Greenhouses gases are substances high above the earth that allow the sun's light to pass through but then lock it in and reflect heat back toward the planet. Many greenhouse gases are natural, while others are man-made, according to the National Oceanic and Atmospheric Administration (NOAA) website.
Methane, which comes from natural gas, landfills and cow flatulence, is one of the top three greenhouse gases, according to the Popular Science website. Methane has a Global Warming Potential (GWP) rating of 21. GWP is determined by how much heat a gas molecule traps, "preventing infrared radiation from escaping the planet," noted Slate magazine writer Brendan I. Koerner.
Carbon dioxide (CO2), which comes mainly from burning fossils fuels, is one of the top three greenhouse gases. Though not as high on the GWP scale as methane, CO2 accounts for the majority of greenhouse gas emissions in the United States -- 85 percent, noted Koerner. Vehicles are the biggest contributor.
Surprisingly, the number-one greenhouse gas is steam, or water vapor, which Popular Science says accounts for 36 to 70 percent of greenhouse gases. The water droplets are dense, keeping the earth's radiation from escaping. The warmer it gets, the more water vapor is created and the more radiation absorption takes place. NOAA refers to this as a "positive feedback loop."
Other Greenhouse Gases
Rounding out the top 10 greenhouse gases, as reported by Popular Science, are sulfuryl fluoride, sulfur hexafluoride, hexafluoroethane, trichlorofluoromethane, trifluoromethane, ozone and nitrous oxide.