US carrying capacity

 I bring these to you because I have always had a hard time finding good discussions of carrying capacity that actually generate a number. 

Population Politics: The Choices that Shape Our Future: The Carrying Capacity of the United States
Dr. Virginia Abernethy (hat tip: DieOff.org), (Book found here)

POPULATION SIZE AND THE STANDARD OF LIVING

Depletion of soil, water, and fuel at a much faster rate than any of these can be replenished suggests that the carrying capacity of the United States already has been exceeded. David and Marcia Pimentel (1991) of the College of Agriculture and Life Sciences, Cornell University, take these three factors into account to estimate that, at a standard of living only slightly lower than is enjoyed today, the sustainable population size for the United States is less than half its present number. Beyond this, we abuse the carrying capacity and should expect sudden shocks that will massively drive down the standard of living... 

[In a scenario envisioning] the United States moving to a solar-energy-based economy, that is, to total replacement of our current fossil-fuel energy dependence. Solar energy is a renewable, steady stream, so it meets a key criterion for sustainability. From renewable sources alone, however, only one-fifth to one-half of the present level of energy use would be available. To maintain a standard of living only slightly lower than we enjoy today, population size would need to decline to about 100 million people...

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.

So using very different methodologies and assumptions, and with the benefits of modern technology assumed, you get something like 1/3rd to almost 1/2 of our current population level is sustainable.

Note that without immigration, we would have a shrinking population, a shrinkage that likely would accelerate during the tough conditions that are likely to prevail during the baby boomer retirement period.  But two powerful interests, business groups (generally represented by the Republicans) and demographic minded politicians (generally Democrats) stand opposition to cutting back the flow.