For the past 200 years, humans have benefited from the abundant, inexpensive, and easily obtained energy of fossil fuels. Energy surpluses such as this are unusual in human history. In systems with little surplus energy, population growth is low and complexity emerges slowly due to the energetic costs it carries. On the rare occasions when energy is readily available, societies respond by growing rapidly. They must become more complex in response to the social, economic, and resource challenges of dense population. More complex societies are more expensive, requiring greater energy per capita. The process of increasing complexity necessitates greater energy production, creating a positive feedback cycle. Past societies have collapsed under such pressures. Population and complexity grew rapidly when the Industrial Revolution replaced economies based on annual solar radiation with economies fueled by fossil energy. The Green Revolution of the 20th century is credited with preventing mass starvation, but it has made food production and sustaining population ever‐more dependent on high‐energy (low‐entropy) inputs. Some believe innovation will overcome the limitations of resources and permit unchecked growth. However, increases in complexity, innovation, and fossil energy are all subject to diminishing returns, and cannot continue to support population at current levels.