I suggest an immigration limit that roughly matches emigration, which I understand is roughly 300,000 people annually, with the crucial exception that older immigrants be allowed in at much higher levels. This should help manage overpopulation both in the United States and countries that provide the source of immigration.
Overpopulation is a problem on regional and global levels. In poor areas of the world where economic development isn't happening, overpopulation makes life more miserable. Where economic development is happening or will happen (which I think is more likely worldwide than stagnation), overpopulation leads to increased consumption of natural resources and increased environmental damage. The stunning per-capita pollution in the United States makes our overpopulation and net immigration a huge threat world-wide.
Immigration that exceeds emigration is a form of environmental externality where many effects of overpopulation are transferred from one nation to another. An immigration surplus is also one of the few forms of environmental problems caused by Third World nations more than the developed world, but that doesn't mean it should be ignored.
The problem keeping environmental groups from addressing this issue is the long and still-existing tie between racism and immigration control. Sierra Club has refused to endorse immigration limits and even (in my opinion) reduced its commitment to reducing global overpopulation because of the implicit and near-explicit racism of many immigration control proponents.
So my partial solution - an immigration policy for the United States that seeks a rough balance between immigration and emigration, EXCEPT for immigrants who are old enough to not have many biological children. I'd guess that immigrants who are 35 or older will have so few biological children that they will have little permanent demographic impact. Immigration from older people could even be increased to match the current overall level of legal immigration.
This solution focuses tightly on the immigration/overpopulation issue. Transfer of older immigrants between nations will not have a permanent impact on where the overpopulation is located. Racists will not be able to support it, and losing their support is absolutely essential. I hope it will be less unattractive to groups on the left with close ties to recent immigrant groups, although I doubt they're going to like it.
This is only a partial solution so long as we have extensive illegal immigration. "Closing the border" is not a realistic solution by itself - we've developed an economy dependent on that cheap labor. Some type of amnesty for most of those already here, and a non-exploitative guest worker program, AND control over employers hiring illegals under these future conditions where they'll be competing for jobs, AND truly securing the border, AND accepting that some things grown and produced here are going to cost a lot more because the workers will be more expensive and less exploited, will all be necessary for a complete solution.
Not holding my breath for all this, but I do think my partial solution is possible. I expect national security may push illegal immigration issues in the right direction, although it will also push in a police-state direction as well. We'll see.
key: population, immigration, politics