Winds of Change
I found it surprising that in “A Path to Sustainable Energy by 2030,” Mark Z. Jacobson and Mark A. Delucchi do not mention the effects of the suggested energy sources on climate. The authors propose to absorb about six terawatts of energy from about 60 terawatts available in the wind, or about 10 percent of its total energy. Because the winds, at least near the U.S., usually flow around highs or lows, where the speed and related Coriolis force tend to maintain the pressure difference, I can easily envision that absorbing the energy will change the rate at which the pressure centers collapse. How this would change the weather, I do not know, but it must make a change to give us some of the energy. Possibly, the weather change would be an improvement, but as a believer in Murphy’s Law, I would be surprised. About 100 years ago dumping garbage into the ocean was justified because the oceans were infinite compared to the effect, so no one calculated how much was allowable. Let’s be smarter this time! Why not do the calculations before we cause more problems?
Paul Roetling
Grand Island, N.Y.

Jacobson and Delucchi have a bold vision and a generally balanced account of the opportunities and difficulties. Three matters of concern should be explored further, however. First, vast solar arrays in deserts would suffer from serious loss of efficiency in sandstorms. Second, extensive studies were published in the 1970s on the design of liquid-hydrogen-powered aircraft, the massive energy systems needed at major airports and the difficulties of servicing aircraft whose systems are at three kelvins. Although these problems might eventually be solved, it is very optimistic to think this could be done by 2030. Third, and probably the biggest worry of all, is the rate of construction of these new energy systems that the plan would require. All new global energy systems have grown over a century or so at an average of 1 to 2 percent a year; the article implies a rate of 5 percent a year. Admittedly, the authors compare previous novel construction rates to their proposals, but these were not on the global scale. New technologies that do not reach about 2 to 3 percent of the market lose momentum, fall into the “snake pit” and are forgotten.
John E. Allen
Kingston University
London

THE AUTHORS REPLY: Sandstorms are limited primarily to the Sahara, Persian Gulf states and Gobi Desert but hardly contribute to reduced solar radiation in North or South America or Australia in comparison. During severe events, solar power is reduced in sandstorm regions, but the solar radiation reaching the ground in the annual average in such regions is still large because the events occur periodically. With respect to the second point, we propose that most transportation modes use electricity. Only in cases where electricity cannot be used do we propose hydrogen or hydrogen-electric hybrid vehicles. Air transport is probably the most difficult sector to address; however, a recent European Commission report suggests that there are “no critical barriers to implementation” of a liquid-hydrogen aircraft fleet (http://ec.europa.eu/research/transport/news/article_786_en.html). Although liquid hydrogen requires more than four times the volume of jet fuel, it is about one-third the weight of jet fuel for the same energy, which more than makes up for the additional weight of the fuel tank. As a result, a fully fueled liquid-hydrogen plane will produce more drag but weigh less than a jet-­fueled plane. With respect to the third point, our plan is for governments to mobilize infrastructure changes at a rapid pace. The examples given in the letter are based on typical market penetrations, not on aggressive efforts.

Vertical vs. Horizontal
Aspects of the system described in Dickson Despommier’s “The Rise of Vertical Farms” seem problematic to me. I did like the concept of recycling nutrients and the use of multispecies agriculture. I have done some experiments with organic hydroponics, but I grew the plants outside using natural light. With a 30-story building, 300 feet on each side, you cannot collect enough natural light to grow plants in the entire building. During June, at the peak of the growing season, the amount of sunlight incident on the south, west and east walls of the building will be about a thirtieth of the sunlight incident on a farm with 30 300-by-300-foot plots. In addition, there is no reasonably priced technology to distribute light to the inner parts of the building.

If the deficit in light were to be made up by photovoltaic-powered lights, each square foot of growing area would require about 20 square feet of panels. This difference in area is a result of the inefficiency of the panels and electrical lights. And if the light deficit in the central part of the building were to be made up by electrical lights, the heat load in the building would be enormous. There would also be an enormous moisture load from the respirating plants, creating the need for a huge ventilation system. If high-quality agricultural land is not available, a more feasible approach may be to use recycled nutrients to grow food at ground level on poor-quality land.
Larry Schlussler
Sun Frost
Arcata, Calif.

Save the Planet
Among several reasons cited in favor of educating women in “How Women Can Save the Planet,” Lawrence M. Krauss correctly states that educated women “are less likely to accept fundamentalist extremism.” I believe that it is especially important for Muslim groups such as the Taliban to know that the founder of Islam, the Prophet Muhammad, 1,400 years ago told his followers that they could earn paradise by educating their daughters. Could we all not come closer to a more peaceful society, a paradise on earth, by rediscovering the wisdom of educating the women of our planet?
Sohail Husain
Yale University School of Medicine