In describing San Francisco’s admirable determination to go green, Amie notes how unusual it is for a municipality to ban plastic bags. But there’s at least one good reason for it to remain unusual: It turns out plastic bags are actually better for the environment than paper ones, in terms of carbon emissions, energy use, and deforestation.

I don’t bring this up in order to make some broad point about unintended consequences—nor to cast into question the effectiveness of government regulation. San Francisco is bound to make some mistakes on its road to eco-friendliness, and as Amie’s list shows, it’s far ahead of most cities.

However, the plastic bags issue does underscore two broad points that greens should keep in mind:

1) Our ingrained sense of “what’s green” may need tweaking.

Culturally, a lot of our perceptions about eco-friendliness are actually legacies from the 1970s and 1980s, when stopping the hole in the ozone layer, ending nuclear power, and preventing sea animals from stray six-packs and plastic bags was synonymous with saving the environment.

Now, as we’ve pivoted toward a massive effort to reduce carbon emissions, we’re finding that some things we previously scorned may not be so bad. It’s something to keep in mind as we design green policies for the future.

2) Sometimes, a bigger carbon footprint may be justified.

This item contradicts what I said above, but each point is worth digesting independently. Plastic bags are better from the standpoint of carbon emissions, but we shouldn’t let an obsession with carbon blind us to other important aspects of the environment.

Hands down, paper bags are better at protecting wildlife than plastic bags—and safeguarding San Francisco’s unique fauna is probably more valuable, on the margin, than keeping a few tons of CO2 out of the air. In that sense, the city may have a real interest in banning plastic to protect its own fragile ecosystem. To get the biggest benefit, Mayor Newsom should probably ban plastic bags and pursue extra carbon reductions elsewhere (like cutting down on transcontinental flights by divorcing his East Coast wife and marrying local … oh wait, he’s already done that).

So where does that leave us? Not too bewildered, I hope. If you want assurance about the green impact of your actions, just remember: marry local and use cloth bags.

Barron YoungSmith writes for Slate and The New Republic.