Ruler of the World

Petroleum is currently the largest source of energy worldwide. It is used extensively in transportation as gasoline for cars; as diesel fuel for trucks, buses, trains and ships; and as airline fuel for planes and jets. Oil is also burned for heating.

Petroleum is a non-renewable, fossil fuel. Burning petroleum releases carbon dioxide, a greenhouse gas (GHG), into the atmosphere. In terms of GHG emissions per kilowatt-hour (kWh) of energy generated, petroleum is "cleaner" than coal (about 25% lower emissions) but "dirtier" than natural gas (36% higher emissions).

Just over one-third (35.4 %) of the power consumed globally is generated by burning petroleum. In the United States, 40% of the energy consumed comes from petroleum. [World and U.S. percentages are for 2006]

Petroleum is the largest source of energy worldwide. Burning petroleum (as heating oil, gasoline, airline fuel, and so on) generates more than one-third (36.5 %) of our power. We consume 29.7 billion barrels of oil each year. That amount of oil contains energy equivalent to 50.5 petawatt hours of electricity. [2006 values]

In terms of carbon dioxide emissions, petroleum is a more efficient fuel than coal but is less efficient than natural gas. Burning petroleum produces about 250 kilograms of carbon dioxide for each megawatt-hour (MWh) of electricity produced. Natural gas combustion creates 180 kg of CO₂ emissions per MWh (about 30% less than petroleum), while burning coal generates roughly 330 kg/MWh of CO₂ (about 30% more than oil).

As this is written in September 2009, oil costs about $70 per barrel. A barrel (42 gallons) of crude oil can be used to generate about 5.8 million Btu's of energy. This works out to about 4 cents per kWh.

The cost of oil is constantly changing. Within one six-month span between July 2008 and January 2009, the price of oil rose above $135 per barrel and later dropped below $35 per barrel!

We use oil for energy in various ways. Some people burn fuel oil for heating. A relatively small amount of the electricity generated worldwide comes from oil-fired power plants. Most petroleum is used in transportation, as gasoline for cars, diesel fuel for trucks and buses, and aviation fuel for jet planes. The cost, energy content, and carbon emissions are different for these various forms of petroleum.

When burned directly for heating, oil costs about 3.2 cents per kilowatt-hour. Electricity generated by an oil-fired plant costs about 4.8 cents per kWh. Most petroleum is used in transportation, however. At $2.50 per gallon, gasoline costs the equivalent of 6.9 cents per kWh. Diesel fuel at $2.70 per gallon is roughly equal to 6.6 cents per kWh.

Various air pollutants are released when oil is burned. Exhaust gases from vehicles powered by gasoline and diesel fuels are a major source of air pollution in urban areas. Oils spills from oil tanker ships or pipelines can cause severe damage to ecosystems and wildlife, especially when they occur along coasts or in Arctic environments where natural degradation occurs slowly due to the cold temperatures. Burning petroleum creates less CO₂ emissions (per kWh) than coal but more than natural gas.

People like to drive cars. People don't like the price of gas to rise. People don't like the pollution from burning fossil fuels: air pollution, greenhouse gas emissions, oil spills. Don't like dependence on foreign sources or supplies from unstable regions or countries that are not allies.

Coal is a non-renewable fossil fuel. Coal is a type of sedimentary/metamorphic rock that is mined from the ground. Coal is burned in vast quantities worldwide to produce electricity. Burning coal emits significant quantities of carbon dioxide, a greenhouse gas, into the atmosphere.

Use of coal can create several environmental hazards, including: damage from mining (especially strip mining), acid rain, various types of air pollution, and high concentrations of hazardous materials (mercury, traces of radioactive materials, etc.) in the ash left over when coal is burned. Coal is a relatively inexpensive source of energy at the present time (the average worldwide cost of electricity generated by burning coal is about 2 cents per kilowatt-hour). Coal emits more carbon dioxide per kWh of energy generated than does oil; and much more than burning natural gas.

Coal is the world's second largest source of energy. Coal is burned to generate electricity and for heating. About one-quarter (27%) of the world's energy comes from coal. The energy content of all the coal we burn is enough to generate 37.4 petawatt hours of electricity. [2006 values]

Burning coal emits significant quantities of carbon dioxide, a greenhouse gas, into the atmosphere. Coal emits more carbon dioxide per kWh of energy generated than does oil; and much more than burning natural gas.

Burning coal emits about 330 kilograms of carbon dioxide for each megawatt-hour of electricity produced. Petroleum emits about 250 kg/MWh of CO₂, while natural gas is relatively "clean" at 180 kg of emissions per MWh.

Different grades of coal generate different levels of emissions. Anthracite coal gives off about 350 kg of CO₂ per MWh, while bituminous coal has lower emissions at 320 kg/MWh.

Coal is a very popular energy source worldwide due to broad availability and low cost. Electricity generated by burning coal costs about 2 cents per kilowatt-hour. When burned directly for heating, coal can cost as little as 1 to 1.5 cents per kilowatt-hour-equvalent of heat energy produced. These costs are lower than other fossil fuels, and much lower than most alternative and renewable fuel sources.

The price of coal delivered to power plants in the USA in 2006 was around $1.70 per million Btu (units commonly used in the USA) of energy content. The price of coal has fluctuated less over time than the price of oil or natural gas. However, as shown in the graph, the price of coal has been rising... by 2008 it had climbed to $2.07 per million Btu (22% higher than the 2006 price).

Coal mining, especially strip mining, can have severe environmental impacts. Burning coal releases various air pollutants, including sulfur compounds which produce acid rain. Modern coal-fired power plants with expensive "scrubbers" vastly reduce emissions of most pollutants (but not carbon dioxide emissions!); however, many coal plants in developing nations and older coal-fired plants do not have such "scrubbers" and continue to emit lots of air pollutants. Solid wastes left over from burning coal contain elevated concentrations of heavy metals (such as mercury) and moderately radioactive isotopes. Underground fires in many coal mines and coal seams can burn for years, emitting pollutants without generating any useful energy.

People like to have cheap electricity. Strip mines, acid rain, and greenhouse gas emissions are less popular..

Natural Gas is a fossil fuel. It is burned in furnaces to heat buildings, used for cooking and heating water, burned in electrical power plants, and occasionally used as a fuel for vehicles.

Natural gas ranks third among worldwide energy sources, just behind coal. Natural gas is burned for heating and cooking and to generate electricity. Almost one-quarter (22.9%) of the world's energy comes from natural gas. We consume 104 trillion cubic feet of natural gas each year. That amount of gas contains energy equivalent to 31.7 petawatt hours of electricity. [2006 values]

Natural gas is a fossil fuel which we burn to produce energy. When it is burned, natural gas emits carbon dioxide. However, natural gas is a much "cleaner" fuel than the other major fossil fuels (petroleum and coal) in terms of CO₂ emissions.

For each megawatt-hour of energy produced, natural gas emits 181 kilograms of CO₂ into the atmosphere. That's about 20-30% less CO₂ emissions than from various petroleum products (fuel oil, gasoline, etc.) and about 35-45% less emissions than various types of coal.

Natural gas is essentially methane, which is also a very potent greenhouse gas. Some natural gas escapes into the atmosphere (during transport, etc.) before it is burned as fuel. This leakage slightly increases the overall greenhouse gas emissions associated with the use of natural gas. You may want to adjust your settings for the CO₂ emissions from natural gas to account for this methane leakage.

Natural gas costs about $14 per thousand cubic feet sold to residential customers. That works out to about 4.7 cents per kilowatt-hour of energy produced. When used to generate electricity (instead of for heating), natural gas costs about 3.7 cents per kWh of electricity delivered to customers.

The price of natural gas had remained quite steady for many years. As shown in the graph, prices throughout most of the 1980s and 1990s hovered around $6 per thousand cubic feet. However, the cost of natural gas rose sharply in the past decade as prices more than doubled during that time.

Combustion of natural gas releases carbon dioxide into the atmosphere. However, petroleum and coal both emit quite a bit more CO2 when burned than natural gas does. Some air pollutants are also released when natural gas is burned, but the scale of this pollution is also much smaller than that caused by burning oil or coal.

Fairly minor amounts of pollutants are created in the process of refining natural gas after it is removed from the ground. Some natural gas inevitably leaks out into the atmosphere during handling and transport. Since natural gas is essentially methane, a very potent greenhouse gas, this leakage contributes somewhat to global warming concerns.

The infrastructure used to extract and transport natural gas can be disruptive to environments. Natural gas drilling rigs, as well as the roads and other facilities needed to support them, can disturb natural environments and recreational opportunities in the vicinity. Pipelines used to transport natural gas can also disturb environments they traverse.

Most people do not have a strong opinion either way about the use of natural gas. Natural gas is "cleaner" in terms of overall pollution and carbon dioxide emissions than the other fossil fuels (petroleum and coal). This gains it some support. Sometimes natural gas is a bit more expensive than other fossil fuels, making it less popular.

Gas can be used for heating, cooking, and electricity generation. This flexibility is an advantage. Because of the low density of natural gas per kWh of energy it contains, natural gas is challenging to store and transport. Although natural gas is used in some areas as a fuel for vehicles, natural gas is not generally as well suited for transportation uses as are petroleum products. These limitations detract from the popularity of natural gas.

Some people dislike the environmental impact of wells used to extract natural gas from the ground. Different nations and regions have unequal supplies of natural gas. Some people disapprove of reliance on any fuel source that comes from another country, especially if the region is politically unstable or has an adversarial relationship with the country needing to import fuel.

Nuclear power plants produce electricity by using the heat generated by "splitting atoms" (nuclear fission reactions). Nuclear power plants do not emit greenhouse gases since they do not burn fossil fuels. Nuclear plants do generate radioactive wastes which present long-term storage, disposal, and security concerns. Slightly less than 6% of the energy consumed worldwide is generated by nuclear power; about 8% of U.S. energy consumption is supplied by nuclear plants (2006 figures).

Nuclear power ranks well behind the three major fossil fuels (petroleum, coal, and natural gas) in terms of the amount of the world's energy demands it supplies. Nuclear power produces roughly the same amount of energy as does hydroelectric power. Less than one-sixteenth (5.9%) of the world's energy comes from nuclear power. Nuclear reactors are used to produce about 8.1 petawatt hours of electricity each year worldwide. [2006 values]

Nuclear power does not directly cause carbon dioxide emissions.

Indirectly, construction of nuclear power plants generates some CO₂ emissions. Diesel-powered trucks and construction equipment burn petroleum, cement manufacturing releases CO₂ from limestone, and so on. This also can be said to some extent for the infrastructure behind all of our power sources, be they windmills or dams with hydroelectric power plants or conventional coal-fired plants.

You may set the CO₂ emissions for nuclear power to zero if you wish to represent just the ongoing emissions from this power source. Or, you may choose to set it to some small value to represent the "overhead" energy (and thus carbon dioxide emission) costs associated with the construction of nuclear power plants. Whichever you choose, you should be consistent across energy sources (hydroelectric, solar, wind, etc.).

The cost of generating electricity using nuclear power ranges from about 1 to 12 cents per kWh.

The main concern associated with nuclear power is the storage, transport, and disposal of radioactive wastes. Fuel rods, typically made with radioactive uranium, must be disposed of when their remaining uranium content falls below a certain level. Other equipment and materials used in nuclear reactors acquire low levels of radioactivity after prolonged exposure to the fuel rods and must also be disposed of at some point. Some types of radioactivity take very long times to dissipate (decades to centuries or longer), so disposal of such substances is unlike any waste handling problem previously encountered by humans.

Some people believe that we can safely bury radioactive wastes in special containers. Those people think the risks involved in such disposal are small compared to the advantages of nuclear power. Other people fear that radioactive wastes are likely to leak out of containers over the vast time spans required for radioactivity levels to fall to safe values. They are concerned that the wastes could get into aquifers and other natural features, contaminating them for very long times.

Nuclear power does not release air pollutants (including greenhouse gases), which makes it "cleaner" than burning fossil fuels such as petroleum, natural gas, and coal. As is the case with most other types of large power plants used to generate electricity, nuclear plants must use large amounts of water for cooling. This can cause environmental problems by straining local water supplies and/or by altering the natural temperature of aquatic environments.

People tend to have strong opinions, either pro or con, regarding nuclear power.

Supporters feel that the dangers associated with disposal of radioactive wastes are small. They highlight the advantages in reduced pollution and reduced carbon dioxide emissions associated with nuclear power as compared to burning fossil fuels. They also assert that fuel supplies for nuclear power are large compared with dwindling sources of fossil fuels. Some claim that nuclear power is the only viable means for some countries to achieve energy independence.

Opponents of nuclear power are generally distrustful of claims that radioactive wastes can be stored and disposed of safely. They claim that time periods associated with radioactive decay are so long that we simply cannot predict what might eventually become of waste products. Some also cite security concerns. If stolen, enriched uranium could be used in a weapon by a terrorist group or a "rogue state". The availability of enriched uranium would be minimal if it was not produced for use in nuclear plants.

Hydroelectric power is a source of renewable energy that is generated by harnessing the power of moving water. Most hydroelectric power is generated at dams that temporarily trap water flowing downhill in rivers. Water flows through turbine generators, producing electricity as gravity pulls the water downwards. Some hydroelectric power is produced in the oceans by converting the motion of waves into electricity.

Hydroelectric power is by far the largest source of renewable energy currently in use around the world. Electrical power is generated by moving water, either ocean waves or river water collected behind dams. Although it is the largest current source of renewable energy, only about one-sixteenth (6.2%) of the world's energy comes from hydroelectric power. That is slightly more than is generated by nuclear power. "Water power" is used to produce about 8.7 petawatt hours of electricity each year worldwide. [2006 values]

Much more of our power comes from fossil fuels than from hydroelectric power. Natural gas generates more than 3 times as much energy as does hydroelectric power, while coal produces more than 4 times as much. Petroleum, our largest energy source, generates almost 6 times as much energy as does hydroelectric power.

Hydroelectric power does not directly cause carbon dioxide emissions.

Indirectly, construction of dams and hydroelectric power plants generates some CO₂ emissions. Diesel-powered trucks and construction equipment burn petroleum, cement manufacturing releases CO₂ from limestone, and so on. This also can be said to some extent for the infrastructure behind all of our power sources, be they windmills or nuclear power plants or conventional coal-fired plants.

Alteration of land and water use associated with hydroelectric power could alter the carbon cycle over some area, but it would be difficult to say whether that would cause a net increase in CO₂ emissions.

You may set the CO₂ emissions for hydroelectric power to zero if you wish to represent just the ongoing emissions from this power source. Or, you may choose to set it to some small value to represent the "overhead" energy (and thus carbon dioxide emission) costs associated with construction of dams and hydroelectric power plants. Whichever you choose, you should be consistent across energy sources (nuclear, solar, wind, etc.).

The cost of generating electricity using hydroelectric power ranges from about 2 to 10 cents per kWh.

Most hydroelectric power plants are located next to dams, which allow the operators to control the water level and flow to the generating turbines. Dams disrupt the natural flow of rivers and sometimes flood large areas. They can have negative aesthetic as well as natural impacts, such as the disruption of fish migrations and the alteration of distribution of river-carried sediments.

Hydroelectric power does not produce any ongoing emissions of pollutants, in contrast to the pollution generated by burning fossil fuels.

Hydroelectric power from ocean waves and currents is a relatively new field, and a variety of approaches are being tested. It isn't yet clear what the overall impact of these facilities will be on seashore aesthetics, ocean life, and marine environments.

Most people like the fact that ongoing operations at hydroelectric power plants are essentially non-polluting, including in terms of carbon dioxide emissions.

People like seashores for recreation. If ocean hydroelectric power plants disrupt that, people might be unhappy.

Some people like the recreational opportunities provided by the reservoirs above dams.

Some people are very strongly opposed to the ecological disruption caused by dams and the areas they flood.

The term "biofuels" encompasses a diverse collection of fuel sources that are derived from living ("bio") creatures. Wood is the most common "biofuel", and is still used for cooking and heating homes throughout much of the world. Ethanol, a type of alcohol, is derived from plants (such as sugarcane and corn) and is used as a fuel for vehicles. Vegetable oil from plants or recycled after use in cooking can be burned in diesel engines. Sawdust, charcoal, and gases (mostly methane) emitted by decaying material in landfills are also common biofuels.

Some of the major biofuels are used in such a decentralized way that it is challenging to estimate how much they are being used throughout the world. For example, wood is gathered for heating and cooking in many places around the world, but it is not easy to estimate how much of this sort of activity is going on.

Some experts estimate that about 4.2% of the the world's energy comes from biofuels, while other place the value closer to 2%. Others claim that biofuels supply up to 15% of the world's total energy and as much as 35% in developing countries.

In 2007, about 3.7% of the energy consumed in the USA came from biofuels.

Burning biofuels generates CO₂ emissions to an extent that is similar to emissions generated by burning fossil fuels. The level of emissions depends on the mix of fuels used, since "biofuels" is an umbrella term that covers a range of fuel types. If you choose to adjust the CO₂ emissions setting, you should consider which biofuel or biofuels you wish to have your subjects use.

Burning wood emits almost as much CO₂ as does burning coal, the worst of the major fossil fuels in terms of CO₂ emissions. Wood emits roughly 300 kg of carbon dioxide for each megawatt of energy produced. Use of wood, if not a waste product like sawdust, brings additional problems associated with greenhouse gases; forests, which absorb CO₂ from the air (removing it from the atmosphere), are often cut down to supply wood for fuel.

Biogas, which is mostly methane collected from landfills (where microbes that "digest" trash give off the gas), does emit carbon dioxide when burned. However, methane is also a greenhouse gas, and a very potent one at that. Releasing it directly into the atmosphere would probably produce even more of a greenhouse effect than would the carbon dioxide generated by burning the biogas.

Biomass used to generate electricity ranges in cost from about 3 to 12 cents per kWh.

Some biofuels are comparable in price to petroleum, while other can cost up to 3 or 4 times as much.

The term "biofuel" actually covers a range of diverse power sources. Not surprisingly, the environmental impacts of these sources vary.

Wood is still widely used in many places, typically by poor people who have little access to other energy sources. Burning wood produces CO₂ emissions and emits other air pollutants. Deforestation to gather wood for fuel brings with it severe erosion, habitat destruction, and a host of other environmental problems. Charcoal is also widely used as a fuel; its environmental impacts are essentially the same as those involving wood.

Ethanol is used extensively in fuels for vehicles. When burned, ethanol emits CO₂ and other air pollutants. Some ethanol is made from crops (particularly corn) that could also be used as food for humans and/or livestock. Since modern agriculture tends to be energy-intensive, it is unclear whether ethanol produced from food crops actually results in a net gain in usable energy or not. Land use for growing any crops for use in biofuels has impacts on the environment.

Some biofuels are produced from waste products. These include sawdust, gases (mostly methane) emitted by decaying material in landfills, and some types of biodiesel. Since these fuels are burned, they do emit carbon dioxide and other air pollutants. On the other hand, they do make use of materials that would otherwise be waste. This can be a mixed bag. Methane is a very potent greenhouse gas, so burning it (even though that emits CO₂, which is also a greenhouse gas) contributes less to global warming than does allowing it to escape into the atmosphere. Sawdust, on the other hand, does not generate greenhouse gas emissions until it is burned; this means that in some respects we are better off with sawdust as a waste material.

Biofuels are renewable energy sources, which is a plus in the eyes of most folks. Some people like the fact that biofuels can be used to reduce dependence on fossil fuel imports from politically unstable or unfriendly countries.

Since use of wood as a fuel generates severe negative environmental impacts (including deforestation) in various ways, most people support efforts to replace the use of wood as a fuel with cleaner alternatives. However, such efforts generally require money, so support is sometimes limited by financial concerns.

Some biofuels turn waste products into useful energy sources; most people wholeheartedly support this. Biofuels do tend to emit carbon dioxide when burned, so biofuels are not necessarily a help in efforts to curtail global warming. Some biofuels are produced from crops that could be used as food instead. Some people find this objectionable in light of continuing global struggles with hunger and malnutrition. Some sources of biofuels (particularly ethanol produced from food crops) may merely transform an energy source into another form without generating a net gain in usable energy.

The world harvests about 433 terawatt-hours (a bit over 0.4 petawatt-hours) of power from the wind each year. Wind power accounts for roughly 0.3% of the world's energy supply.

If you choose to have your subjects use more wind power, you should consider how much wind power could be scaled up from its present level. We clearly could harvest much more energy from the wind; however, it is probably unreasonable to assume that a very large portion of our total energy use could be generated by wind power alone.

The world harvests about 433 terawatt-hours (a bit over 0.4 petawatt-hours) of power from the wind each year. Wind power accounts for roughly 0.3% of the world's energy supply.

If you choose to have your subjects use more wind power, you should consider how much wind power could be scaled up from its present level. We clearly could harvest much more energy from the wind; however, it is probably unreasonable to assume that a very large portion of our total energy use could be generated by wind power alone.

Wind power does not directly cause carbon dioxide emissions.

Indirectly, construction of wind turbines generates some CO₂ emissions. Diesel-powered trucks and construction equipment burn petroleum, manufacturing metal turbine blades requires energy, and so on. This also can be said to some extent for the infrastructure behind all of our power sources, be they dams for hydroelectric facilities or nuclear power plants or conventional coal-fired plants.

Alteration of land use associated with hydroelectric power could alter the natural carbon cycle over some area, but it would be difficult to say whether that would cause a net increase in CO₂ emissions.

You may set the CO₂ emissions for wind power to zero if you wish to represent just the ongoing emissions from this power source. Or, you may choose to set it to some small value to represent the "overhead" energy (and thus carbon dioxide emission) costs associated with manufacturing and construction of wind turbines. Whichever you choose, you should be consistent across energy sources (nuclear, solar, hydroelectric, etc.).

The cost of generating electricity from wind energy ranges from about 4 to 9 cents per kWh.

Some people consider wind turbines, especially when clustered in wind farms, an eyesore that damages the visual appeal of the landscape. Some other people don't mind, or even like the look of wind turbines which remind them of progressive, "green" energy policies.

Some people are concerned about the deaths of birds and bats that collide with the blades of wind turbines. This could be especially bad for migrating flocks of birds. Though experts agree that some birds and bats are killed by wind turbines, different studies reach different conclusions about the severity of this problem.

Wind turbines can also be noisy, which could interfere with or disturb nearby recreational uses of land.

Ongoing generation of wind power does not emit greenhouse gases or other air pollutants, making it far cleaner in this regard than any of the fossil fuel sources. Wind power does not generate radioactive wastes, a potential advantage over nuclear power.

Some people do not like the visual, aesthetic impact that large clusters of wind turbines have on the landscape. Noise generated by spinning wind turbines can disturb neighbors. However, wind farms are often located in places where there are few people... including offshore, on land that is simultaneously used for agriculture, and so on.

Some people like the look of wind turbines, which can remind them of societal progress towards more use of renewable energy sources that don't emit greenhouse gases or other pollutants. Some people like "home grown" energy sources that reduce reliance of fossil fuel imports from politically unstable or unfriendly countries.

There is also some concern about bird deaths caused by collisions with wind turbines, especially migratory flocks. Wind turbines can also be noisy, which can make neighbors of wind power facilities unhappy.

Solar energy is used for heating and to generate electricity from sunlight. Energy from the Sun's rays is used directly to heat water and to warm buildings. Most of the energy (more than 90%) we get from sunlight is used directly for heating. A much smaller fraction of solar power is used to generate electricity. We can only collect solar power during the daytime in places where it is sunny. Because of this limitation, solar power must either be stored (in batteries, as heat, by pumping water uphill, etc.) or its use supplemented with other power supplies that are available when sunlight is not.

Large, centralized solar power plants generate electricity either from banks of photovoltaic (PV) solar panels or via a concentrating solar power (CSP) collector, which uses mirrors to concentrate sunlight and generate electricity by heating a fluid. Solar water heaters and photovoltaic panels, mounted on rooftops of residences and other buildings, are also used as a distributed source of power.

Solar power currently makes up only a small fraction of our energy supply, about 0.5%. Worldwide, we produce about 747 terrawatt-hours (0.75 petawatt-hours) of energy via solar power.

The vast majority of our solar power generation is used for heating, including heating water. Currently, only a small fraction of solar energy we harvest is used to generate electricity.

The use of solar power is growing very rapidly. Use of solar power has been growing by about 35% per year during the past few years.

Solar power does not directly cause carbon dioxide emissions.

Indirectly, manufacture and construction of solar power generating equipment and facilities causes some CO₂ emissions. Industrial processes used to manufacture solar panels require power. Diesel-fueled trucks haul materials to constructions sites. And so on! This also can be said to some extent for the infrastructure behind all of our power sources, be they windmills or nuclear power plants or conventional coal-fired plants.

Alteration of land use associated with solar power may alter the natural carbon cycle over some area, but it is difficult to say whether that causes a net increase in CO₂ emissions.

You may set the CO₂ emissions for solar power to zero if you wish to represent just the ongoing emissions from this power source. Or, you may choose to set it to some small value to represent the "overhead" energy (and thus carbon dioxide emission) costs associated with manufacture and construction of solar power collecting equipment and facilities. Whichever you choose, you should be consistent across energy sources (nuclear, hydroelectric, wind, etc.).

The cost of generating electricity using photovoltaic panels to collect solar energy ranges from about 25 cents to $1.60 per kWh.

Using a concentrating solar power (CSP) system, the cost ranges from 12 to 45 cents per kWh.

Solar power has very minor negative environmental impacts compared to most other energy sources.

Some people consider large clusters of solar panels an eyesore that damages the visual appeal of the landscape. Some other people don't mind, or even like the look of solar power collectors, which remind them of progressive, "green" energy policies. If the use of solar power was scaled up considerably, there would have to be many more solar energy collectors. Placement of such a large number of collectors could become an issue.

Concentrating solar power (CSP) collectors use mirrors to focus sunlight on a boiler. Birds that fly through concentrated beams of light can be injured or killed by the heat. It is not clear how large of a problem this actually is.

Ongoing generation of solar power does not emit greenhouse gases or other air pollutants, making it far cleaner in this regard than any of the fossil fuel sources of energy. Unlike nuclear power, solar power does not generate radioactive wastes.

Most people have a generally favorable view towards solar power, although its relatively high cost can dampen this enthusiasm. Most people like the renewable nature of this power source and view it as a progressive, "green", futuristic option.

If we scale up the use of solar power substantially, we will need many more solar panels or other solar power generating facilities. Some people are concerned that this increase may be visually unappealing or that it may somehow impact land use in a negative way.

Some people like the fact that solar power can be "home grown" in most locations, reducing reliance on fossil fuel imports from politically unstable or unfriendly countries. Some people like the fact that some solar power can be generated in a de-centralized, distributed way by installing photovoltaic solar panels and solar water heaters on residences.

Some applications of solar power, especially residential installations, may bring with them minor complications (dealing with batteries, DC electricity, or meters for selling power to the grid) that make them slightly more cumbersome to use than simply flicking on a switch. The unconstant nature (no power at night, reduced power on cloudy days) of solar power also detracts somewhat from its appeal, often requiring that the use of solar power be supplemented with other, more constant energy sources.

Earth is hot inside. Geothermal power taps this heat to provide us with energy, both directly as heat and indirectly to generate electricity. Roughly 60-75% of geothermal power is used directly for heating, while 25-40% is used for electricity generation.

In the past, only high-temperature geothermal sources in the vicinity of tectonic plate boundaries (think hot springs and geysers) could be used effectively. Modern technology is making low-temperature geothermal energy practical. These newer systems typically run water or some other fluid through a geothermal heat pump to transfer heat from the ground into buildings. Some of these systems can also be used for cooling in summertime if the air is warmer than the ground.

Geothermal power currently makes up only a small fraction of our energy supply, just a bit over one-tenth of one percent. Worldwide, we produce about 188 terrawatt-hours (0.2 petawatt-hours) of energy via geothermal power.

New technologies are expanding the locations where geothermal power is viable. Some researchers believe use of geothermal power could increase sevenfold using current technologies (to ~1.3 pWh per year), or by a factor of 14 with foreseeable technological advances (to ~2.6 pWh per year).

Geothermal power does cause some carbon dioxide emissions. Typically, hot fluids that are drawn from the ground in the process of generating power release some carbon dioxide into the air.

Carbon dioxide emissions from geothermal power are much lower than the emissions generated by burning fossil fuels such as petroleum, coal, and natural gas. Coal, the highest in CO₂ emissions among the fossil fuels, produces about 10 times more CO₂ per kilowatt-hour of energy generated than does geothermal power. Natural gas, the "cleanest" of the fossil fuels, emits about 6 times more CO₂ per kilowatt-hour than does geothermal power.

When used to generate electricity, geothermal power emits about 24-33 kg of carbon for each megawatt-hour of electricity produced. Geothermal systems that are used directly for heating (instead of electricity generation) have even smaller carbon footprints. Carbon emissions from geothermal power vary based on the type of system used, whether the system generates electricity or uses heat directly, and the temperature of the ground beneath the geothermal installation.

The cost of generating geothermal energy ranges from about 4 to 10 cents per kWh.

The negative negative environmental impacts of geothermal power are quite minor compared to most fossil fuel-based energy sources. There is little, if any, pollution associated with geothermal power and the greenhouse gas emissions from geothermal sources are much smaller than those from burning fossil fuels (though some types of geothermal power production do emit far more CO2 than solar, wind, hydroelectric and nuclear power). Geothermal power is a renewable energy source.

Some hot geothermal energy sources can also be used recreationally. People like to bathe in hot springs and to gaze in awe at geyser eruptions (e.g. Yellowstone National Park in Wyoming, USA). Use of such features for energy could disrupt their suitability for recreational use; for instance, by disrupting the flow of hot water to geysers. However, there are many geothermal features that are not suitable for recreational use that can be tapped for energy without conflicting with other uses. Also, it is sometimes possible to use geothermal resources both recreationally and to generate power without one use disrupting the other (e.g. swimming pools heated from geothermal sources).

Some groups have attempted to enhance natural geothermal systems, for example by pumping cool water into hot areas underground and later extracting the hot water. Such efforts have the potential of disrupting natural system. In some cases minor seismic activity (very small earthquakes) has been triggered this way. Though it is unlikely that major quakes could be set off this way, minor disturbances could "damage the plumbing" of underground aquifers, potentially damaging water supplies that people rely on.

Few people are aware of geothermal power as an energy source, so most folks don't have a strong opinion about it one way or the other. Since geothermal power is a renewable energy source that emits far fewer greenhouse gases or other pollutants than fossil fuel energy sources, it is generally viewed favorably by those who are familiar with it.

Geothermal power technologies that rely on high temperatures can only be used in certain locations. This limitation somewhat diminishes the appeal of geothermal power. However, newer geothermal heat pump technologies do not require high temperatures, and thus can be used in most locales. Some people like the fact that geothermal power reduces reliance on fossil fuel imports from politically unstable or unfriendly countries. Some people like the fact that some geothermal power can be generated in a de-centralized, distributed way by installing heat pumps on residences.

High-temperature geothermal power plants could compete with recreational users for access to geothermal energy sources in some instances. People like to use hot springs for bathing and like viewing geysers (e.g. Yellowstone National Park in Wyoming, USA). Sometimes these resources can be used both recreationally and to generate power without one use disrupting the other. Also, there are many geothermal features that are not suitable for recreational use that can be tapped for energy.

Scientists have determined that we need to prevent average global temperatures from rising more than 2° C if we are to avoid the worst impacts of global warming and worldwide climate change. To do that, we need to reduce our CO₂ emissions substantially for many years. There are many "paths" we could follow, in terms of our energy use (and carbon dioxide emissions) trends over the coming years, that would reduce emissions enough. Some involve drastic cuts soon (better for the environment, but harder to convince people to do), while others delay the reductions (easier to convince people to accept, but risky if we cannot actually follow through).

Scientists have created several scenarios that would lead us to the necessary cuts in carbon dioxide emission levels. A typical "middle of the road" scenario would require that total worldwide emissions of CO₂ be reduced to 30 gigatons (billions of tons) or less. About 70% of the human-caused increase in atmospheric greenhouse gases is attributed to energy use. Our target for CO₂ emissions from energy generation should, therefore, be about 21 gigatons of CO₂ (GtCO₂). Our current emissions (for energy generation) are around 32.9 GtCO₂. Your energy policy needs to reduce emissions by nearly 12 gigatons, or about 36%.

You may be familiar with a kilowatt-hour as a unit of energy; your electric bill probably describes your energy use in kilowatt-hours. A kilowatt is 1,000 watts (same idea as kilogram = 1,000 grams and kilometer = 1,000 meters). If you had a 1,000 watt light bulb (that would really light up a room!) and left it on for 1 hour, it would use 1 kilowatt-hour of energy ( 1,000 watts x 1 hour = 1,000 watt-hours = 1 kilowatt-hour). Here's a more realistic example: a 40 watt bulb left on all day and night would consume slightly less than one kilowatt-hour ( 40 watts x 24 hours = 960 watt-hours = 0.96 kilowatt-hours). The abbreviation for kilowatt-hours is kWh.

The best unit for expressing yearly global energy use is the petawatt-hour (abbreviated pWh). One petawatt-hour is equal to a trillion kilowatt-hours:

1 pWh = 1,000,000,000,000 kWh

In 2006, worldwide energy use was around 138.5 pWh. By 2009, it had risen to about 146.5 pWh. Sometimes it is useful to use the following "in-between" units:

• 1 megawatt-hour (MWh) = 1,000 kWh
• 1 gigawatt-hour (GWh) = 1,000,000 kWh
• 1 terrawatt-hour (tWh) = 1,000,000,000 kWh

A Btu (British thermal unit) is an English-system unit for measuring energy, primarily heat energy. One kilowatt-hour (kWh) is the equivalent of 3,412 Btu.

Petroleum is often described in terms of barrels of oil. A barrel of oil stores the equivalent of 1,700 kWh of chemical energy. Refineries typically produce 19-20 gallons of gasoline per barrel of oil. A gallon of gasoline contains the equivalent of 36 kWh of energy.

Coal delivery to power plants is often measured in "short" tons (tons in the English system of units, or 2,000 pounds; as opposed to a metric ton = 1,000 kg = 2,205 pounds). A short ton of coal contains about 5,911 kWh of energy.

Natural gas is measured in terms of its volume. In the USA, prices and energy content of natural gas are often quoted per 1,000 cubic feet. A thousand ft³ of gas contains about 301 kWh of energy. One cubic meter of gas has about 10.6 kWh. Natural gas quantities are sometimes quoted in "therms". One therm is 100,000 Btu (about 29 kWh), roughly the amount of energy in 100 cubic feet of gas.

Energy is lost, generally as waste heat, when we convert energy from one form to another. Although large, modern electrical power plants are remarkably efficient, only about a third of the chemical energy stored in fuels actually makes it to your home electrical outlet; the rest is lost when fuel is burned and as electricity flows over wires to residences and businesses. Likewise, roughly 30% of the energy contained in gasoline or diesel fuel is converted into mechanical energy which moves the vehicle; the rest is lost as heat. "Raw" petroleum is refined into many different products, from gasoline and diesel fuel to heating oil to aviation fuel. Each of these derived products has a dirrerent energy content and associated cost.

Different fuels have different levels of inherent energy content (most of this inherent energy content is lost as waste heat and so forth before we can use it). Here are some commonly encountered types of energy sources, as well as common units used for each, and the inherent amount of energy contained in each:

• 1 short ton coal = 5,911 kWh
• 1 barrel (42 gallons) crude oil = 1,700 kWh
• 1,000 cubic feet natural gas = 301 kWh
• 1 gallon heating oil or diesel fuel = 41 kWh
• 1 gallon gasoline = 36 kWh

It is often difficult to make "apples-to-apples" comparisons between energy sources. This is especially true when trying to compare the "true cost" of energy from various sources.

Construction of electrical power plants is expensive, often costing many millions of dollars. Although these facilities are often used for 20 years or longer, power plant construction and maintenance costs can represent a large portion of the cost of electricity generation. The cost of fuel is the other major cost associated with most, but not all, types electricity production. Fossil-fuel plants (mainly coal and natural gas) burn large amounts of fuel, as is readily apparent to anyone who's seen trainloads of coal deliveries lined up near a coal-fired plant. Nuclear power plants need radioactive fuel rods to produce power. Compared to fossil fuels, fuel rods for nuclear plants are delivered much less frequently but have a much higher cost. Many renewable energy sources (wind, solar, hydroelectric, geothermal) have ongoing fuel costs at or near zero. Power plant construction and maintenance costs can account for more than half of the cost of electricity from a coal-fired plant; about a quarter of the cost of electricity from natural gas; and upwards of 80% of the price of electricity from nuclear plants.

Infrastructure to support the transmission and delivery of various fuels can be costly. Electrical transmission wires crisscross the countryside; pipelines carry natural gas and oil over land; fleets of enormous oil tankers transport petroleum across oceans. Existing vehicles mostly have engines which burn gasoline or diesel fuel; conversion of these vehicles to battery power or ethanol-burning engines is costly.

Conversion of energy and fuels from one form to another often results in a net loss of energy and/or increase in price. Only about a third of the energy contained in coal, for example, makes it to your home electrical outlet; the other two thirds is lost as heat during generation or to resistance in transmission wires. Expensive refineries convert petroleum into gasoline, while automobile engines typically waste about 70% of the potential energy stored in the fuel.

Costs of fuels can fluctuate wildly over months and years. For example, a barrel of oil sold for less than $50 in early 2007; shot up to more than $130 per barrel in the summer of 2008 (an increase of more than 280%); then dropped back below $40 per barrel in early 2009 (a 75% price decrease from the 2008 peak).

Most energy sources include "hidden" costs and experts debate whether it is "fair" to include certain expenses when calculating the price of a given source of energy. Governments tax different energy sources in various ways. Should the price of cleanup from a giant tanker's oil spill be factored into the cost of petroleum? Which long-term storage and disposal methods are adequately safe for spent nuclear fuel rods, how much will that cost, and who decides what "safe enough" is? How much of the huge, ongoing military costs associated with warfare in the Middle East should be attributed to the region's oil supplies? Does our pricing scheme adequately reflect the costs of government subsidies for research and development of alternative energy sources? How do we put a dollar value on environmental damage caused by acid rain indirectly produced by coal-burning plants?

There are some negative environmental impacts associated with just about every source of energy. That's why most of the options for the settings you can select with the environmental impacts ( ) popup menus are negative (Slightly Bad, Somewhat Bad, or Very Bad) and only one is positive (Slightly Good).

The relative environmental impact of using solar power or wind power instead of burning coal or oil would indeed be positive - less air pollution, fewer coal mines, lower chances of oil spills, etc. However, even relatively "green" technologies tend to involve some amount of environmental damage - we still need to mine ores to produce the metal used to build wind turbines and supports for solar panels, we still must run electrical power transmission lines across the landscape, and so on.

You do have the option to set the environmental impact to "Slightly Good" if you wish. Perhaps you think that wind turbines actually enhance the look of a landscape, or that the recreational opportunities provided by a reservoir behind a dam outweight any negative impacts of damming a river. The choice is up to you, Your Highness!