A carbon footprint has historically been defined as "the total set of greenhouse gas (GHG) emissions caused by an organization, event, product or person. Greenhouse gases can be emitted through transport, land clearance, and the production and consumption of food, fuels, manufactured goods, materials, wood, roads, buildings, and services. For simplicity of reporting, it is often expressed in terms of the amount of carbon dioxide, or its equivalent of other GHGs, emitted.

However, carbon footprints are much specific than ecological footprints since they measure direct emissions of gasses that cause climate change into the atmosphere.

The carbon footprint is a very powerful tool to understand the impact of personal behaviour on global warming. Most people are shocked when they see the amount of CO2 their activities create.

A carbon footprint is a measure of carbon dioxide emissions associated with an entity's activities. A carbon footprint includes direct emissions, such as from driving a car, as well as whatever emissions are required to consume any goods and services. Often, a carbon footprint includes the measure of other greenhouse gas emissions as well. The United States, with only 4 percent of the world's population, contributes 25 percent of the world's greenhouse gases. The average American produces about 20 tons of carbon dioxide each year. A large carbon footprint has detrimental effects on the environment.

Greenhouse Gases

The main greenhouse gases resulting from human activity are: carbon dioxide (CO2), methane and nitrous oxide. In modern times, the concentrations of these gases have increased dramatically. Their effect is to contribute to the warming of the Earth’s atmosphere by trapping more heat from the Sun.

The increase in CO2 is mostly from fossil fuels, used in transport, energy generation and manufacture. Deforestation is also a contributor to emissions (18% of the total). A further consequence of deforestation is that it reduces the Earth’s capacity to capture carbon.

Methane is the by-product of agricultural activity, waste management and natural gas production. Methane concentrations have actually decreased in the last decades. Nitrous oxide is emitted through the use of fertilisers and fossil fuel burning. It is also released naturally in the oceans.

Water vapour is an abundant and important greenhouse gas. It is not the cause of global warming but it is affected by it. Human activity has an impact on the water cycle through global warming. It affects weather patterns and the movement of water vapour in the atmosphere.

The impact of greenhouse gases is in the disruption of the balance between the incoming radiation received from the Sun and the heat dissipated into outer space. Each greenhouse gas has a different capacity to retain extra heat and therefore impacts differently on global warming.

CO2 has the greatest effect, greater than all other greenhouse gases combined. The total of all contributions of greenhouse gases and aerosols results in a steady accumulation of excess heat in the atmosphere.

Effects of Carbon Footprint

Greenhouse Gas Emissions

Electricity generation and transportation-related activities account for well over half of the 14 percent increase in greenhouse gas emissions in the United States from 1990 to 2008. The Federal Transit Administration estimates that switching to public transportation instead of driving would allow the average American to reduce his or her carbon footprint by 10 percent. Americans could also reduce their collective carbon footprint by changing their incandescent bulbs to compact fluorescent lights, preventing the emission of 9 billion pounds of greenhouse gases.

Climate Change

Climate change is the ultimate effect of large carbon footprints. Greenhouse gases, whether natural or human-produced, contribute to the warming of the planet. From 1990 to 2005, carbon dioxide emissions increased by 31 percent. By 2008, the emissions had contributed to a 35 percent increase in radiative warming, or a shift in Earth's energy balance toward warming, over 1990 levels. The decade from 2000 to 2009 was the warmest decade on record worldwide, according to the U.S. Environmental Protection Agency's Climate Change Indicators Report.

Depletion of Resources

Large carbon footprints deplete resources on large and small scales, from a country's deforestation activities to one home's increased use of air conditioning. The more those with large carbon footprints use resources, the more greenhouse gases increase and spur further climate change. The Environmental Protection Agency suggests that consideration of different energy supplies and conservation of current ones will be needed to balance energy demand. Reducing carbon dioxide emissions as much as possible and off-setting the remaining emissions by planting trees, for example, or supporting alternative energy efforts, will help to reduce the negative effects of carbon footprints.

Measuring Carbon Footprints

An individual's, nation's, or organisation's carbon footprint can be measured by undertaking a GHG emissions assessment. Once the size of a carbon footprint is known, a strategy can be devised to reduce it, e.g. by technological developments, better process and product management, changed Green Public or Private Procurement (GPP), carbon capture, consumption strategies, and others. The mitigation of carbon footprints through the development of alternative projects, such as solar or wind energy or reforestation, represents one way of reducing a carbon footprint and is often known as Carbon offsetting.

The main influences on carbon footprints include population, economic output, and energy and carbon intensity of the economy. These factors are the main targets of individuals and businesses in order to decrease carbon footprints. Scholars suggest the most effective way to decrease a carbon footprint is to either decrease the amount of energy needed for production or to decrease the dependence on carbon emitting fuels.