Once upon a time, the automobile was actually considered a form of pollution control, since it cleared the streets of all the disease-infested horse manure that had plagued urban centers in the late nineteenth and early twentieth centuries. But as car usage spread... and spread... and spread... it eventually spawned its own pollution problems, not least the threat to the climate posed by burning all that gasoline. Sean Casten has an interesting post over at Grist noting that many of today's much-adored green technologies may, too, impose unforeseen ecological hazards of their own one day. A few examples:

1. The solar industry depends on massive volumes of silicon, which must be mined from quartz and purified of its oxygen with a healthy dose of coal and/or charcoal. Do we comprehend the increased size of quartz mines and (char)coal use to meet a solar-dependent grid?

2. Any central power generation technology requires prodigious amounts of copper in the wires, which must be mined and purified, often with significant acid leaching.

3. Any battery-intensive future—whether for automotive or electricity storage—is implicitly a world that puts us homo sapiens in much closer contact with large concentrations of heavy metals, from lead to cadmium or lighter metals like lithium.

4. Fuel cells require large volumes of rare earth metals (platinum, rhodium, etc.) that tend to be concentrated in parts of the globe not always known for political pleasantry.

First, caveats. None of these strikes me as a good reason to stay mired in our current, fossil fuel-burning course, given everything we know about the dangers of drastic climate change. Moreover, some of the dangers listed seem quite manageable: A decent recycling program can, in theory, alleviate many of the dangers associated with a battery-intensive future, while some of the newer designs for concentrated solar thermal plants use even fewer toxic materials than current photovoltaics do. I'll take these headaches over oil dependency any day.

But fine, point taken. Resources are finite, externalities are everywhere, and we should be cognizant that any glittery new solution for our energy needs may have its own pitfalls. One of the big takeaways here is that the cleanest option for weaning ourselves off carbon is to promote energy efficiency and cut waste, as much as possible. According to McKinsey, the United States could make around 40 percent of needed emission cuts through 2030 via efficiency alone—and mostly at negative cost. I suspect those numbers are overstated, but it gives a rough sense for how much wasted energy is sloshing around the system. As a bonus, curtailing that waste (usually) doesn't involve vast quanities of lead and cadmium.

Vaguely related to Casten's post is this discussion by Rob Goodspeed about whether a switch to an electric-car infrastructure might someday obviate efforts to curb sprawl and promote, say, walkable neighborhoods and mass transit. After all, if we're all puttering around electric cars powered by the wind, and not using any gasoline, then why not drive as much as we damn well please and launch ourselves into the exurbs? And maybe we should! (Although one could note that poorly designed sprawl creates all sorts of other externalities besides carbon-dioxide emissions, from water-quality degradation to congestion.) Still, it's worth observing that even a greener, fossil fuel-free energy economy won't let us evade the planet's resource limitations—sustainability will always be a lurking question.

--Bradford Plumer