1) There is a total of approximately 12.4 billion acres of agricultural land (cropland, rangeland, irrigated land) on the major continents. Given that the average Net Primary Productivity (NPP) of agricultural land is approximately 2900 kcal m-2 y-1 calculate the annual global NPP associated with these lands (your answer should be reported in SI units, i.e. Joules)
2)
Assume that animals (humans and domesticated animals) have a 10% energy conversion efficiency and that we need approximately 2800 kcal d-1 to survive. How many i) Human Vegetarians and ii) Human Omnivores could the planet presently support (for simplicity assume that 50% of the energy content of the omnivore diet comes from meat or meat products). Comment on the implication of this calculation.
UNITS YOU MAY WANT TO USE:
1 ha = 2.47 acres; 1 ton = 1017 kg (1.017 tonne); 1 ly (langley) = 1 cal cm-2; 1 cal = 4.184 J
To solve these questions, we need to follow a step-by-step approach:
1) Calculate the annual global NPP associated with the agricultural lands:
Step 1: Convert the total agricultural land from acres to square meters.
12.4 billion acres * 2.47 ha/acre * 10,000 m²/ha = 307.028 trillion m²
Step 2: Calculate the NPP in kcal.
307.028 trillion m² * 2900 kcal/m²/year = 891.115 trillion kcal/year
Step 3: Convert kcal to Joules.
1 kcal = 4.184 kJ
891.115 trillion kcal * 4.184 kJ/kcal = 3.722 * 10^15 kJ
Therefore, the annual global NPP associated with these lands is approximately 3.722 * 10^15 kJ.
2) Calculate the number of Human Vegetarians and Human Omnivores the planet can support:
Step 1: Convert the daily energy requirement from kcal to Joules.
2800 kcal * 4.184 kJ/kcal = 11,651 kJ
Step 2: Calculate the available energy for human vegetarians and omnivores.
For vegetarians:
Energy available = annual global NPP * efficiency
Energy available = 3.722 * 10^15 kJ * 0.1 = 3.722 * 10^14 kJ
For omnivores:
Assuming 50% of the energy comes from meat or meat products, 50% of the NPP is available to humans:
Energy available = annual global NPP * 0.5 * efficiency
Energy available = 3.722 * 10^15 kJ * 0.5 * 0.1 = 1.861 * 10^14 kJ
Step 3: Calculate the number of Human Vegetarians and Human Omnivores the planet can support.
For vegetarians:
Number of Human Vegetarians = Energy available / daily energy requirement
Number of Human Vegetarians = 3.722 * 10^14 kJ / 11,651 kJ = 31,925,814,321
For omnivores:
Number of Human Omnivores = Energy available / daily energy requirement
Number of Human Omnivores = 1.861 * 10^14 kJ / 11,651 kJ = 15,971,377,793
Thus, the planet can presently support approximately 31.9 billion Human Vegetarians and 15.97 billion Human Omnivores.
Implication: This calculation suggests that the planet can sustain more human vegetarians than omnivores. The energy efficiency of the vegetarian diet allows for a larger population to be supported with the available resources, compared to an omnivorous diet. Transitioning towards plant-based diets may be beneficial for the sustainability of the planet's resources.