And it's not as if global emissions would be the same. People move to the U.S. from the Third World so they can afford a car, air conditioning, and the like. One plausible estimate is that Mexican immigrants emit four times as much carbon in the U.S. than if they had stayed home.
You could argue that, well, these immigrants won't earn enough money to buy cars and air conditioning, but do you really want to go there? Logically, either an immigrant will prosper in America and emit a huge amount of carbon or will fail to prosper. Neither prospect looks good for America. My best guess is that the median illegal immigrant will do well enough to buy a big vehicle with spinning rims on credit, without ever making enough to pay enough taxes to make himself a net benefit.
Moreover, as we saw during the Housing Bubble, immigration drives people into distant exurbs, leading them to commute more and (in California) to run their air conditioners more.
What's interesting is that relationship between immigration and carbon emissions goes almost totally unspoken. Judging from Google searches, almost nobody every mentions it. I don't think many even think about it. The level of intellectual sophistication is about this:
Carbon emissions are Bad.
Immigration is Good.
Therefore, anybody who says that immigration leads to more carbon emissions is Bad.
Here's a recent LA Times article on global warming and immigration. Of course, it's not about how immigration causes global warming, it's a reverse bankshot about how global warming could cause immigration:
Now, scientists are predicting another consequence of climate change –mass migration to the United States.OMG! “As many as 7 million Mexicans could migrate to the U.S. by 2080.”
As many as 7 million Mexicans could migrate to the U.S. by 2080 as climate change reduces agricultural production in Mexico, according to a new study being published this week in the Proceedings of the National Academy of Sciences.