L22 - Feeding the world: what is the carrying capacity of Earth? Flashcards

1
Q

Name the historical thinker of the 1800s who first warned about large scale famine. Describe his argument

More recently who also warned about a similar outcome?

A

Thomas Malthus:
- “An Essay on the Principle of Population”
- Predicted exponential population growth outstripping linear increase in food production
- Famine, disease and mortality ensues

UK government’s chief scientific adviser warned of “perfect storm” driven by population growth + resource shortage in 2030

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2
Q

Why did Malthus’ prediction not come true?

A

DIdn’t account for key factors:
- Expansion of agricultural land use
- Major advances in plant breeding
- Advances in animal husbandry

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3
Q

How has food production per capita changed?

Use wheat production over the last 40 years as an example

A
  • Increasing food per-capita for the last 200 years
  • x 6 Wheat production in UK from 1960 to 2000
  • x 2 Wheat production in World from 1960 to 2000
  • Due to increased yield and production area
  • Note, significant increases in food per-capita not global e.g. Africa
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4
Q

How has population size varied historically?

A
  • Sudden increase during 19th and 20th centuries
  • 4.4 billlion in 1982
  • 8 billion in 2024
  • 1990s rate of increase of 1.6% annually
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5
Q

Why has food production increased significantly over the last 200 years?

When was the green revolution and what was it?

A

1850 - 1950:
- Increased food production largely due to expansion of cultivated land
- E.g. prairies + steppe of USA + Russia became “bread baskets” of northern hemisphere
- Little opportunity of further expansion in temperate regions
- Potential of expansion in tropics

1950 onwards:
- Increased yield via “Green Revolution”
- Yield per hectare dramatically increased
- Due to fertilisers, herbicides + fungicides

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6
Q

Describe an example of where the effects of the “Green Revolution” has been studied

A

Broadbalk experiment at Rothamsted:
- Ran since 1843 w/ different wheat varieties + practices modelling changes in farming trends

Results:
- Yield correlates with amount of N applied
- Fallow practice increased yield from 1925 to 1960
- Drastic yield increase between 1960 - 1980 from weedkiller introduction
- Further increase between 1980 - 2000 with fungicide introduction

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7
Q

Give the equation for the total biomass produced by a crop per metre squared, Y(g)

A

Y_g = (S_t)(ε_i)(ε_c)(η) / k

S_t = Total incident solar radiation over growing season

ε_i = efficiency of radiation interception

ε_c = efficiency of conversion of radiation to biomass

η = harvest index (proportion of biomass into grain)

k = energy content of (grain) biomass

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8
Q

Describe how major agronomic advances have increased yield by referring to the S_t term in the biomass equation

Draw the relevant diagrams

A

S_t greatly increased:
- plant varieties that can withstand cold + establish canopy earlier
- Irrigation + fertilisers increase LAI by providing water + nitrogen (specifically RuBisCo) for leaves

(See diagrams of relevant curves on pg 4)

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9
Q

Describe how breeding practices have increased yield by referring to the η term in the biomass equation

Give an example crop to demonstrate this

A
  • “Green Revolution” crop breeding introduced many desired genotypes

E.g. IRRI designed new rice variety:
- Short stems
- Increased no. of grains per panicle
- Vigorous root growth
- Reduced time to maturity
- Fewer tillers, each with more grains

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10
Q

Outline the calculation the DeWit first made to calculate the upperbound for the carrying capacity of the Earth

A

1) S_t = 4.4 x 10^9 (In temperate regions)

2) ε_i = 1 (Assume plants capture all light, valid for e.g. palm, not for wheat)

3) ε_c = 2% from photosynthesis

4) η = 1 (Focused on total biomass, not grain)

5) k = 15 x 10^3 J/g

Y_g = 59 T/ha/yr

  • Same calculation for tropics gives Y_g = 116 T/ha/yr
  • Assuming 2700kcal per person
  • Assuming all land suitable for crops
  • DeWit calculated 1022 billion people as upper bound!
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11
Q

What were the 5 other considerations that reduced DeWit’s initial estimation of the upper bound of the carrying capacity

Give the carrying capacity that most studies now predict

A

1) Accounting for living + recreational area needed per person reduced estimate to 146 billion

2) Much of earth’s surface not suitable for crops/habitation

3) Water and fertiliser not infinite

4) Land needed for other uses e.g. timber

5) Land needed for nature

Most studies predict carrying capacity of around 8 billion people
- Greatly influenced by human consumption patterns

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12
Q

Influence of population on land use:

Describe the current population trends in wealthy and poorer nations

What figure is global population estimated to stabilise at?

What factors could reduce population growth and prevent a potential land use crisis?

A
  • Birth and death rates converging in many wealthy nations
  • Total population growth slowing
  • Estimates of stabilisation at 9-12 billion by 2050

Factors thought to reduce population growth in developing nations:
- Economic development
- Women’s education
- Access to medicine + contraception

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13
Q

Describe the influence of our eating habits on land use

A
  • Eating meat increases land use
  • 10-16 kcal of plant carbohydrate needed to produce 1 kcal of meat or dairy
  • 30% meat diet increases plant carbohydrate requirement to 5.5 x veggie
  • Wealthy nations often eat in excess
  • 2000 - 2700 kcal needed w/ exercise
  • 1560 kcal needed w/o exercise
  • In 1966 Americans purchased 3200kcal per day, 600 kcal wasted

> 1 billion people in developing countries on less than 1560 kcal/day

  • Consuming + wasting less food would reduce agricultural pressure
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