Atmospheric Concentrations of Greenhouse Gases

Putting everybody wise to the risks on Earth.

The Earth’s temperature depends mainly on the amount of energy received from the sun, the portion reflected back into space, and the extent to which the atmosphere retains heat. Natural forces (e.g., volcanoes and changes in the Earth’s orbit) and human activities (e.g., emissions of so-called “greenhouse gases ” [GHGs] and land use change) affect the amount of energy held in the Earth-atmosphere system and therefore affect the Earth’s climate. Human activities in all countries have altered the chemical composition of the atmosphere by the emissions and accumulation in the atmosphere of GHGs. The primary gases that retain heat in the atmosphere are water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and certain manufactured gases such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and sulfur hexafluoride (SF₆).

Once emitted, gases remain in the atmosphere for varying amounts of time. Very “short-lived” compounds, such as particulate matter (PM), remain airborne on average for only hours or days. CH₄ also has a relatively short average lifetime, though much longer than PM, remaining in the atmosphere for roughly 12 years. The half-life of CO₂ emissions is roughly 100 years (5 to 200 years: IPCC, 2001), but about a quarter of emissions today will still be in the atmosphere after hundreds of years and about one-tenth for hundreds of thousands of years (Archer and Ganopolski, 2005; Archer et al., 1998). Finally, many of the synthetic gases such as halocarbons (or gases that contain the halogens chlorine, fluorine, bromine, or iodine) are extremely long-lived, remaining in the atmosphere for hundreds or even tens of thousands of years. When emissions—from the U.S. (the U.S. Greenhouse Gas Emissions indicator) as well as other countries—remain in the atmosphere over long periods, they accumulate and are measured as atmospheric concentrations. U.S. GHG emissions from 1890 to 2000 are estimated to have contributed about one-fifth of the increase in global GHG concentrations (den Elzen et al., 2005).

(from http://www.epa.gov/)