Weisz replies: My article examines the magnitude of available energy sources for the support of the growing human population. Expressing that magnitude in terms of human lifetimes indicates the urgency for remedial actions in technology and social behavior, since those actions themselves take a matter of lifetimes to accomplish.

Bernard Cohen and Eric Swager both point to the large potentials of nuclear energy. My article points out the large potential longevity capabilities of nuclear fission energy for “hundreds of years,” and leaves the number of hundreds unspecifically large!

Cohen also mentions the potential of harvesting uranium from the ocean, where it is present in a few parts per billion concentration. However, important basic thermodynamic and mass transport rate constraints limit the economics and feasibility of concentrating highly dispersed matter. I have discussed those constraints relative to the analogous proposal by Fritz Haber, inventor of ammonia synthesis, for harvesting gold from the oceans to pay Germany’s World War I debts. 1  

An energy unit—be it an erg, joule, BTU, or other—describes a definitive amount of energy. Many discussions concerning future energy alternatives predict their energy costs in currency units, that is, dollars. Unfortunately, the value of the dollar itself depends on many factors: human choices, accounting procedures, economic policies, and, most importantly, the then prevailing energy availability.

David Goldstein discusses an energy efficiency defined by the ratio of the economic parameter gross domestic product to the amount of energy consumed. GDP is measured in currency units. It adds the dollars transacted in the goods sector, which includes agriculture, mining, and manufacturing, and those in the service sector—for example, informational, financial, and insurance services and entertainment. Energy consumption for services is lower than for goods. Thus, as long as I have adequate per capita energy for my food, housing, and other essential goods, I can spend many more dollars on services and entertainment, which results in a higher GDP-to-energy ratio. Current per capita energy availability is adequate, with the US having the highest availability; thus people are free to expend currency for many activities beyond necessities.

My analysis deals with the basic scientific and arithmetic choices and limitations of energy resource supply alternatives that are available to sustain the prevailing course of energy demands and the conceivable lengths of time. It serves as a basic guide to understanding the existing challenge of providing for humanity’s future energy supply.

It is extremely unlikely that the long prevailing trend of increasing energy demand will be voluntarily modified to substantially impact the adequacy or lifetime of current energy supplies. By way of example, just halting the growing rate in energy demand due to population growth alone would require a 20% reduction in total energy use—that is, in all energy-consuming technologies and human activities—to be continually repeated in less than 20-year intervals.

Refrigerators are an example of greatly improved energy efficiency. However, the technological capabilities of efficient heat removal have also created broad demand for air-conditioning, which has become a “necessity” humans are unlikely to give up.

David Goldstein is engaged in some important activities in both the energy efficiency and human behavior categories; he has my enthusiastic support. Like him, I also have experienced how ongoing creative achievements are silently absorbed. They generate new and more energy uses that do not noticeably affect net reduction in total energy demand. I was part of a Mobil Corp team that created new catalytic technologies for generating fuels and other products while using 20–30% less petroleum. That work has become a silent dent in the statistics of rising energy demand.

As to policy and governments, the most important current need is education of all citizens and policymakers (which, in a democracy, should be synonymous). That education must include the most basic ingredients of the physical sciences and arithmetic and their relevance to society’s functioning and survival.

P. B.