July/August 2000 Vol. 56, No. 4, pp. 28-38
By Steve Fetter
Although the evidence for human-induced global warming is still the subject of intense debate, the majority of the world's climate researchers believe that the process is well under way.
The earth is a natural greenhouse. It would not be habitable if natural greenhouse gases--chiefly water vapor--did not trap heat in the atmosphere. But industrialization and rapid population growth have significantly increased the concentration of greenhouse gases--especially carbon dioxide, which is released by fossil fuel burning and deforestation.
In response to the belief that increasing concentrations of greenhouse gases might lead to harmful changes in climate, the Framework Convention on Climate Change was negotiated in Rio de Janeiro in 1992. Its objective: to achieve "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."
Most studies of climate change focus on the global impact of a doubling of carbon dioxide in the atmosphere from the preindustrial level of 275 parts per million to 550 parts per million. According to the Intergovernmental Panel on Climate Change, the scientific body established to advise parties to the convention, a doubling would, over the long term, increase the average global surface air temperature by 1.5-4.5 degrees Celsius (2.5-8 degrees Fahrenheit), with a best estimate of 2.5 degrees Celsius (4.5 degrees Fahrenheit).
Uncertainties about how cloud cover, ocean currents, and vegetation would change as the atmosphere warms are at the root of the wide range in estimates. But even "small" changes could have big consequences. An increase of 1.5 degrees Celsius, for example, would be greater than any change in the last 10,000 years; one of 4.5 degrees would rival the increase that occurred at the end of the last ice age.
The European Union argues that the increase in average global temperature should not be allowed to exceed 2 degrees Celsius (3.5 degrees Fahrenheit) and that greenhouse gas concentrations should be stabilized at less than an "equivalent doubling" of carbon dioxide. (Equivalent doubling reflects the fact that other greenhouse gases--methane, nitrous oxide, and hydrocarbons--must be factored in.) This would require reductions in greenhouse emissions far beyond existing commitments or proposals.
In particular, it would require a fundamental transformation of the global energy system during the next half century. Traditional fossil fuels--mainly coal, oil, and natural gas--would have to be largely replaced by energy sources that emit little or no carbon dioxide.
The great transformation
Energy experts predict that total global consumption of primary energy--energy used for space heating, transportation, and generating electricity--will double or triple over the next 50 years, from about 400 exajoules (EJ) per year in 1998 to 800-1,200 EJ per year in 2050.
(An exajoule is a billion billion joules. One exajoule is about equal to the energy content of 30 million tons of coal, or the gasoline consumed by a million automobiles during their lifetimes, or the annual energy consumption of West Virginia or Portugal.)
Fossil fuel consumption would have to be limited to about 300 EJ per year in 2050 to permit stabilization of anthropogenic greenhouse gases at an equivalent doubling. Carbon-free energy sources would then have to supply the difference: 500-900 EJ per year.
That's daunting. In 1998, carbon-free sources supplied less than 60 EJ. Carbon-free energy would need to grow tenfold over the next 50 years--from 15 percent of the total commercial supply to 60-75 percent in 2050.
Possibilities and improbabilities Only two sources of carbon-free energy--hydropower and nuclear fission-- currently produce a significant fraction of the world's energy supply, with each accounting for about 27 EJ per year (7 percent of the current energy supply), virtually all of it used to generate electricity. All other carbon-free sources-- geothermal, wind, solar, and commercial biomass combined--supplied only about 4 EJ (1 percent) in 1998.
Carbon-free energy production has been growing recently at about 2 percent per year--much less than the 5 percent rate needed to stabilize carbon dioxide levels at an equivalent doubling. Moreover, most of the recent growth has been caused by an expansion of nuclear and hydro capacity, both of which are expected to taper off in the coming decades.
Further expansion of hydropower is limited by geography and people's tolerance for dams. Hydropower's contribution might increase to about 60 EJ per year by 2050, but even that is doubtful.
Among the other carbon-free sources of energy often mentioned with enthusiasm are fusion and tapping various forms of geothermal and ocean energy. But each of these has significant technical and economic drawbacks; they almost certainly will not supply a significant fraction of the world's energy before 2050.
This leaves five carbon-free energy sources that could potentially make a substantial contribution to the energy supply in 2050: fission, biomass, solar, wind, and "decarbonized" fossil fuels.
Fission
Of the carbon-free sources that could make a major contribution to the future supply of electrical energy, only nuclear fission is deployed on a significant scale today. In 1998, 434 nuclear reactors with a capacity of 350 gigawatts supplied over 2,300 terawatt-hours of electricity--17 percent of the world's electricity and more than 6 percent of commercial primary energy.
In some nuclear-intensive scenarios, the number of reactors would increase to 1,000-2,000 by 2050, with an installed capacity of 1,100-1,700 gigawatts. At that level, nuclear energy could supply 70-110 EJ per year--about 30-40 percent of the world's electricity.
If this energy were supplied entirely by light-water reactors (the type now in widest use) operating on a once-through cycle (in which the spent fuel is treated as waste), 5-7 million tons of natural uranium would be consumed by 2050. For comparison, it is estimated that at least 20 million tons of uranium can be mined at reasonable prices. Conventional uranium resources could easily support a high-growth scenario for at least 50 years using a once-through cycle.
Over the longer term, heavy reliance on nuclear energy would require a transition to fuel cycles that use uranium more efficiently--or that exploit unconventional uranium resources.
For decades, recycling unburned plutonium and uranium in breeder reactors has been thought to be the "solution" to possible eventual uranium shortages. Recycling is potentially perilous, though, because it increases the risk that plutonium could be diverted to the manufacture of weapons.
Less often discussed is tapping the oceans, which contain about 4.5 billion metric tons of dissolved uranium. Recent studies suggest that uranium could be extracted from seawater for as little as $100 per kilogram. If anything like that proves true, plutonium recycling and breeder reactors might forever remain economically unattractive...
Steve Fetter is a professor in the School of Public Affairs at the University of Maryland, College Park. A more detailed, technical, and fully annotated version of this essay can be found at www.puaf.umd.edu/papers/fetter.htm.
©2000 The Bulletin of the Atomic Scientists