Greenhouse gases (GHGs) are gases that trap heat in the atmosphere. The main ones, says the U.S. EPA (, are:

- Carbon dioxide (CO2) - Enters the atmosphere through burning fossil fuels (coal, natural gas and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement).

- Methane (CH4) - Emitted during the production and transport of coal, natural gas and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.

- Nitrous oxide (N2O) - Emitted during agricultural and industrial activities, as well as during combustion of fossil fuels and solid waste.

- Fluorinated gases - Hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons and halons).

These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential (GWP) gases.



Each GHG has a different GWP and persists for a different length of time in the atmosphere.

The EPA says certain GHGs are more effective at warming the than others. The two most important characteristics of a GHG in terms of climate impact are how well the gas absorbs energy (preventing it from immediately escaping to space), and how long the gas stays in the atmosphere.

The GWP for a gas is a measure of the total energy that a gas absorbs over a particular period of time (usually 100 years), compared to carbon dioxide, the agency explains. The larger the GWP, the more warming the gas causes.

In the U.S., heavy trucks account for about 21 percent of transportation-related greenhouse gas emissions, say officials at Red Dot, a company that designs and manufactures heavy duty mobile HVAC systems and components ( Light duty vehicles account for around 28 percent; passenger cars about 33 percent.

These figures include direct emissions from fossil fuel combustion, as well as hydrofluorocarbon emissions from mobile air conditioners, they note.