Mr. Engber has forgotten an important factor in his argument. We CAN become parents and instill environmentalist values in our children without adding to the population or our carbon footprint, if we adopt.
If more people chose to adopt rather than to go to extreme measures to become pregnant (which always carries the risk of multiple births--even more of a carbon footprint!) this would certainly result in a threefold benefit to our world. 1: smaller population. 2: fewer children around the world raised in orphanages and foster care. 3: if the parents are environmentalists, more votes for green!
International adoption does result in a particular individual (the adoptee) probably increasing her carbon footprint, but in order to be a net gain to the world, her carbon footprint would have to exceed what it would have been in her country of origin PLUS what her parents' biological child would have produced: an unlikely scenario.
So forget the case for a childless society--let's advocate for adoption as a green choice. Millions of children around the world could benefit, now and in the future!