(Replying to PARENT post)
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.
(Replying to PARENT post)
This might still be worth if it prevents illegal migration of 1 million people per year from countries with much lower CO2 emissions per capita (e.g. Venezuela 6t/year) to the USA (16.5 tons/year).
http://greennews.ie/carbon-footprint-of-trumps-proposed-us-m... estimates the carbon footprint of the „Wall“ as 48m tons - once. With my simplistic numbers above it saves up to 10.5m additional tons per year (i.e. N times that amount in N years). So its carbon footprint is amortized in less than 3 years.
That said, I have no political opinion about the „Wall“ (being in Europe), I just like analyzing both sides‘ arguments.
(Replying to PARENT post)
For example, "the wall". Obviously advocates of the wall for the most part don't give a crap about the environmental impact. But even on the Left I don't hear this cited often as a reason to oppose it (amongst many). The CO2 emissions from constructing a several hundred mile long concrete or steel border would be enormous. (Steel is also a major emissions source.)
Even urbanists, who generally talk a lot about denser construction as having environmental benefits and are kind of "lefty", don't generally talk about the counterposing cost of concrete & steel emissions. I'm not saying it doesn't pencil out, but let's please always talk about it.