What Are Greenhouse Gases?	Behind the struggle to address global warming and climate change lies the increase in greenhouse gases in our atmosphere.	A greenhouse gas is any gaseous compound in the atmosphere that is capable of absorbing infrared radiation thereby trapping and holding heat in the atmosphere.	By increasing the heat in the atmosphere greenhouse gases are responsible for the greenhouse effect which ultimately leads to global warming.    	The sun bombards Earth with enormous amounts of radiation which strike Earth's atmosphere in the form of visible light plus ultraviolet (UV) infrared (IR) and other types of radiation that are invisible to the human eye.	About 30 percent of the radiation striking the Earth is reflected back out to space by clouds ice and other reflective surfaces. The remaining 70 percent is absorbed by the oceans the land and the atmosphere according to NASA.	As they absorb radiation and heat up the oceans land and atmosphere release heat in the form of IR thermal radiation which passes out of the atmosphere into space. The balance between incoming and outgoing radiation keeps Earth's overall average temperature at about 59 F (15 C).	This exchange of incoming and outgoing radiation that warms Earth is often referred to as the greenhouse effect because a greenhouse works in much the same way.	Incoming UV radiation easily passes through the glass walls of a greenhouse and is absorbed by the plants and hard surfaces inside. Weaker IR radiation however has difficulty passing out through the glass walls and is trapped inside warming the greenhouse.	The gases in the atmosphere that absorb radiation are known as greenhouse gases (sometimes abbreviated as GHG) because they are largely responsible for the greenhouse effect. The greenhouse effect in turn is one of the leading causes of global warming.	The most significant greenhouse gases are water vapor (H2O) carbon dioxide (CO2) methane (CH4) and nitrous oxide (N2O) according to the Environmental Protection Agency (EPA).	Fluorinated gases including hydrofluorocarbons perfluorocarbons and sulfur hexafluoride are created during industrial processes and are also considered greenhouse gases. Though they are present in very small concentrations they trap heat very effectively making them high global-warming potential (GWP) gases.	Chlorofluorocarbons (CFCs) once used as refrigerants and aerosol propellants until they were phased out by international agreement are also greenhouse gases.	Three factors affect the degree to which any greenhouse gas will influence global warming:	Carbon dioxide has a significant impact on global warming partly because of its abundance in the atmosphere: In 2011 CO2 composed 84 percent of U.S. greenhouse emissions according to the EPA. Additionally CO2 stays in the atmosphere for thousands of years.	However methane is about 21 times more efficient at absorbing radiation than CO2 giving it a high GWP rating even though it stays in the atmosphere only about 10 years.	Some greenhouse gases like methane are produced through agricultural practices including livestock manure management. Others like CO2 largely result from natural processes like respiration and from the burning of fossil fuels like coal oil and gas.	Since the beginning of the Industrial Revolution in the early 1800s humans have been burning fossils fuels at an ever-increasing rate. In the United States like most other industrialized nations burning fossil fuels is the single greatest manmade source of greenhouse gases.	According to the EPA the production of electricity is the source of 33 percent of U.S. greenhouse gas emissions. Over 70 percent of U.S. electricity comes from plants that burn fossil fuels usually coal and natural gas.	Transportation is a close second contributing about 28 percent of U.S. greenhouse gas emissions. Almost all cars trucks ships trains and airplanes run on gasoline or diesel fuels.	Manufacturing and other industries contribute about 20 percent of U.S. greenhouse gas emissions followed by residential and commercial sources (11 percent) and agriculture (8 percent).	It's worth noting that forestry and other land-use practices offset some of these greenhouse gas emissions according to the EPA. Because trees and other plants absorb CO2 from the atmosphere they reduce our overall greenhouse gas emissions by roughly 14 percent.	Worldwide however the output of greenhouse gases is a source of grave concern: From the time the Industrial Revolution began to the year 2009 atmospheric CO2 levels have increased almost 38 percent and methane levels have increased a whopping 148 percent according to NASA  � and most of that increase has been in the past 50 years.	If these trends continue scientists government officials and a growing number of citizens fear that the worst effects of global warming  � extreme weather rising sea levels plant and animal extinctions ocean acidification major shifts in climate and unprecedented social upheaval  � will be inevitable.	Follow Marc Lallanilla on Twitter and Google+. Follow us @livescience Facebook & Google+. 	For the latest information on greenhouse gases visit: