Gristmill‘s ur-pundit David Roberts makes a good point concerning this post, discussing whether buying a Prius or other super-efficient vehicle really reduces climate-warming emissions. What Dave is wrestling with is whether the oil "saved" by driving a Prius is really just bought by someone else—leading to no net reduction in overall emissions.
A few things to say here. First, as astute commenter Patrick Niemeyer notes, the Prius has other benefits besides using less fuel—it’s also a phenomenally clean car. Driving a Prius reduces your overall emissions of smog-forming compounds, particulate matter, and volatile organics. For cars, cleanliness is, quite obviously, a good thing.
But that doesn’t really get to the heart of Dave’s point—which is that super-efficient cars may ultimately be irrelevant if all the oil gets burned sooner or later. I still think that’s not quite right. By reducing aggregate demand for transportation fuels in any given year, cars like the Prius offer a couple of clear, tangible benefits to the climate.
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Let’s say that for every 10 gallons of gas you save by driving a Prius, demand for gasoline from the global transportation system falls by six gallons—that is, six gallons of gas aren’t used in a given year, and are either placed in storage or not pumped out of the ground. Now, of course, that gasoline may be burned next year, or the year after that. But the pace at which carbon is emitted to the atmosphere does matter: the longer the GHGs are in the atmosphere, the more heat they trap. Delaying carbon emissions may reduce the pace of climate forcing, and buy a little time to deal with the consequences.
Which suggests reducing aggregate demand for transportation fuels can slow, however slightly, the pace of climate change. Which is a good thing.
The second benefit stems from this fact: all else being equal, reducing aggregate demand for transportation fuels will reduce the price of oil. And lower prices reduce the incentive to pull more oil out of the ground.
For the "easy oil"—the stuff that gushes from the ground, or can be coaxed out without too much trouble—this probably doesn’t matter. (If you can produce a barrel of oil for under $10 with current technology, it’s a pretty sure bet that it will get used up sooner or later.) But for the hard oil—stuff that takes a lot of technology, effort, and energy to produce—price matters a lot. Oil from, say, Alberta’s tar sands might be extremely profitable to extract when oil’s at $60 per barrel, but a money sink if oil prices fall below $40 per barrel. Producing the really difficult oil is an especially big climate problem, since it often takes a lot of energy (and climate-warming emissions) to do so.
Now, if prices rise and people think they’ll stay high, a lot of the relatively hard oil will get produced—and sooner rather than later. And there’s a huge supply of "hard" oil in the world. By reducing demand, you make it less economically feasible to extract the really costly oil—which reduces, or at least delays, the climate impacts of the transportation system.
Of course, it could be that, super-efficient cars or no, the world’s "easy" oil will be used up eventually; and following that, prices will rise, and we’ll start extracting the stuff that’s harder to get at. And it’s even feasible to argue that it’s better to face that day of reckoning sooner rather than later—we’ll have to face the music sometime.
Nevertheless, a world of hybrid cars will probably come to that juncture, with all of the world’s easy oil already tapped, a little bit later than it otherwise would; and it might—just might—prevent some of the really difficult oil from being produced at all—especially if renewable energy technologies get more cost- and energy-efficient in the interim.
To me, either result—slowing the pace of global warming emissions, and/or keeping some hard-to-extract oil in the ground—would be a boon for the climate. Whether it’s a cost-effective way to get to that benefit is a question for another time.