Get out your propeller beanies, folks. I’m going into full-on geek mode.

On Monday I mentioned that — despite my family’s best efforts to cut back on our CO2 emissions by reducing how much we fly — the world has conspired to defeat us. Sure, we’re flying less, but the rest of our extended family is flying more as a consequence.

One commenter asked if I shouldn’t forget all the personal sacrifice folderol, and just work to convince Boeing to build more efficient planes.

Oh, if only it were so easy …

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First off, Boeing is making more efficient planes — and sales are, for lack of a better term, taking off. Of course, it’ll take years for the new planes to make a substantial dent in fuel consumption of the world’s airliner fleet; but over time, yes, we can expect the world’s airlines to gradually take us from place to place using less fuel per trip.

This is really a continuation of a long-standing trend: measured per passenger mile, air travel has grown steadily more efficient for decades. Better engines consume less fuel per mile, while computerized ticketing & scheduling systems have made sure that each seat is filled, reducing waste. As a result, flying a mile is now about three times more fuel efficient than in 1970. (I could barely believe this when I first read it; but see table 2-12 here.)

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But from a climate perspective, it doesn’t really matter how efficient each trip is. What really matters is the total impact of flying. Unfortunately, even as the efficiency of any given trip has soared, total fuel consumption has taken flight as well: the U.S. consumes 70 percent more jet fuel today than in 1970. (See Table 5-11 here.)

In the chart below, the blue line represents energy consumed per passenger mile — the lower the line goes, the more efficient the air transportation system is. The pink line represents total jet fuel consumption; the higher it goes, the more fuel is used, total, by all airplanes.

Planes:  fuel efficiency vs. total consumption

As you can see, even as plane travel got more efficient, the total impact of flying continued to rise; efficiency gains were overwhelmed by gains in population and — perhaps more importantly — income, particularly among wealthier people who tend to fly the most. Each flight was more efficient, as measured per passenger mile; but the total number of passenger miles flown soared skyward.

The big point here: efficiency alone just isn’t enough to carry the day. If we’re actually going to protect the climate, we’ve got to do much more than just wait for our planes to get more efficient. We’ve also got to focus on how much we fly, both individually and as a culture — which is going to mean a combination of personal choices and the right kinds of incentives to reduce the impact of our travel.