> " The CO2 emissions from constructing a several hundred mile long concrete or steel border would be enormous. (Steel is also a major emissions source.)" You

> " The CO2 emissions from constructing a several hundred mile long concrete or steel border would be enormous. (Steel is also a major emissions source.)" You probably don't hear this approach taken very often because most people won't consider it persuasive when they remember that we have nearly 50,000 miles of interstate highway and many many times more miles of lesser highways and roads. The concrete and steel needed for a couple hundred miles of wall would be a drop in the bucket of infrastructural concrete and steel across the country. You can of course argue that every drop counts, but that sort of argument isn't going to impress many people, which explains why you don't hear it very often. Arguments against the necessity of the wall have more bite.