At first glance, the links between global population growth and climate change seem obvious. The more people we have on this planet, the greater their collective impact on the climate.
However, closer examination with a longer time horizon reveals relationships between population size and climate change that may help us better understand humanity’s plight as the world’s population reaches 8 billion people – a milestone the United Nations marked on November 15, 2022.
Back to the stone age
For much of human evolution, our ancestors were exposed to large climatic fluctuations between ice ages and intermittent warmer periods. The last of these ice ages ended about 10,000 years ago.
Before the ice caps melted, sea levels were about 400 feet (120 meters) lower than they are today. This allowed humans to migrate across the world. Wherever they went, our ancestors reshaped the landscapes, first by clearing the forests, then by the first agricultural practices that emerged in a number of regions from the end of the last ice age.
Paleoclimatologist William Ruddiman has suggested that these early actions – cutting down trees and expanding agriculture – caused an initial small increase in carbon dioxide in the atmosphere. This has contributed to a stable climate over the past 10,000 years by thwarting downward trends in carbon dioxide levels that could have triggered another glaciation event.
By reshaping landscapes, our ancestors actively constructed the niches they inhabited. This process is an important aspect of evolutionary change, creating important feedback dynamics between evolving species and their environment.
As humans evolved, the demands of a growing population, associated knowledge creation, and energy consumption created a feedback cycle that my colleagues and I call the Anthropocene Engine. This engine has transformed the planet.
Reigniting the engine of the Anthropocene
The engine of the Anthropocene has been running for at least 8,000 years. This led to the rise of modern civilizations and ultimately to the environmental challenges we face today, including climate change. How does the Anthropocene engine work?
First, populations had to reach a critical number of people to succeed in creating enough knowledge about their environment to be able to begin to actively and deliberately transform the niches in which they lived.
Prosperous agriculture was the product of this knowledge. In turn, agriculture increased the amount of energy available to these early societies.
More energy supports more people. More people drove to early settlements and later to cities. This allowed the specialization of tasks and the division of labor, which in turn accelerated the creation of more knowledge, which increased the energy available and also allowed the size of the population to grow. And so on.
Although the details of this process differ across the world, they are all driven by the same Anthropocene engine.
The problem of exponential growth
As an evolutionary biologist and historian of science, I have studied the evolution of knowledge and complexity for over three decades and developed mathematical models with colleagues to help explain these processes. Using the universality of processes underlying the Anthropocene engine, we can capture these dynamics in the form of a growth equation, which includes the links between population growth and increased energy consumption. .
A consequence of positive feedback cycles in dynamical systems is that they lead to exponential growth.
Exponential growth can start very slowly and be barely noticeable for a while. But this will end up having dramatic consequences where resources are limited.
Driven by the engine of the Anthropocene, the human population has grown exponentially and individual societies have approached collapse several times over the past 8,000 years. The demise of the Easter Island civilization and the collapse of the Maya Empire, for example, have been linked to the depletion of environmental resources as populations grew. The dramatic decline in Europe’s population during the Black Death in the 1300s was a direct result of the overcrowded and unsanitary living conditions that facilitated the spread of Yersenia pestis, or plague.
Biologist Paul Ehrlich warned of uncontrolled growth in his 1968 book “The Population Bomb”, predicting that growing global demand for scarce resources would lead to societal collapse without changes in human consumption.
But overall, humanity has always found a way to avoid catastrophe. Knowledge-based innovations, such as the Green Revolution – whose large-scale effects Ehrlich had not foreseen – allowed people to set the record straight, leading to more cycles of innovation and (almost) of collapse.
An example is the sequence of energy regimes. It all started with wood and animal power. Then came coal, oil and gas.
Fossil fuels fueled the Industrial Revolution and, with it, greater wealth and advances in health care. But the era of fossil fuels has had dramatic consequences. It nearly doubled the concentration of carbon dioxide in the atmosphere in less than 300 years, causing the unprecedented rate of global warming humanity is experiencing today.
At the same time, inequalities have become endemic. Poorer nations that have contributed little to climate change suffer the most from global warming, while just 20 wealthier countries are responsible for around 80% of emissions.
The next energy transition to avoid collapse is underway with the rise of renewable energy sources like wind and solar. But studies – including a report released ahead of the 2022 UN Climate Change Conference in November – show that humans are not changing their energy use fast enough to control climate change.
Use knowledge to reset the cycle again
Every species, if left unchecked, would grow exponentially. But species are subject to constraints – or negative feedback mechanisms – such as predators and limited food supplies.
The engine of the Anthropocene has allowed humans to emancipate themselves from many negative feedback mechanisms that would otherwise have controlled population growth. We have intensified food production, expanded trade between regions, and discovered medicines to survive disease.
Where does that leave humanity now? Are we approaching the inevitable collapse due to climate change of our own making, or can we make a new transition and discover innovations that reset the cycle?
The introduction of negative feedbacks into our socio-economic and technical systems – not in the form of radical population control or war, but in the form of norms, values and regulations on excessive greenhouse gas emissions greenhouse – can help to control climate change. Humanity can use knowledge to keep itself within its environmental limits.