"What determines population growth? What has been the cause of the unprecedented growth in world population in our recent history? Many socio-economic reasons are given as explanations: medical advances, improvements in public health, sanitation and hygiene, increased food availability and agricultural productivity, extension of cultivation, and development of trade and transportation. Surprisingly, high quality energy sources are rarely mentioned or quickly discounted. Yet an argument can be made that each of the above factors contributing to population growth is aided and influenced by high quality energy supplies. Cheap and abundant fossil fuels have been a necessary precondition for the past century’s population growth. And while not all countries benefit directly from the consumption of high quality energy supplies, most countries benefit from the impact of high energy societies on low energy societies. What if energy consumption, or more precisely, energy resource availability, somehow determines population growth? Perhaps energy resources determine the Earth’s carrying capacity, or how many people the Earth can support? Perhaps different energy resources have different effects on population growth? If we hypothesize that the Earth’s population is ultimately determined by availability of energy resources, and if some of those energy resources are at or near their peak rates of production, then that may affect rates of population growth. If the correlation is strong enough, the number of people the Earth can support may also be at or near its peak. Therefore the number of people in 2050 may be very different from widespread United Nations (UN) forecasts. Growing populations consume more energy. Availability of energy allows populations to grow. Energy consumption exerts demands on energy resources making them scarcer. They become harder to extract. Nearby forests are depleted, coal mines must dig deeper, oil has to be drilled in more complex environments. In other words, energy resource extraction experiences declining marginal returns. This has led to the exploitation of new energy sources, which in turn expands the Earth’s carrying capacity. Then populations grow once more."