L22 - Feeding the world: what is the carrying capacity of Earth? Flashcards
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?
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
Why did Malthus’ prediction not come true?
DIdn’t account for key factors:
- Expansion of agricultural land use
- Major advances in plant breeding
- Advances in animal husbandry
How has food production per capita changed?
Use wheat production over the last 40 years as an example
- 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
How has population size varied historically?
- Sudden increase during 19th and 20th centuries
- 4.4 billlion in 1982
- 8 billion in 2024
- 1990s rate of increase of 1.6% annually
Why has food production increased significantly over the last 200 years?
When was the green revolution and what was it?
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
Describe an example of where the effects of the “Green Revolution” has been studied
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
Give the equation for the total biomass produced by a crop per metre squared, Y(g)
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
Describe how major agronomic advances have increased yield by referring to the S_t term in the biomass equation
Draw the relevant diagrams
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)
Describe how breeding practices have increased yield by referring to the η term in the biomass equation
Give an example crop to demonstrate this
- “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
Outline the calculation the DeWit first made to calculate the upperbound for the carrying capacity of the Earth
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!
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
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
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?
- 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
Describe the influence of our eating habits on land use
- 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
