Economic activity and energy use are closely coupled. Today the yearly increase in economic activity is larger than the totality of world economic activity of 200 years ago; the population is huge compared to then and will continue to increase; and the energy we now use affects the world's entire environment in many ways. This is the source of the global warming problem.
Large scale, anthropogenic environment problems have happened before and governments have responded collectively to the dangers. More than 30 years ago Rachel Carlson's famous book, "Silent Spring," awoke us to the dangers of DDT, and DDT was phased out. More recently, scientific evidence showing depletion of ozone over the Antarctic (the ozone hole) resulted in the Montreal Treaty phasing out chlorofluorocarbons. The response to these two environmental problems was relatively easy because the economic impact of the required response was limited.
We now face a more complex problem, Global Climate Change. Response to this problem will be much more difficult because its source is the carbon-based energy systems (oil, gas and coal) that drive our economy. This issue first leaped into the consciousness of governments at the 1992 World Summit in Rio de Janeiro, although it had been worrying the scientific community for 40 years. We know that energy use is coupled to economic development. A growing population in the developing world aspires to a standard of living approaching that of the rich nations, and this requires that energy use increase.
The greenhouse gases emitted in energy production drive global warming, and limiting global warming requires that the use of carbon based fuels decrease at the same time that energy use increases. This can be done, in principle, by using a combination of conservation and efficiency and carbon-free energy sources; renewables (hydropower, wind, geothermal, biomass, solar), fusion, and nuclear. However, the nations of the world have yet to make a real start on the necessary changes. The not yet ratified Kyoto Protocol is only the barest of beginnings. Here I want to discuss the problem, the numbers, the options, and conclude with some recommendations.
The picture of the earth taken by the Apollo astronauts on their way to the moon (Figure 1) has, I believe, had a profound effect on thinking about global ecology. This picture shows our world hanging in a black void, our nearest neighbor a dead moon 240,000 miles away, and no signs of life anywhere else in our solar system. We live on a thin skin on our world about 13 miles thick from the top of the highest mountain to the depths of the deepest ocean, tiny compared to our world's 8000-mile diameter. It is what we put into this thin skin that is responsible for global climate change.
[FIGURE 1 OMITTED]
The scientific evidence for climate change is clear. The last 100 years has seen a rapid increase in temperature (Figure 2) that is unprecedented and which tracks the increase in greenhouse gases that we have put into our atmosphere mainly from carbon-based fuels. The evidence comes from actual temperature data, growth rings in trees, gas bubbles trapped in glaciers, coral reefs, etc. The 1990's were the warmest decade in recorded history, and there is more carbon dioxide in the atmosphere now than has been there in the last 400,000 years.
[FIGURE 2 OMITTED]
After the Rio de Janeiro World Summit, governments set up the Intergovernmental Panel on Climate Change (IPCC) to analyze the problem. The scientific community, led by the IPCC, has been working to predict the consequences of the increasing level of greenhouse gases. Modeling the world climate system is complex because of the interaction of the atmosphere, landmasses, and the oceans, but the models continue to improve and now give a quite reasonable agreement with what has happened in the past (Figure 3). Using the newest models to predict the future (Figure 4) gives a "business as usual" expectation that by the year 2100 the world average temperature will increase by something between 3.5[degrees]C and 6[degrees]C and that this increase will be smaller near the equator and higher near the poles. Such a large temperature increase would be very bad. The oceans will rise from one to two meters, driven simply by thermal expansion of the water. The increase will be more if there is a significant amount of melting of the icecaps. All the mid-latitude glaciers will disappear. Rainfall patterns will change. There will be more extreme weather events. Most dangerous of all is the potential for large-scale climate instabilities driven by things that we do not yet fully understand. Will we see that large temperature change? It is very unlikely because business will almost certainly not go on as usual. Already there are signs of change that one hopes will begin to control the emission of greenhouse gases. While the Kyoto Protocol, about which there has been so much argument, is only a pale imitation of an effective program, it is a beginning.
[FIGURES 3-4 OMITTED]
There is a tendency in this world of technology-driven rapid change to believe that the scientists and engineers will come up with technological quick fixes for almost anything that will solve whatever problem comes along. However, in this case, nature prevents such a quick fix because greenhouse gases have a long residence time in the atmosphere (Table 1). It will, in fact, take hundreds of years to undo what we can do in a few decades. Bjorn Lomborg, in his most interesting book, "The Skeptical Economist," agrees that there is a problem, but thinks we can wait before trying to solve it. Once the effects are manifest, it is too late.
If a physicist is to preserve his scientific credentials he has to include at least one equation. I will use only one which I call the energy identity. It relates population (P), per capita income (I/R), and energy intensity (E/I) (the amount of energy required to produce a given amount of GDP) to energy.
E = P x I/P x E/I
[Where: E = energy; P = population; I = world income; I/P per capita income; E/I energy intensity]
Put in these terms, the energy required by a country, a region, or the entire world is easy to predict. Simply multiply the expected population, the desired per capita income, and the expected energy intensity and you have the answer. Note that when I talk about energy, I will mean what is called primary energy. This is different from the energy each of us uses directly. For example, if you plug an appliance into an electrical supply, you use a certain amount of electrical energy; but the primary energy required is larger than that by a factor of about 1.5 to 3, depending on the efficiency of the power plant. Look first at the population term. The United Nations' population projection (Figure 5) in their medium-growth scenario predicts that the world population will grow from its present 6 billion to about 10 billion in the year 2050, and to about 11.5 billion in the year 2100. Almost all the growth is predicted to occur in the developing countries. This growth may slow down with the empowerment of women as well as with an increase in per capita income. Be that as it may, the first term in our equation is predicted to nearly double in this century.
[FIGURE 5 OMITTED]
Next, look at the predictions of GDP and per capita GDP (Table 2). The uncertainties here are clearly large and depend strongly on assumptions about economic growth. They are from the International Institute of Applied Systems Analysis (IIASA) middle-growth projection (their Scenario B). World GDP is projected to grow nearly eight-fold, again with most of the growth occurring in the developing world. The numbers are reasonable and assume that amount of growth is 1.3% per year in the industrialized world and 3.2% in the developing world.
Per capita GDP in the year 2000 in the industrialized world is twenty times that in the developing world, and the ratio is expected to fall to six to one by the year 2100. The ratio would be even smaller if the developing world's population increase were smaller than projected, and there are signs that this may come to pass. Whether the consumption level indicated by the industrialized nations' year 2100 per capita GDP is appropriate is not a question for here.
Next I turn to the third term in the equation. Energy intensity has been declining by about 1% a year for decades. The decline is related to increased efficiency in primary energy use, and to economic changes that increase the amount of services compared to the amount of heavy industry. The IIASA projections (Table 3) assume a continuation of this trend throughout the century. By the year 2100, the developing world is projected to achieve an energy intensity that is the same as the industrialized world has today, while the industrialized world will have improved by a factor of three.
We can now put all of these terms together to get the world projected primary energy use (Figure 6). Under these assumptions, primary energy use will go from 14 TW-years now to 27 TW-years in the year 2050, to 40 TW-years in the year 2100 (a terawatt is one million megawatts). Almost all the increase occurs in the developing world as their standard of living improves. By the year 2020, the developing world is projected to be using as much primary energy as all of the rest of the world combined. The Summary Table (Table 4) shows the relations among the numbers.
[FIGURE 6 OMITTED]
There remains one missing piece that is necessary to complete the puzzle. What is the goal? Do we wish to return to the level of greenhouse gases in the atmosphere in the pre-industrial era (280 parts per million or ppm), or will we accept some higher level? Personally, I regard a 50% increase over the pre-industrial level to about 450 ppm as probably acceptable, twice the pre-industrial level as risky, and anything more as unacceptably dangerous.
In what follows I will use 450 ppm as the target and focus on what we need to do by the year 2050 to be on track to stabilize the greenhouse gases in the atmosphere at that level (not then, but later). We now, worldwide, use 11 TW of carbon-based and 3 TW of carbon-free primary power (Table 5). By 2050, of the predicted 27 TW we should be using only seven terawatts of carbon-based and 20 TW of carbon-free power. This is a daunting challenge; to produce more carbon-free power in the next 50 years than exist now in totality, and to do it at the same time that carbon-based power is being reduced, which will make powerful economic interests very unhappy. Carbon emissions will have to go down further as time goes on, but I will stay with requirements for 2050 because anything that can have a major impact has to begin being deployed soon.
This is the context in which I said earlier that the Kyoto Protocol was an insignificant baby step. If carried out successfully, the industrialized world will decrease its use of carbon-based energy by a half to one terawatt-year per year, while the developing world will increase its use by two to three terawatt-years per year. This is not much of an advance toward a goal that requires the reduction of worldwide use of carbon-based energy.
There are only two large-scale options available now that can make a major impact on greenhouse gas emissions in the next 30 to 40 years. They are conservation and efficiency, and nuclear power. The renewables on which so many in the environment movement pin so much hope are in their infancy. What has to be done to begin to bring greenhouse gases under control has to be done on a very large scale and soon. But, while development of the renewables should be strongly supported, the problem cannot wait on their maturity. The world economy will be using an additional 13 TW in the next 50 years, and to make an impact on this requires large-scale action. Remember that one terawatt of primary power represents the power going into 400 to 500 GW electric power plants. Only mature technologies can have a real impact in the next several decades.
Hydropower: It is currently widely deployed but it is near its limits. Hydropower might expand by a factor of two but it generates its own environmental problems as, for example, with the Three Gorges dams in China.
Biomass: In theory, plants get all of their structural material from the carbon dioxide in the atmosphere, so if you grow plants and then burn them to produce energy, you put back into the atmosphere only the same carbon dioxide that you took out in the first place. In practice it is not quite so simple. It takes energy to produce the required fertilizer, to dry the crops, and to transport them. Biomass is a net carbon dioxide generator and no one has done a really thorough analysis on just how much it will generate. In addition, food crops are already feeling pressure from water limits and it may be hard to find the necessary water to grow energy crops. Growing forests is a different matter; that takes carbon dioxide out of the atmosphere and keeps it in the trees.
Wind: It is useful where steady winds exist as, for example, in western Denmark and California. There are about 16 GW of wind power installed now but, because of the intermittency of winds, they only produce about four gigawatt-years per year of energy, an average efficiency of 25%. They are cost effective and should be deployed widely but cannot be used for base-load power.
Geothermal: This is useful in volcanic areas like northern California or Hawaii. However, sites are limited.
Solar Photo Voltaic: Unfortunately the sun does not shine for all of the 24 hours, even when the days are clear. A solar plant that produces an average power of one gigawatt with low-cost 15% efficient solar cells (not yet available) produces three gigawatt of power at noon and zero at midnight, and it occupies 20 million square meters of land. Such power can be very useful in supplying peak daytime loads in regions without extended heavy cloud cover (not in India during the monsoon, for example). Costs are high now, but will come down by 2020 to a predicted $1.50 per peak watt.
Fuel Cells: These are the hope of the future for transport. They can efficiently convert chemical energy to electricity. I believe we will first see them in trucks and buses around 2010 using hydrocarbon fuels, improving efficiency by a factor of roughly three. Hydrogen cells that produce nothing but water as a byproduct will come later, perhaps by 2020. However, hydrogen does not exist freely in nature and has to be produced by using some other form of energy. The emissions from that energy have to be factored into the environmental equation. Hydrogen may be the perfect energy storage medium for solar voltaic systems.
Conservation and Efficiency: The rationale for emphasizing conservation and efficiency is simple--if you don't use it, it produces no greenhouse gases. The potential for huge savings is there. Built into the forecast that energy use will double by the year 2050 is the assumption that energy intensity will continue its historic rate of decline of 1% per year (Figure 7). If we don't do that well, while assuming the same population and per capita GDP, we will use more energy; if we do better, we will use less. If the decline in energy intensity could be improved to 2% per year, the same population and GDP assumptions would require seven terawatt-years less than that which is required under the standard projections. To be on our track to stabilize C[O.sub.2] in the atmosphere at 450 ppm, the world would then need only 13 TW of carbon-free power compared to the 20 TW required under the standard assumption. The practical minded might note that avoiding the cost of 7 TW-yrs per year of primary energy would save the world economy more than $1 trillion per year.
[FIGURE 7 OMITTED]
Making energy intensity decline by 2% per year will not be easy, but it can be done. China has set an example where carbon dioxide emissions have actually declined by 8% while their economy has grown (Figure 8). They have accomplished this by closing some of their least efficient power plants, shutting down some of their inefficient heavy industry, and beginning the switch of households to cleaner fuels for heating and cooking. It is also worth noting that China's reforestation program is removing significant amounts of carbon dioxide from the atmosphere.
[FIGURE 8 OMITTED]
The best opportunities are in the developing world where energy use is expanding fastest. The industrialized world should assist the developing countries in installing the most efficient energy systems as those countries' energy needs grow. The best natural gas-fired power plants are nearly twice as efficient as old coal plants in producing output energy from primary energy and, in addition, for the same amount of primary energy, natural gas produces one half of the carbon dioxide of coal. It is, thus, possible to get the same energy output with a quarter of the C[o.sub.2] emissions.
Conservation and efficiency should be the centerpiece of the early programs to improve energy intensity. There are also huge opportunities in transportation, lighting, heating, insulation and appliances. This is the most economically efficient way to attack the greenhouse gas problem.
Nuclear Power: This is the most controversial option. There are about 400 nuclear power plants operating in the world today using about one terawatt of primary power to produce 400 GW of electrical output. All of this is carbon free. With a coordinated effort to design the next generation of improved reactors, nuclear power could be expanded by about a factor of three to five over the next 50 years. However, if such an expansion is to occur, public concerns about radiation, accidents, and radioactive waste disposal must be addressed.
Radiation exposure from operating plants is not something to worry about (Table 6). Natural radioactivity from cosmic rays bombarding the earth from space, and from radioactive materials that exist all around us, deliver an annual radiation dose to each person of about 240 millirem (mr) per year. Of this amount, 40 mr comes from materials in our own bodies, such as potassium-40 and carbon-13. Medical x-rays give an average annual dose to each individual of 60 mr. A nuclear power plant gives a negligible amount of radiation in comparison, about four one-thousandths of a mr. Incidentally, a typical coal-fired power plant gives about the same radiation from natural impurities of coal.
The Chernobyl accident, a truly horrific incident, occurred in a type of reactor that has never been allowed to be built outside of the old Soviet Bloc. This type of reactor can become unstable under certain conditions. At Chernobyl, the operators systematically disconnected all of the reactor's safety systems as part of a misguided test and brought the reactor into the unstable region with the consequences that we all know. Even so, radiation levels outside Chemobyl's immediate vicinity were not significant compared to natural radiation.
The Three-Mile Island accident in the U.S. is the worst to have occurred in the water-moderated reactors that produce most of the world's nuclear power. The operators of Three-Mile Island made almost every error that could be made in operating the reactor and thereby melted the reactor core. However, because of the reactor's design and the reactor's containment, the largest dose that anyone outside of the plant itself received was a few mr.
Safety is extremely important in nuclear power. It must be designed in from the beginning and plant operations must be overseen by tough regulators. Given that, the public health impacts of nuclear power (Table 7) are less than those of any other energy source except wind power. Even the solar cells used in photovoltaic systems have much more severe public health consequences than does nuclear power, because of the toxic chemicals used in their manufacture.
Safe disposal of spent fuel from nuclear power plants is appropriately a major concern of the public. This spent fuel is intensely radioactive when it comes out of a reactor and must be handled with great care. It has three main components that could be handled separately in principle, but are now lumped together. The three are uranium (95%), fission fragments (4%), and the long-lived components (1.5%). The uranium is no more of a hazard than the ore that was mined to make the fuel. The fission fragments need to be stored for only a few hundred years until their radioactivity falls below the level of concern. The long-lived component, if left untreated, must be isolated for hundreds of thousands of years. (Table 8)
At present, the favorite method of protecting the public from this material is called geological disposition which keeps all of the components together and stores them in special mines such as Yucca Mountain in the United States. This method will work, but, if that is to be the solution, it will have to be in an international context because not all nations have appropriate geological formations.
People who are concerned about geological storage should consider the case of the OKLO natural reactor in Gabon, Africa. This natural formation began operating as a nuclear reactor about two billion years ago when the ratio of the fissionable uranium-235 to uranium-238 was larger, because of the different lifetimes of these isotopes, and when climatic conditions were correct. OKLO produced about 100 MW of power for many millions of years, and no trace of its existence can be found a few tens of kilometers away.
The scientific community is looking at an alternative to handling the long-lived component called transmutation. The long-lived component can be recycled in reactors to fission it into shorter-lived elements. If this works out, it appears possible to shorten the required period of isolation to a few thousand years, less than the lifetime of the pyramids, man-made structures that have lasted a very long time. It is too early to say if this will work out or to say what impact it might have on the cost of nuclear power.
It is worth noting that at least in the United States nuclear power cost includes an allowance for disposition of the spent fuel. A charge of 0.1 cents per kilowatt-hour is added to the price of nuclear electricity and that money goes to the government for the eventual disposition of waste. The fund is now $20 billion. It is not clear if that is the correct charge, but it is interesting to note that no such charge is imposed on any of the carbon-based power sources. If one were imposed, there would be a great deal of money to use for conservation, efficiency and carbon-free alternatives.
It is worth noting that there is no equivalent charge on energy for disposing of the C[O.sub.2] from hydrocarbon power systems. There has been talk of C[O.sub.2] sequestration; pumping it deep underground for long-term storage. There is some experience with this technique from the use of C[O.sub.2] injection for enhanced oil recovery from wells. Based on this, there are estimates that range from one to four cents per kilowatt-hour to sequester the C[O.sub.2] from a coal-fired power plant.
CONCLUSIONS AND RECOMMENDATIONS
It has been said by some anthropologists that our behavior today is conditioned by our experiences hundreds of thousands of years ago when human ancestors were primitive, few in number, and scattered. Then one worried about the tiger behind the next tree, not about the one miles away. If so, I want you to know that the climate change tiger is behind the next tree.
Energy use is going to inexorably increase, driven by the needs of the developing world. These countries will not and should not live in energy poverty while the industrialized nations live in energy plenty.
It must be recognized that scientists and engineers have no quick fix for the problem of global warming because of the long residence time of greenhouse gases in the atmosphere.
A goal has to be set for the allowable increase in carbon dioxide and other greenhouse gases, for without a goal there can be no coherent program. Politicians will want the problem to go away. Economists will want the goal set as high as possible so that the cost of mitigation is minimized. Scientists will want the goal set as low as possible to minimize the consequences. A consensus has to be reached to define a long-term program and, whatever plan is decided upon, it should be designed to minimize the cost of achieving the desired results.
A concerted effort to drive down energy intensity through conservation and efficiency should be the first element of any such plan. Conservation and efficiency may even save money in the long run, and it is obvious that energy not used is carbon-free and non-polluting. Emissions trading and reforestation should be practiced to the maximum possible extent. It is much more economical to introduce new efficient power plants, for example, in places where demand increases, than to shut down and replace a less efficient plant that still has considerable useful life left.
Nuclear power should be expanded. Concerns about radiation and waste storage have been blown out of all proportion. Environmentalists in countries like Belgium, Germany and Sweden, where nuclear power is to be phased out, should be prepared to tell their neighbors why more years of their lives should be lost by being non-nuclear. Some people consider nuclear power to be the devil himself; but I would ask them which devil would they prefer to live with, nuclear power or climate change?
Research and development on renewables, fuel cells, hydrogen systems, and so forth, should be expanded. They do have a role to play and, while they are not ready now for large-scale deployment, they should be readied with all deliberate speed.
Effective action in a market-based world can only be accomplished with aware governments that will put in place sensible policies involving both incentives and penalties. It is time to stop posturing. The United States government does not seem to have any awareness of the problem and too many nations in Europe, through their positions on emissions trading and reforestation, seem more interested in making sinners suffer than in solving the problem.
Action is required. Surely the economists and politicians can design an incentive system that would encourage entrepreneurial innovation. To a physicist it is easy; a carbon tax would do the job quite effectively, discouraging the most polluting fuels and encouraging the least polluting. A waste disposal charge, like that imposed on nuclear power, would do well also and could raise considerable money.
In conclusion, please remember this: global warming will be our grandchildren's problem, but they are too young to do anything about it.
TABLE 1: Removal Time and Percent Contribution to Climate
Agent Rough Removal Time Approximate
Carbon Dioxide >100 years 60%
Methane 10 years 25%
Tropospheric 50 days 20%
Nitrous Oxide 100 years 5%
Fluorocarbons >1000 years <1%
Sulfate Aerosols 10 days -25%
Black Carbon 10 days 15%
TABLE 2: Comparison of GDP (trillions of constant U.S. $) and Per
Capita GDP in Years 2000 and 2100 (thousands of constant U.S.$
per person) (IIASA Scenario B)
GDP GDP/ GDP GDP/
Industrialized 20.3 22.2 71 70.5
Reforming 0.8 1.8 16 27.4
Developing 5.1 1.1 116 11.5
WORLD 26.2 4.2 202 17.3
TABLE 3: Energy Intensity (Watt-yr/dollar) (IIASA Scenario B)
Watt-year/dollar 2000 2050 2100
Industrialized 0.3 0.18 0.11
Reforming 2.26 0.78 0.29
Developing 1.08 0.59 0.30
WORLD 0.52 0.36 0.23
TABLE 4: Summary Table
Item 2000 2050 2100
Primary Power (Terawatts) 14 27 40
Population (Billions) 6.2 10 11.6
Energy Intensity 0.52 0.36 0.23
1. IIASA "Scenario B" (middle growth)
2. U.N. Population Projection (middle scenario)
3. A 1% per year decline in energy intensity is assumed
TABLE 5: Requirements for 2050 (assuming a scenario aimed at
stabilizing atmospheric C[O.sub.2] at 450 ppm)
Source of Primary Power 2000 2050
Carbon Based 11 TW 7 TW
Carbon Free 3 TW 20 TW
TABLE 6: Radiation Exposures
Source Radiation Dose
Natural Radioactivity 240
Natural in Body (75kg) * 40
Medical (average) 60
Nuclear Plant (1GW electric) 0.004
Coal Plant (1GW electric) 0.003
Chernobyl Accident (Austria 1988) 24
Chernobyl Accident (Austria 1996) 7
* Included in the Natural Total
TABLE 7: Public Health Impacts per TWh (Krewitt et al.,
Risk Analysis, Vol. 18, No. 4, 1998)
Coal Lignite Oil Gas Nuclear
Years of life lost 138 167 359 42 9.1
Years of life lost
(A) Normal 16
(B) Accidents 0.015
Respiratory 0.69 0.72 1.8 0.21 0.05
Cerebrovascular 1.7 1.8 4.4 0.51 0.11
Congestive 0.8 0.84 2.1 0.24 0.05
Restricted 4751 4976 12248 1446 314
Days with 1303 1365 3361 397 86
Cough days in 1492 1562 3846 454 98
Respiratory 693 726 1786 211 45
Chronic 115 135 333 39 11
Chronic cough 148 174 428 51 14
Nonfatal cancer 2.4
Years of life lost 58 2.7
Years of life lost
Respiratory 0.29 0.01
Cerebrovascular 0.7 0.03
Congestive 0.33 0.02
Restricted 1977 90
Days with 543 25
Cough days in 621 28
Respiratory 288 13
Chronic 54 2.4
Chronic cough 69 3.2
TABLE 8: The Spent Fuel Problem
Component Fission Uranium Long-Lived
Percent of total 4 95 1
Radio-activity Intense Negligible Medium
Untreated required 200 0 300,000
Hoffert, Martin I. et. al. (1998) "Energy implications of future stabilization of atmospheric C[O.sub.2] content." Nature 395, 881-4.
Intergovernmental Panel on Climate Change (2001) Third Assessment Report--Climate Change 2001: The Scientific Basis. Summary for Policy Makers.
Krewitt, Wolfram et. al. (1998) "Health Risks of Energy Systems," Risk Analysis 18, No. 4, pp. 377-384.
National Research Council (2001) Climate Change Science, An Analysis of Some Key Questions, National Academy Press, ISBN 0-309-07574-2.
Streets, David G. et. al. (2001) "Recent reductions in China's greenhouse gas emissions," Science 294, pp. 1835-7, Nov. 30.
U.S. Global Change Research Program, National Assessment Synthesis Team (2000) Climate Change Impacts on the United States; ISBN 0521000742.
TABLE AND FIGURE CREDITS
Table 1: Climate Change Science, An Analysis of Some Key Questions, National Academy Press, ISBN 0-309-07574-2, p 3 (2001).
Table 2: International Institute of Applied Systems Analysis, Vienna, Austria, 2001 (Scenario B).
Table 3: International Institute of Applied Systems Analysis, Vienna, Austria, 2001 (Scenario B).
Table 5: For this and other scenarios see, for example, M. I. Hoffert, et. al., Nature 395,881 (1998).
Table 7: Krewitt, et. al., Risk Analysis 18, No. 4 (1998).
Figure 1: View of the earth from Apollo 17, December 7, 1972, NASA.
Figure 2: National Assessment Synthesis Team," Climate Change Impacts on the United States; U.S. Global Change Research Program; ISBN 0521000742 (2000).
Figure 3: Intergovernmental Panel on Climate Change, Third Assessment Report--Climate Change 2001: The Scientific Basis. Summary for Policy Makers, p 11.
Figure 4: Intergovernmental Panel on Climate Change, Third Assessment Report--Climate Change 2001: The Scientific Basis, Synthesis Report, Q-9.
Figure 5: United Nations' World Population Projections to 2150; 1998 (Medium Scenario).
Figure 6: International Institute of Applied Systems Analysis, Vienna, Austria, 2001 (Scenario B).
Figure 8: D. G. Streets, et. al., Science 294, p 294, Nov. 30, 2001.
Paul Pigott Professor in the Physical Sciences, Stanford University
Director Emeritus, Stanford Linear Accelerator Center