A carbon footprint is the measure of the amount of greenhouse gases, measured in units of carbon dioxide, produced by human activities. A carbon footprint can be measured for an individual or an organization, and is typically given in tons of CO2-equivalent (CO2-eq) per year. For example, the average North American generates about 20 tons of CO2-eq each year. The global average carbon footprint is about 4 tons of CO2-eq per year (Figure 1).
Primary and secondary footprints
An individual’s or organization’s carbon footprint can be broken down into the primary and secondary footprints. The primary footprint is the sum of direct emissions of greenhouse gases from the burning of fossil fuels for energy consumption and transportation. More fuel-efficient cars have a smaller primary footprint, as do energy-efficient light bulbs in your home or office. Worldwide, 82% of anthropogenic greenhouse gas emissions are in the form of CO2 from fossil fuel combustion (Figure 2).
The secondary footprint is the sum of indirect emissions of greenhouse gases during the lifecycle of products used by an individual or organization. For example, the greenhouse gases emitted during the production of plastic for water bottles, as well as the energy used to transport the water, contributes to the secondary carbon footprint. Products with more packaging will generally have a larger secondary footprint than products with a minimal amount of packaging.
Greenhouse gases and the greenhouse effect
Although carbon footprints are reported in annual tons of CO2 emissions, they actually are a measure of total greenhouse gas emissions. A greenhouse gas is any gas that traps heat in the atmosphere through the greenhouse effect. Because of the presence of greenhouse gases in our atmosphere the average temperature of the Earth is 14 ºC (57 ºF). Without the greenhouse effect, the average temperature of the atmosphere would be -19 ºC (-2.2 ºF).
Many greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and water, occur naturally. Other greenhouse gases, such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are synthetic. Since the beginning of the Industrial Revolution, atmospheric concentrations of greenhouse gases, both natural and man-made, have been increasing. Burning fossil fuels and land-use changes such as deforestation interfere with the natural carbon cycle, moving carbon from its solid form to the gaseous state, thus increasing atmospheric concentrations of carbon dioxide.
Carbon dioxide equivalent
Each greenhouse gas—carbon dioxide, methane, CFCs, etc.—has a different atmospheric concentration, and a different strength as a greenhouse gas. A potent greenhouse gas with a very small atmospheric concentration can contribute to the overall greenhouse effect just as much as a weaker greenhouse gas with a much larger atmospheric concentration. Because of this variability, carbon footprints are measured in tons of CO2-eq, or the tons of CO2 that would cause the same level of radiative forcing as the emissions of a given greenhouse gas.
Individual carbon footprints
An individual’s carbon footprint is the direct effect their actions have on the environment in terms of greenhouse gas emissions. In general, the biggest contributors to the carbon footprints of individuals in industrialized nations are transportation and household electricity use. An individual's secondary carbon footprint is dominated by their diet, clothes, and personal products (Figure 3).
Worldwide, the fossil fuels used for transportation contribute to over 13% of greenhouse gas emissions (Figure 4). Cars with an average fuel efficiency produce nearly 20 pounds of CO2-eq for every gallon of gasoline burned.
Air transportation has a larger carbon footprint than driving. The average round-trip flight across the U.S. emits about 6,000 pounds of CO2-eq, and short-haul flights emit more CO2-eq per mile traveled than medium- to long-haul flights.
Home heating and cooling
In the U.S., 20% of greenhouse gas emissions come from home energy use. Heating and cooling usually consume more energy than any other home appliances. The relative contributions of heating and cooling to an individual’s carbon footprint vary by region. In colder states, as much as two-thirds of a household’s energy bill is from heating. Heating an average American home with natural gas or electricity produces a carbon footprint of 6,400 or 4,700 pounds CO2-eq, respectively. In warmer areas, summertime air conditioning constitutes the bulk of a household's energy bill. Air conditioning a typical home produces a carbon footprint of about 6,600 pounds CO2-eq.
Worldwide, agriculture contributes to nearly 14% of total greenhouse gas emissions. In the U.S., the food we eat accounts for 17% of our total fossil fuel consumption. The carbon footprint of an average American diet is 0.75 tons CO2-eq, without accounting for food transportation. On average, food travels 1,500 miles between the production location and the market. Meat products have a larger carbon footprint than fruits, vegetables, and grains: the carbon footprint of the average meat eater is about 1.5 tons CO2-eq larger than that of a vegetarian.
Offsets and emissions trading
There are many ways for individuals and organizations to reduce their carbon footprint, such as driving less, using energy efficient appliances, and buying local, organic foods as well as products with less packaging. The purchase of carbon offsets is another way to reduce a carbon footprint. One carbon offset represents the reduction of one ton of CO2-eq. Companies who sell carbon offsets invest in projects such as renewable energy research, agricultural and landfill gas capture, and tree-planting.
Critics of carbon offsets argue they will be used to absolve any guilt over maintaining “business as usual” in our lifestyles. Additionally, the current offset market is voluntary and largely unregulated, raising the possibility that companies will defraud customers seeking to reduce their carbon footprint.
Emissions trading schemes provide a financial incentive for organizations and corporations to reduce their carbon footprint. Such schemes exist under cap-and-trade systems, where the total carbon emissions for a particular country, region, or sector are capped at a certain value, and organizations are issued permits to emit a fraction of the total emissions. Organizations that emit less carbon than their emission target can then sell their “excess” carbon emissions. This market mechanism is expected to bring down the costs of meeting emissions targets.
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