8 billion humans: How population growth and climate change are connected as the 'Anthropocene engine' transforms the planet

At first glance, the connections between the world’s growing population and climate change seem obvious. The more people we have on this planet, the larger their collective impact on the climate.

However, a closer look with a longer time horizon reveals relationships between population size and climate change that can help us better understand both humanity’s predicament as the global population nears 8 billion people – a milestone the United Nations expects the world to hit on about Nov. 15, 2022.

Looking 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 sheets melted, sea levels were about 400 feet (120 meters) lower than today. That allowed humans to migrate around the world. Everywhere they went, our ancestors reshaped landscapes, first by clearing forests and then through early agricultural practices that emerged in a number of regions starting just as the last ice age ended.

Paleoclimatologist William Ruddiman has suggested that these early actions – cutting down trees and expanding farming – caused a small initial rise in carbon dioxide in the atmosphere. That contributed to a stable climate over the past 10,000 years by counteracting trends of declining carbon dioxide levels that might have triggered another glaciation event.