This is a golden oldie from 2007. The author is Alexander Cockburn.
Excerpt: "Then as now, a buoyant market throve on fear. The Roman Catholic Church was a bank whose capital was secured by the infinite mercy of Christ, Mary and the Saints, and so the Pope could sell indulgences, like checks. The sinners established a line of credit against bad behavior and could go on sinning. Today a world market in "carbon credits" is in formation. Those whose "carbon footprint" is small can sell their surplus carbon credits to others, less virtuous than themselves."