In 'The Environmental Argument for Reducing Immigration Into the United States', Philip Cafaro and Winthrop Staples III argue that, given Americans' disproportionate "environmental footprint", environmentalists should want to halt population growth in the U.S., and hence to severely restrict immigration into the U.S.
Despite the authors' constant mentions of "population growth", migration of course doesn't (in itself) alter the global population. So what they're really objecting to is allowing poor people from dysfunctional countries the opportunity to increase their wealth (and hence consumption) through hard work in a place where their hard work will be better rewarded. Because material consumption is bad for the environment. So we should do what we can to keep people poor, including blocking their access to countries with better infrastructure, institutions, etc. After all, if they're stuck in a failed state, with no roads and barely enough reward from their work to put food on their family's plates, they'll use less gas! Yay! (Right?)
Cafaro and Staples seem unimpressed by such welfare-based objections. Besides the tradeoff with environmental values, they offer two responses that engage directly with concern for human welfare:
(1) They claim that "mass immigration drives down the wages of working-class Americans", and so is "unjust" to the latter. But immigration may actually raise native wages, and in any case the welfare gains so drastically outweigh any losses that a little redistribution to compensate any disadvantaged groups could easily bring about an outcome that's better for everyone. (General lesson: whenever an otherwise good policy might seem to unfairly "burden" the poor, just redistribute the proceeds. Environmentalists, of all people, should be aware of this from populist objections to gas taxes.)
(2) They claim that migration to developed countries "makes it easier for common citizens and wealthy elites in other countries to ignore the conditions that are driving so many people to emigrate in the first place." I have no idea why they believe this. It seems at least as likely that mass emigration ("brain drain") would call attention to the country's problems. Though I'm pretty skeptical that the likelihood of implementing needed reforms is going to be much affected, either way, by emigration levels.
Stepping back: If we want to get the most welfare "bang" for our ecological "buck", barring the global poor access to economic opportunities is surely not the way to go. (It's less extreme than outright killing them, but I think ultimately misguided for fundamentally similar reasons.) We should strive for improved efficiency in less humanly damaging ways: emissions taxes, reduced animal (esp. cattle) farming, increased urban density / efficient transit, etc. Not to mention investing in scientific research to uncover new solutions -- investments which are more easily made by a wealthier, better educated populace.