1. Ecological Sustainability: what can models tell us?
Dan Everett, CS department
These notes available in outline form:

2. What is sustainability?
Humans living in a way that does not diminish Earth’s capacity to sustain life
Alternatively: living within Earth’s ecological carrying capacity
Are we going through a global ecological crisis?

3. Why use computer models?
A valid model matches reality and can be used to accurately predict the future
Unfortunately, we do not have a valid model of human interaction with Earth
A useful model illustrates key processes that govern the real system

4. Models, bias, and objectivity
To create models, we must make assumptions
This introduces the possibility of bias
However, invalid models will not validate! (model predictions do not match reality)
By rejecting incorrect ideas, we hope to eventually discover objective reality

5. What models cannot do Inform us about our values
Predict results of conscious human decisions

6. Positive feedback
When a system is changed, the system reacts to reinforce the change:
Example:
I drink alcohol
It makes me feel better
I drink more

7. Population with positive feedback
When population size increases, there are more new births
More births increases population faster
Result: exponential growth

8. Feedback and growth
Positive feedback tend to produce exponential curve:
Rises with increasing speed
Doubles in constant time
(Doubling time =70 / percent increase)

9. Exponential growth & finite limit
Is this a valid/useful model of humans on Earth? If so:
Where is the limit?
What happens when we reach the limit?

10. Negative feedback
When a system is changed, the system reacts to resist the change
Example:
I drink more alcohol
I feel sufficiently inebriated
I stop drinking

11. Adding negative feedback
As population approaches the carrying capacity, growth rate decreases
Population continues to increase, but more slowly

12. 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.

13. Negative Feedback With Delay
System responds to the limit, but only after a delay.
Result: system overshoots and oscillates around the limit.

14. Overshoot and collapse
Previous model assumes carrying capacity is constant
What if a severe overshoot degrades the environment?
Carrying capacity might be permanently reduced
Image:

15. Humans are different…
Human carrying capacity is hard to define, because…
Technological changes affect food production
Complex social factors affect population

16. Eighteenth Century China
“Golden Age” of Qing Dynasty
Peace and stability
“Mandate” for social welfare
Regional “ever-normal granaries”
“Hydraulic conservancy” for flood control, irrigation, canals

17. Ecological Cost of Golden Age
Prosperity and stability encouraged population growth
Marginal lands brought under cultivation
Watershed forests cut down…
… increasing erosion and sedimentation

18. 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 1873—1900
Result: Massive famines

19. China Today Current population is much larger than Golden Age…
…but is current population sustainable?
Massive famines in 1960’s unrelated to population size
Government is intent on population stabilization

20. Population stabilization: a 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!

21. 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
--Unfortunately, US coverage of the Cairo conference focused entirely on the abortion debate.

22. 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.”

23. Projecting population future
Population trends are easy to predict if we know fertility and mortality rates
Fertility and mortality are very hard to predict!

24. 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

25. A critical generation
One billion Earthlings are now in their adolescent years
The fate of these young people may be critical to our planet’s future

26. The IPAT formula I = PAT I = environmental Impact P = population size
Proposed by Paul Ehrlich
I = environmental Impact
P = population size
A = Affluence
T = Technology factor

27. IPAT: a conceptual model
Population is not the only factor
An American has more environmental impact than a Bangladeshi or Chinese
To reduce environmental impact we must control P, A, T or all three
Problem with IPAT: no defined measure of total impact I

28. Ecological Footprint model
Definition of total impact:
Ecological fooprint is the total land area that would be needed to support a city, country, or other population unit.

29. Calculating National Ecological Footprints
Economic categories are evaluated
Land is needed to supply food, space for built structures, wood supply, or to absorb carbon emissions
Add these amounts up for all economic categories
Result is the ecological footprint

30. Results of Ecological footprint
Were everyone on Earth to live as an average North American…
It would require three Earths to sustain this lifestyle.

31. The Nightmare Scenario
World3 model created by MIT systems group for the Club of Rome
Model updated, 1990
Graphic:

32. Deconstructing World3 World3 is complex: over 100 variables
Assumptions are hidden in equations
Critique: “Malthus in, Malthus out”

33. Deconstructing World3

34. Deconstructing World3

35. What has happened? Nonrenewable resources begin to run out
Capital is diverted to resource extraction
Less capital for agriculture
Yields fall, leading to famine and death
Malthus in, Malthus out!

36. The Cornucopians
Economist Julian Simon bet ecologist Paul Ehrlich that prices of nonrenewable resources would fall
Ehrlich lost and had to pay Simon $1000
Cornucopians argue that human ingenuity will surmount all “limits” to growth.

37. Are there limits? Simon declined this bet.
Ehrlich and climatologist Steven Schneider offered to bet Simon $1000 on each of 15 ecological indicators getting worse over time.
Simon declined this bet.
The limits to growth are not industrial resources, but ecological resources
The real limit may be the ability of Earth to absorb pollution

38. Ecological safety factor?
Many scientists believe that humans should use at most 50% of Earth’s ecological capacity
This gives us a safety margin in case our calculations are off
It also leaves some room for other living things to share our planet

39. The Big Question:
How much do humans have to change in order to live within Earth’s carrying capacity?

40. And the answer is…
Ecological overload factor if every Earthling comes up to US lifestyle: 3
Additional population increase from
6 to 9 billion:
Further improvement needed to
leave 50% of Earth alone
TOTAL IMPROVEMENT NEEDED:

41. What does this mean?
We need at least a 9-fold reduction in the amount of pollution caused by each dollar of economic activity
Design school: Factor Ten
Another design school: Zero Waste

42. Is Zero Waste possible? Nature does it!
Bill McDonough: divide materials into industrial nutrients and ecological nutrients
Recycle industrial nutrients
Compost biological nutrients
Voila! Future technology!

43. A Democracy Deficit? Those most vulnerable are far away…
And have little power to promote change
Needed: effective planetary democracy
Struggle over the global trade system – the front line of the battle to save the Earth?

44. Acknowledgements
Material on China from Mike Davis, Late Victorian Holocausts