For example, when you drive a car, each gallon of gasoline you burn produces carbon in the form of carbon dioxide. Depending on the fuel efficiency of your vehicle and the miles traveled, a gasoline-powered car can easily generate its own weight in carbon dioxide each year. The average American is responsible for about 20 tons of carbon dioxide emissions each year, a far greater per capita number than that of any other industrialized country. In fact, the US accounts for more than 20% of the world's total greenhouse gas emissions. You can reduce your carbon footprint by driving a more efficient car, or driving less. You can also plant trees or help preserve forests to offset your emissions, since trees are a sink for carbon.

The carbon footprint calculator estimates CO2 emissions for energy use and transportation, and for organizations paper use, because these types of activities are responsible for a significant percentage of U.S. emissions, and are measurable based on readily available information. Your total carbon footprint would account for the energy used to produce all the products and services you consume, as well as all your other activities, and would be substantially larger. Home energy use and transportation represent approximately 40% of all U.S. emissions, so for an average person the emissions from these two activities would have to be multiplied by 2.5 to determine the person's total carbon footprint.

Guide to Carbon Dioxide and other Greenhouse Gases

Carbon dioxide (CO2) is not the only man-made greenhouse gas - it is simply the one that has accumulated the most in the atmosphere and is presently having the greatest cumulative warming effect on our planet. Human sources of carbon dioxide primarily include the burning of fossil fuels (coal, oil, and natural gas), and deforestation. The amount of carbon dioxide in the atmosphere has increased 30% since pre-industrial times.

SafeClimate typically quotes greenhouse gas units in terms of carbon dioxide (1lb carbon dioxide = 0.2729 lbs of carbon), as well as converting other greenhouse gases into units of carbon dioxide based on their relative global warming potentials. This standardized approach simplifies things and makes for easier and more meaningful comparisons.

Other greenhouse gases, produced by human activities, include:

· Methane (CH4), emitted by agriculture, ranching, landfills, and energy exploration. Human activities have increased the concentration of methane in the atmosphere by about 145%.

· Nitrous Oxide (N2O), produced by various agricultural and industrial practices, including the use of nitrogen fertilizers, nylon production, and the burning of organic material and fossil fuels. Human activities have increased the level of nitrous oxide in the atmosphere by about 15% above natural levels.

· Tropospheric ozone (O3), ozone in the lower part of the atmosphere, created by the reaction of sunlight with human-produced pollutants from vehicles and power plants. Tropospheric ozone has probably doubled in the Northern Hemisphere since pre-industrial times.

· Chlorofluorocarbons (CFCs), and other halocarbons such as perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and hydrofluorocarbons (HFCs) -- chemicals used in refrigeration, air conditioning, and other industrial processes. The production of chlorofluorocarbons is rapidly being eliminated because of their destructive effect on the ozone layer. Yet other halocarbons, such as HFCs, are now being produced as substitutes, many of which are also greenhouse gases. PFCs are used in semiconductor manufacturing and are a byproduct of aluminum smelting. SF6 is an insulating gas used in the transmission of electricity. SF6 is the most potent greenhouse gas ever measured, roughly twenty-five thousand times more powerful than carbon dioxide.

All of these gases, aside from the halocarbons, are also produced by natural causes - but it is their rapid build-up in the atmosphere over the past few centuries, due to human activities, that is now causing global warming.

The Ecological Footprint

There are many other ways to visualize our individual and overall human impact on the environment. Some environmental and government groups feature a broader concept than the carbon footprint -- the ecological footprint, which is an estimate of how much land and water is needed to produce all the resources an individual consumes, and dispose of all the waste and pollution he or she generates. Because of increasing population and levels of consumption and pollution, human beings are leaving bigger and bigger ecological footprints - at a rate that is increasingly harmful to the planet.

For example, Redefining Progress estimates that the typical American uses 25 acres to support his or her lifestyle, almost five times more than is sustainable. This non-profit group provides tools to calculate your own ecological footprint, and links to many other such calculators. More information on ecological footprints is provided by Sustainable USA and the British group, Best Foot Forward.