Agricultural Land and Human Carrying Capacity FYI
Globally, approximately 3.5% (518,000,000 hectares) of the Earth’s surface (land surface area is roughly 14.8 billion hectares) is considered Class 1 agricultural land suitable for long term cultivation and 10.69%, or 1.5 billion hectares (1.4 billion per UN) is arable.
In rounder numbers, if 1 hectare of Class 1 land could support one person comfortably (prosperously) by 16th century expectations without working all the livelong day, then Earth’s carrying capacity is about 500 million agrarians (H.T. Odum’s best guess of Earth’s maximum human carrying capacity if all environmental productivity that can be consumed is used to support humans and mutualists).
Oh, but agriculture is the number 1 driver of the Anthropocene mass extinction event. Something less than E.O. Wilson’s half Earth may be needed to allow room for Nature. My guess is <20% for one species (and mutualist domesticants, crops and animals), so start with a ‘humans get 20%’ policy and see if that stops anthropogenic species extinction AND ALLOWS FOR NEW SPECIES TO EVOLVE TO REPLACE BIOMASS AND DIVERSITY LOSS to end the Anthropocene and restore (maximize the empower of the Gaian system) the planet’s life-support system (as distinct from maximizing the number of humans/crops/livestock/pets by maximally diverting all environmental productivity to support humans to the demise of non-mutualist species).
Roughly 18% of all cultivated land is irrigated worldwide (Class 1 more so), and it’s estimated that irrigated land produces 40% of the world’s food. Irrigation, other than by using water containing no salts (e.g. rainwater) on land that is not flooded annually (that flushes out salts), has the long term (hundreds to thousands of years) outcome of salinating soils to a point no crops can be grown (e.g. Fertile Crescent and California’s San Joaquin and Sacramento Valley which are in the process of having their soils salinated).
So figure up to 400million hectares of Class 1 land that could produce a crop. But not annually (unless annually flooded and flood waters deposit silt/nutrients) or all human/animal manure is collected and added to water in a rice paddy, or bat/bird guano is mined sustainably and spread on a few garden plots, or a green manure crop can be grown, or livestock grazed well beyond croplands and protected overnight from predators in corrals where manure, containing nutrients gathered from a large area (as developed in West Africa and by Early European Farmers/Anatolian Neolithic Farmers) are added to cropland/garden plots. Annual cropping of land is the exception.
Energetically, what works elsewhere is swidden agriculture (slash-and-burn). After burning, a nutrient-rich layer of ash remains, and some cropping, from good to not-worth planting again, can be done for 1–5 years on Class 1 land. A fallow period of 15–40 years necessarily follows (range 5–50 years). Figure 27 years fallow and 3 years of diminishing returns cropping (after the first year fighting weeds is often not worth the effort, so what often works is 27 yers fallow and one good crop), or 1/9th of land is cropped at any one time (or 1/27th). So of <400 million hectares, 44 million hectares cropped/year (or 15 million hectares) supports 44 million to 15 million prosperous non-slave/peasant/surf humans who have plenty of leisure time (comparable to forager-nomads).
The Roman’s centuries long deforestation of the Mediterranean region allowed for open-field agriculture where strips were ploughed to allow planting of annual crops (barley, oats, legumes, wheat, rye) with fallow periods between. Ploughing allows weeds/manure to be turned over while unintentionally increasing soil erosion. As productivity declined, new forest was cleared, expanding/maintaining agricultural productivity (for a time — Egypt, with annual flood/silt deposition, was Rome’s only sustainable food source until taken by the Byzantine Empire 639–641 CE).
A two-field system involved crop rotation on the farmed field followed by a fallow year. This developed into a medieval three-field system with one field in grain, another in nitrogen-fixing legumes (peas, beans, lintels) and one fallow field that would then be ploughed to plant a grain crop. A four-field system in northern Europe added a green manure crop of nitrogen-fixing clover and one in a root crop, turnips whose tops provided forage.
Ploughs were pulled by oxen and later horses and mules, so all the strip/field systems depended on a grazing commons that included fallow fields (and households depended on a woodlot commons for fuel) which, if not managed to prevent overstocking/overgathering, led to a tragedy of the commons.
Due to ongoing soil erosion and mining of soils of phosphorus, potassium, and other nutrients, intensive farming by slave/surf/animal power was unsustainable long term beyond a few centuries. Swidden systems have proven sustainable long term provided the needed fallow period is not too short which the demands of distant elites for more production selects for, leading to failure of the food production system, e.g. Roman agriculture whose overcropping merely partially nitrogen deficiency and not the eventual over extraction of any of 17 other nutrients, e.g. potassium and phosphorous, i.e. medieval agriculture was unsustainable.
If the rate of soil erosion is greater than the rate of soil formation, then the land is not suitable for agriculture. If the extraction of any of 18 essential nutrients from the soil exceeds the rate of addition, such as from parent materials or amendments, of the one in short supply, then the land cannot be sustainably farmed.
Human Carrying Capacity
Maximally using all Class 1 lands to maximize human numbers would not end the Anthropocene mass extinction event. To maybe live within limits, allow humans to claim 20% of lands including arable and Class 1 lands within a watershed or sub-watershed area to maybe leave enough room for other species that also require Class 1 lands. Not all arable or Class 1 land could be claimed.
If one-fifth of Class 1 lands and one-fifth of other arable/orchard/pastoral lands could be claimed as part of the human’s 20%, then this gets the human population into the 7–35 million range, which may yet be too many on a severely degraded planet where systems will require about 500 years to recover (75 years may be enough for some grasslands, longer for old-growth forests and most soils to recover).
For Nature’s (and posterity’s) sake, humans cannot claim all of Earth’s environmental productivity (20% for one species and mutualists may be too much impact to end the Anthropocene). Per best guess of systemic management principles that apply to other species and presupposed to applied to humans, the human population should be in the range of 7–35 million, so on a degraded by overshoot planet, 7 million might still be too many.
Fortunately, 1.5 million humans would be a large enough population to preserve present ethno-linguistic-cultural diversity in 10k founding populations. Six million humans would allow each of 10k cultures to have four founding population eggs in the Earth basket to increase the probability of one persisting.
While a human population of 7 million might end anthropogenic extinctions (currently about 8000 times higher than baseline), the continued impact of 7 million humans on a compromised Earth may not allow for respeciation, so the Anthropocene would merely be extended. An initial population of 5 million humans who are endeavoring to renormalize over an 8–20 generation period could have a better outcome than billions endeavoring to sidestep extinction by keeping on keeping on and failing (game over) may have.
“For the first time in history a conviction has developed among those who can actually think more than a decade ahead that we are playing a global endgame. Humanity's grasp on the planet is not strong. It is growing weaker. Our population is too large....” — Edward O. Wilson, Half Earth: Our Planet's Fight for Life 2016