small, positive feedback rules
• As population increases, negative feedback takes
  over -- the system feels its limits.

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Negative Feedback With Delay

• System responds to the limit, but only after a
  delay.
• Result system overshoots and oscillates around
  the limit.

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Overshoot and collapse

• Previous model assumes carrying capacity is
  constant
• What if a severe overshoot degrades the
  environment?
• Carrying capacity might be permanently reduced
• Imagehttp//www.dieoff.com/page80.htm

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Humans are different

• Human carrying capacity is hard to define,
  because
• Technological changes affect food production
• Complex social factors affect population

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Eighteenth Century ChinaMike Davis, Late
Victorian Holocausts

• Golden Age of Qing Dynasty
• Peace and stability
• Mandate for social welfare
• Regional ever-normal granaries
• Hydraulic conservancy for flood control,
  irrigation, canals

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Ecological Cost of Golden Age

• Prosperity and stability encouraged population
  growth
• Marginal lands brought under cultivation
• Watershed forests cut down
• increasing erosion and sedimentation

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Nineteenth Century China

• Crushing cost of flood control
• Yellow River changed course (1855)
• European depredations (Opium wars, etc.)
• Internal decay and corruption
• Rebellion and civil war
• Extremely severe El Ninos 18731900
• Result Massive famines

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China Today

• Current population is much larger than Golden
  Age
• but is current population sustainable?
• Massive famines in 1960s unrelated to population
  size
• Government is intent on population stabilization

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Population stabilizationa sensitive topic

• Deeply held religious beliefs about procreation,
  families, and society
• People resist interference in their family lives
• Residue of mistrust from colonialism
• which we need to understand better!

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Age-Structured Population Model

• Track population fertility by age category
• Uses available demographic data

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The Demographic Transition
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• ALL future population growth will occur in the
  developing world!

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Age distribution
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Demographic Momentum

• In a population with the age distribution of a
  rapidly growing population
• Even after fertility has reached replacement
  levels, the population will continue to increase,
  because there are so many people of childbearing
  age
• Eventually, the distribution will even out.

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A Global Population Consensus

• 1994 UN Summit on Population and Development
• Population stabilization is an important part of
  sustainable development
• Not a separate goal, but in context of better
  health care and development
• Key to success empowering women
• http//www.iisd.ca/cairo.html --Unfortunately, US
  coverage of the Cairo conference focused entirely
  on the abortion debate.

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Population accomplishments

• World population quadrupled in the last century
• Average life expectancy rose from 46 to 66 years
• Developed countries fertility is 1.5
• Developing countries fertility dropped from 6.2
  to 3.1
• Many countries have passed laws increasing status
  of women
• Families with fewer children invest more in their
  care Children are an investment, not a lottery.

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UN projections

• World population may have passed its inflection
  point in 1970.
• Herman Kahn called this time The Year Zero
• Population may increase 50 in 21st century

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The idea of exponential growth persists

• It is easier for us to believe that poor
  countries are the danger to the worlds future
• IPAT and Ecological Footprint models suggest that
  economic growth is a greater danger than
  population growth

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A critical generation

• One billion Earthlings are now in their
  adolescent years
• The fate of these young people may be critical to
  our planets future