How to house 100 billion humans on Earth without destroying surface ecology?

October 15^th, 2007 was proclaimed as Blog Action Day, with the environment as this year’s topic.

In previous blog entries (Question which I asked myself en route to Olamot 2006 and Housing 100 billion humans on Earth – another take), I considered the problem of housing 100 billion (10¹¹) humans on Earth without ruining its ecology or making it uninhabitable for plants and animals. Now I would like to consider whether it would be feasible to house this number of humans deep in Earth, leaving most of its surface free of humans, for animals and plants to freely roam.

The total surface area of the Earth is 510 million km², of which about 150 million are land. Let’s assume that we’ll use only 200 million km², of which 150 million km² are below land, and 50 million km² – below water.

We’ll also assume that the average volume needed per capita, including recreation, food producing farms, and factories, is area of 1km² times height of 5 meters. Thus, to house 100 billion humans over an area of 200 million km², we’ll need 500 layers.

Since the plan involves digging of giant caves inside the Earth, each layer height will have to consist of 5 meters of inhabitable space plus unknown height of supporting structure, which we’ll assume to be 15 meters. Thus, the total height of 500 layers would be 10km.

Assuming that the top layer will be at small depth from the surface, the bottom layer will be at depth of about 10km. The geothermal gradient of the Earth’s crust is approximately 20 Kelvins (20°K) per 1km depth. So, the temperature at bottom of the living space will be about 230°C (about 500°K). Therefore, a mechanism for cooling the caves will be needed. This mechanism will, however, provide the humans with geothermal energy, estimated to be 63mW/m². Over an area of 200 million km², total geothermal energy will be 12.6×10¹²W, or 126W per capita.

Of course, the problem of constructing and maintaining structures able to withstand the pressures at depth of 10km as well as survive the movements of the Earth’s crust’s tectonic plates, needs to be solved.