What are greenhouse gases?

Greenhouse gases accelerate atmospheric warming and can persist for long periods of time. The major greenhouse gases are: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydroflourocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆). Most of these gases occur naturally in the environment, but at much lower concentrations than current levels. The sources of these gases that are associated with human activity include fossil-fuel combustion, land-use conversion (deforestation), industrial processes, fertilizers, refrigerants, and landfills.

Carbon dioxide, methane, and nitrous oxide are continuously emitted to and removed from the atmosphere by natural processes on Earth. Human activities, however, cause additional quantities of these gases to be emitted, thereby changing their atmospheric concentrations. Biological activities such as respiration by plants and animals, and seasonal cycles of plant growth and decay are examples of processes that cycle carbon and nitrogen between the atmosphere and animal and plant life. Such processes generally do not alter atmospheric greenhouse gas concentrations over decadal timeframes.

Carbon Dioxide (CO₂)

Carbon dioxide is the most prevalent human influenced greenhouse gas in the atmosphere, and accounts for about 85% of total greenhouse gas emissions. In nature, carbon is cycled between various atmospheric, oceanic, land, marine, and mineral reservoirs. The largest fluxes of CO₂ occur between the atmosphere and the biosphere (plants and animals), and between the atmosphere and surface water of the oceans. "Concentrations of carbon dioxide have already risen from a pre-industrial level of 280 ppm to around 379 ppm in 2005" (IPCC 2007). The IPCC states that "the amounts of carbon dioxide… now in the atmosphere, far exceed pre-industrial values going back 650,000 years" and "the present atmospheric CO₂ increase is caused by human-caused emissions of CO₂" (IPCC 2007). Forest clearing, other biomass burning, and some non-energy production processes such as cement production, also emit notable quantities of carbon dioxide. Source: IPCC 2007. Climate Change 2007: The Physical Science Basis.

Methane (CH₄)

Methane is the second-most prevalent greenhouse gas, accounting for 8% of U.S. emissions. By far, the largest natural source of methane is wetlands, accounting for 76% of natural methane emissions (IPCC 2001). The largest source of methane emissions due to human activity is from landfills, closely followed by enteric fermentation by ruminant livestock (EPA 2005). In landfills, methane is produced through anaerobic (i.e., without oxygen) decomposition of organic waste. Methane is also emitted during the production and distribution of natural gas and petroleum, and is released as a by-product of coal mining and incomplete fossil fuel combustion. "Methane concentrations have risen from 715 parts per billion (ppb) to 1,774 in 2005" (IPCC 2007), an increase of 150%. Source: IPCC Climate Change 2001: The Scientific Basis US Emissions Inventory 2005: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 IPCC 2007. Climate Change 2007: The Physical Science Basis.

Nitrous Oxide (N₂O)

Nitrous oxide is the third-most prevalent greenhouse gas emitted in the U.S. and accounts for about 5% of total emissions. "Primary human-related sources of N₂O are agricultural soil management, animal manure management, sewage treatment, mobile and stationary combustion of fossil fuel, adipic acid production, and nitric acid production. Nitrous oxide is also produced naturally from a wide variety of biological sources in soil and water, particularly microbial action in wet tropical forests" (EPA). The atmospheric concentration of nitrous oxide (N₂O) has increased by 16 percent since 1750, a concentration that has not been exceeded during the last thousand years (IPCC 2001). Source: IPCC Climate Change 2001: The Scientific Basis http://www.epa.gov/nitrousoxide/sources.html

Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride (SF₆)

Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆) are powerful greenhouse gases accounting for about 2% of all greenhouse gas emissions. HFCs, primarily used as replacements for ozone depleting substances like Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), currently have a small global impact; however, it is anticipated that their emissions will increase in the future. PFCs and SF₆ are predominantly emitted from various industrial processes including aluminum smelting, semiconductor manufacturing, electric power transmission and distribution, and magnesium casting. Currently, the global impact of PFCs and SF₆ is also small; however, they have a significant growth rate, extremely long atmospheric lifetimes, and are strong absorbers of infrared radiation, and therefore have the potential to influence climate far into the future (IPCC 2001). Source: IPCC Climate Change 2001: The Scientific Basis http://www.epa.gov/highgwp/sources.html

GHGs and Global Warming Potential

Because GHGs vary in their ability to trap heat in the atmosphere, some are more harmful to the climate than others. Each GHG has a "global warming potential" (GWP), which refers to its heat-trapping ability relative to that of CO₂. For example, methane is 21 times more potent than CO₂, so methane has a GWP of 21. The GWPs shown in the table to the right are 100-year GWPs, referring to their effect relative to CO₂ over a 100-year timeframe.

Source: Intergovernmental Panel on Climate Change, Climate Change 1995: The Science of Climate Change (Cambridge, UK: Cambridge University Press, 1996).

How did we determine that a tree sequesters a ton of carbon over its lifetime?

The calculations to make the determination that a typical tree sequesters about a ton of carbon in its lifetime were done using UFORE (the Urban Forest Effects model) and GROWOUT, computer programs developed by US Forest Service researchers that predict future conditions in an urban forest. UFORE compiles information from a city's urban forest inventory and combines that information with local, hourly weather and air pollution concentration data. The output that UFORE provides to the user is in a format that can be easily input into GROWOUT. GROWOUT then performs further calculations to provide the user with predictions about the conditions and value of their future urban forest. Outputs from GROWOUT include information such as total population of trees by year, percent tree cover over a designated area such as the city boundaries, and carbon storage by trees and its value. In this case, the model output indicates that in good conditions, 729 kg, or 1607 lbs. of carbon are stored, and 226 kg or 498 lbs. of carbon emissions are avoided by one tree over a typical 57 year life span of an urban tree.