I much appreciate Bill’s thoughtful review of The Quest. He points to a central question—what will fuel transportation in the future? In the aftermath of the 1970s energy crises, a sharp division emerged between oil and other energy sources. In much of the world, oil was squeezed out of electric generation and primarily became a transportation fuel. Thus, when one talks about wind and solar today, that has very little to do with transportation. Transportation and electric generation, as Bill observes, are two separate worlds, with very little trade between them.

Autos have become much more efficient over the last few decades and will become more efficient in the years ahead. Nevertheless, the challenge of fueling the world’s growing population of cars will be huge. I try to phrase the problem simply in The Quest: how will the world cope, in terms of energy, in going from an auto population of 1 billion cars to 2 billion? The most obvious answer is by becoming more efficient. And that will certainly be the case, where, in the United States for instance, the new car will go from about 30 miles per gallon today to 54 miles per gallon by early in the next decade. But that still implies oil as a fuel.

Over the past decade, we have seen a sequence of initiatives to reduce oil dependence in transportation—hydrogen, biofuels, electric car, natural gas vehicles. Biofuels have made an impact. In the United States, on a volumetric basis, they constitute almost 10 percent of motor fuel. But there is a clear limit with ethanol and the other first generation biofuels. The second generation of biofuels is proving harder to attain than anticipated by some a few years ago.

Around 2008, the electric car gained new prominence and commitment, although it is not a new concept. (See the wonderful picture in The Quest of Thomas Edison and Henry Ford discussing the automobile in 1896, prior to Edison’s efforts to promote the electric car!).

From today’s vantage point, electricity seems destined to play a larger role in transportation in the years ahead.  Wide dispersion of electric vehicles would break down the wall between transportation and electric generation and reduce the market share of oil. But informed expectations vary widely as to whether the electric car will be a niche vehicle or a mass market product, and it will probably be five to 10 years before that is clear. Battery innovation and cost are key.

But now there is a new competitor. Or rather, another old competitor with new vigor—natural gas. In some parts of the world, natural gas has long been an alternative fuel.  What is new is the North American shale gas revolution. Its prospect of abundant volumes of low-cost natural gas has made natural gas a new contender in the transportation market. Given current costs, it does appear that natural gas will expand its role in fueling fleets of buses and delivery and service trucks and could well gain share for large long-distance trucks. But the need and cost for refueling infrastructure constitute a major barrier to wide use in motor cars.

All this leads to a very interesting question: Will natural gas gain share in transportation going directly into fuel tanks or by generating electricity that goes into electric vehicles?

Both natural gas and coal are converted into liquids in a few situations around the world today. But the costs—and energy costs—raise major barriers to any notable expansion. For instance, China has stepped back from what were, a few years ago, ambitious targets for coal to liquids. The reason was cost.

So all of this turns attention back to the importance of continuing to improve the energy efficiency of automobiles and gives very strong impetus to the now-familiar hybrid technology as a key enabler.

But there is still that basic challenge of 2 billion cars. Economic growth and rising incomes are what will propel the world from 1 billion to 2 billion cars. That is a massive growth. Moving down that road will require a good deal of innovation along the various avenues that Bill described in his review.

Daniel Yergin is chairman of IHS Cambridge Energy Research Associates and a highly-regarded expert on energy issues. He was awarded the Pulitzer Prize in 1992 for his best-selling book, The Prize: The Epic Quest for Oil, Money, and Power. His most recent book is The Quest: Energy, Security, and the Remaking of the Modern World.