Our energy footprint is not subject to area constraints. It is a theoretical area of forest that would be needed to sequester the excess carbon (as carbon dioxide, CO2) that is being added to the atmosphere by the burning of fossil fuels to generate energy for travel, heating, lighting, manufacturing, etc. If we fail to sequester the excess, it will build up in the atmosphere and create the potential for a possibly catastrophic rate of global warming or other environmental stress. To evaluate sustainability, we must decouple the real demands on Earth generated by our food, wood products and degraded land needs from the theoretical demands generated by burning fossil fuels. They reflect different kinds of sustainability problems and are not cumulative.
In order to understand why there is a problem with sustainability of our lifestyle, we need to think globally. Any good almanac or encyclopedia will provide information about the areas of the Earth that are in any way ecologically available. When areas of true desert, and those covered by water or permanent ice are eliminated, this ecologically available land area, according to my almanac source, is slightly less than 29,000,000,000 (billion) acres. A significant part of this area, such as tundra, semi-arid regions, areas above timberline, and swamplands is not practically accessible for our food, wood products and land degradation demands. United Nations estimates of areas of arable land, cropland, and pasture (FAO, 1995) and U. S. Environmental Protection Agency estimates of world forest cover (Brown et al., 1996) indicate that there are only about 22 billion acres of usable land. This sounds like a lot, but there are 6 billion people on Earth today and most reasonable projections conclude there will be about 10 billion people on Earth by 2050. Because our concern is for a sustainable future, we need to think in terms of these 10 billion rather than today. Thus, by 2050, the ecologically usable surface of the Earth will allow an average total footprint of slightly more than 2 acres per person. This number is fairly well constrained because the usable land area on Earth is not going to change on a human time scale, and population will probably not be significantly less than the projected 10 billion persons.