The estimate of maximum sustainable population size takes into account both the source and sink functions of Earth. At least two effects of pollutiongreenhouse warming and the ozone hole are poorly understood. One can only estimate the extent of change to which present levels of pollutants commit us already, the lead time before effects become manifest, and the damage that is being done. Nevertheless, the shift away from a fossil-fuel based economy, adopted in order to minimize greenhouse gas emissions and/or as a market response to high prices, will be one of the severest constraints.
Others, more sanguine, peg the U.S. carrying capacity at a higher level. Economist Robert Costanza of the Marine Biological Institute (University of Maryland) and editor of Ecological Economics thinks the carrying capacity is closer to being 150 million persons (Carrying Capacity, 1991).
In the United States, humankind is already managing and using more than half of all the solar energy captured by photosynthesis. Yet even this is insufficient to our needs, and we are actually using nearly three times that much energy, or about 40% more energy than is captured by all plants in the United States [italics in the original]. This rate is made possible only because we are temporarily drawing upon stored fossil energy; the very use of these fossil fuels, plus erosion and other misuse of our natural resources, are reducing the carrying capacity of our ecosystem. SOURCE
Carrying Capacity of the USA
Carrying capacity (maximum sustainable population size) of the United States: