Four words from the Pope could have the greatest effect per effort of any act on our environmental problems: "You can use contraception."

It won't solve all problems, but it will help a lot.

There is plenty of scripture he could call on to justify it. One, for example, is that the prophecy of having as many descendants as grains of sand on the beach couldn't have meant all at once, because we wouldn't all fit. Decreasing the chance of that many descendants undermines the prophecy.

Italy's low birth rate means Catholics are using birth control anyway, so people have figured it out.