How many humans the Earth can support | Corey Bradshaw | TEDxSydney

Key Concepts

Global population trends, biomass distribution, agricultural revolution impact, ecological footprint, carrying capacity (K), population rate of change (r), facilitation phase, negative phase, climate change, refugee crisis, family planning.

Global Population Trends and Historical Context

The speaker begins by presenting a graph of global population trends over the last 12,000 years. To provide context, significant historical events are overlaid:

• 10,000 years ago: Bass Strait flooding, separating Tasmania from mainland Australia.
• 5,000 years ago: A significant population increase and technological advancements in indigenous Australia.
• 4,500 years ago: Construction of the oldest pyramids in Egypt.
• 3,500 years ago: Oldest fossil dingo dates.
• ~3,500 years ago: Thylacines and devils went extinct on the mainland.
• 146 BCE: Romans sacked Carthage.
• 1788: Permanent European arrival in Australia.
• 1971: Indigenous Australians included in the national census.

The speaker notes that approximately 7% of all people who have ever lived are alive today, representing about 130 billion people.

Biomass Distribution

Despite the large human population, humans are not the dominant biomass. Biomass is defined as the average weight of an individual multiplied by the total number of individuals in a population. The speaker outlines the distribution of biomass:

• Plants: Dominate global biomass.
• Bacteria, Fungi, Archaea: Significant contributors.
• Viruses: Outweighed by humans due to their small size.
• Animals:
  • Marine Arthropods: Most abundant animal biomass.
  • Fish: Second largest animal biomass.
  • Segmented Worms, Terrestrial Arthropods (mostly insects), Mollusks, Cnidarians (sponges and jellies): Follow in decreasing order.
• Vertebrates:
  • Livestock: Dominant vertebrate biomass.
  • Humans: Second largest vertebrate biomass.
  • Non-segmented Worms (Nematodes):
  • Wild Mammals and Birds: Significantly smaller biomass compared to livestock and humans.

The speaker emphasizes that this distribution is a recent phenomenon, driven by the agricultural revolution.

Impact of the Agricultural Revolution

The speaker highlights the dramatic shift in vertebrate biomass distribution since the onset of the agricultural revolution (approximately 12,000 years ago). Before agriculture, wild vertebrates constituted the majority of vertebrate biomass. Today, livestock, particularly cows, dominate, followed by pigs and chickens. This shift is termed "agriculture," which is defined as the process of extracting productivity from the ground and converting it into meat. The speaker notes that every biodiversity metric, including live coral cover, wetland area, free-flowing rivers, and large predatory fish populations, reflects this decline.

Human Population Demography and Carrying Capacity

The speaker delves into human demography, focusing on the period since 1950, when global data collection became more consistent. The highest rate of population increase occurred in the 1960s, following the post-World War II baby boom. The speaker overlays the 17 largest human mortality events (including the Napoleonic Wars, World Wars, and COVID-19) on the population trajectory, demonstrating that these events had minimal impact on the overall growth trend.

Basic Demography Primer

The speaker introduces basic demographic concepts:

• Time (x-axis): Representing forward progression.
• Population Size (y-axis): Representing the number of individuals.
• Rate of Population Change (r): Calculated as the natural logarithm of the ratio of population sizes between two time intervals. A negative 'r' indicates a population decline.

For most species, there is a negative relationship between population size and the rate of population change. The point where 'r' equals zero represents stability.

• Carrying Capacity (K): Defined as the point where the negative relationship between population size and 'r' crosses the stability line (r=0). It represents the maximum population size an environment can sustainably support. Large populations experience increased competition, leading to lower fitness and a decline in 'r', while small populations experience less competition, higher fitness, and an increase in 'r'.

Human Carrying Capacity

The concept of carrying capacity is more complex for humans due to our ability to engineer ecosystems through food production, transportation, housing, and technology. The speaker introduces the concept of the ecological footprint, measured in "Earths" consumed per year, as an indicator of resource use. In 1970, humanity crossed the threshold of consuming one Earth per year. Currently, we consume 1.7 Earths per year. If everyone consumed at the rate of an average Australian, it would require four Earths. This unsustainable consumption is likened to living off a bank account balance by withdrawing more than is deposited, eventually leading to bankruptcy.

Phases of Human Population Growth

Analyzing the relationship between population rate of change and population size since 1800 reveals three distinct phases:

1. Facilitation Phase: A positive relationship between growth rate and population size.
2. Transition (around 1950): An abrupt shift with little change.
3. Negative Phase (since 1962): A strong negative correlation between growth rate and population size, aligning with the pattern observed in most other species. This phase coincides with crossing the "one Earth" line in ecological footprint.

Extrapolating the negative phase relationship to 'r' equals zero predicts a maximum carrying capacity of 11-12 billion people by as early as 2065. This scenario represents a state where births perfectly offset deaths, indicating a harsh environment.

A sustainable population size, based on the facilitation phase relationship, would be approximately 2.5 billion. Using ecological footprint data, aiming for 0.5 Earths per year consumption, yields a similar estimate of 2.4 billion (8.2 billion current population divided by 3.4, the factor by which we exceed sustainable consumption). Independent analyses suggest that 3.3 billion people could live in economic comfort with an equal distribution of wealth.

Population Growth and Climate Change

Current UN projections align with the high-growth trajectory, indicating a continued increase in population. However, growth is not uniform globally. By the end of the century, over a third of all children will be born in sub-Saharan Africa, a region highly vulnerable to climate change.

The speaker addresses the relationship between population growth and climate change, noting that the tightest correlation between temperature anomalies and population size occurs during the negative phase, after crossing the "one Earth" line. While climate change is not solely a product of population size, per capita consumption has also increased by 41 gigajoules per person per year since the late 1960s. Machine learning models indicate that human population size is the primary driver of climate change, with consumption increases playing a secondary role.

Refugees and Displacement

In 2024, approximately 120 million people were forcibly displaced from their homes, with 43 million becoming refugees. In Africa, a 1% increase in population growth is associated with a 3-5 times increase in refugees. Without addressing climate change, warfare, and famine, projections suggest 80-120 million refugees could originate from Africa alone each year by the end of the century. The speaker argues that the rise of right-wing populist movements is partly driven by xenophobia resulting from increasing displacement.

Conclusion and Call to Action

The speaker concludes that the population issue cannot be ignored, as it disproportionately affects the most vulnerable populations. The speaker advocates for empowering women globally by providing free, non-coercive, culturally sensitive family planning options. The speaker states that if we don't have family planning that we want, the environment is going to do it for us.