5 Notorious Greenhouse Gases

Water vapor is the most potent of the greenhouse gases in Earth’s atmosphere, and it’s sort of a unique player among the greenhouse gases. The amount of water vapor in the atmosphere cannot, in general, be directly modified by human behavior—it’s set by air temperatures. The warmer the surface, the greater the evaporation rate of water from the surface. As a result, increased evaporation leads to a greater concentration of water vapor in the lower atmosphere capable of absorbing infrared radiation and emitting it downward.

Of the greenhouse gases, carbon dioxide (CO₂) is the most prominent. Sources of atmospheric CO₂ include volcanoes, the combustion and decay of organic matter, respiration by aerobic (oxygen-using) organisms, and the burning of fossil fuels, clearing of land, and production of cement by humans. These sources are balanced, on average, by a set of physical, chemical, or biological processes, called "sinks," that tend to remove CO₂ from the atmosphere. Plant life, which takes up CO₂ during the process of photosynthesis, is an important natural sink. In the oceans, marine life can absorb dissolved CO₂, and some marine organisms even use CO₂ to build skeletons and other structures made of calcium carbonate (CaCO₃).

Methane (CH₄) is the second most important greenhouse gas. It is more potent than CO₂, but exists in far lower concentrations in the atmosphere. CH₄ also hangs around in the atmosphere for a shorter time than CO₂—the residence time for CH₄ is roughly 10 years, compared with hundreds of years for CO₂. Natural sources of methane include many wetlands, methane-oxidizing bacteria that feed on organic material consumed by termites, volcanoes, seepage vents of the seafloor in regions rich with organic sediment, and methane hydrates trapped along the continental shelves of the oceans and in polar permafrost. The primary natural sink for methane is the atmosphere itself; another natural sink is soil, where methane is oxidized by bacteria.
As with CO₂, human activity is increasing the CH₄ concentration faster than it can be offset by natural sinks. Human sources (rice cultivation, livestock farming, the burning of coal and natural gas, biomass combustion, and decomposition in landfills) currently account for approximately 70 percent of total annual emissions, leading to substantial increases in concentration over time.

The next most significant greenhouse gas is surface, or low-level, ozone (O₃). Surface O₃ is a result of air pollution; it must be distinguished from naturally occurring stratospheric O₃, which has a very different role in the planetary radiation balance. The primary natural source of surface O₃ is the subsidence of stratospheric O₃ from the upper atmosphere toward Earth’s surface. In contrast, the primary human-driven source of surface O₃ is in photochemical reactions involving carbon monoxide (CO), such as in smog.

Additional trace gases produced by industrial activity that have greenhouse properties include nitrous oxide (N₂O) and fluorinated gases (halocarbons). The latter includes sulfur hexafluoride, hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). Nitrous oxides have small background concentrations due to natural biological reactions in soil and water, whereas the fluorinated gases owe their existence almost entirely to industrial sources.