When matter absorbs energy, the atoms and molecules that make up the material become excited; they move around more quickly. The increased movement raises the material’s temperature. But atoms and molecules on Earth are not just absorbing sunlight, they are also radiating thermal infrared energy (heat). Excess carbon dioxide and other greenhouse gases force the Earth’s energy budget out of balance by absorbing thermal infrared energy (heat) radiated by the surface. Greenhouse gases absorb thermal infrared energy (heat) with wavelengths in a part of the energy spectrum that other gases, such as nitrogen and oxygen, do not.

The major naturally occurring greenhouse gases are water vapor, carbon dioxide, methane, and nitrous oxide. They act somewhat like glass in a greenhouse or a type of blanket that insulates the Earth. However, they are not in a “layer” but are distributed throughout the troposphere.

Because greenhouse gas molecules radiate heat in mall directions, some heat spreads downward and ultimately comes back into contact with the Earth’s surface, where it is absorbed. The temperature of the surface becomes warmer than it would be if it were heated only by direct solar heating. This supplemental heating of the Earth’s surface by the atmosphere is the natural greenhouse effect. The natural greenhouse effect raises the Earth’s surface temperature to about 15 degrees Celsius on average—more than 30 degrees Celsius warmer than it would be if it didn’t have an atmosphere.

Note: The Greenhouse Effect is a natural phenomenon somewhat different from a greenhouse. A greenhouse works primarily by preventing warm air (warmed by incoming solar radiation) close to the ground from rising. Greenhouse gases are not in a “layer” but are dispersed throughout the troposphere and there is no “lid” on the troposphere. The atmospheric Greenhouse Effect works by preventing heat loss to space.

Decaying organic materials in oceans, swamps, and bogs release greenhouse gases like methane (CH4) and carbon dioxide (CO2). Natural sources of CH4 include fires, geologic processes, and bacteria that produce CH4 in a variety of settings (most notably, wetlands). Cows even release methane. A source of nitrous oxide (N2O) is through bacterial production. These sources produced through life processes are called biogenic, and they release greenhouse gases that are part of the natural chemistry of Earth and its atmosphere. Some natural sources of emissions could have dramatically altered Earth’s atmosphere at different times in the past. Volcanoes release tremendous amounts of gases and particles into the air. Volcanoes have been known to change weather patterns for years after erupting. Since little can be done to control natural emissions, the focus is on controlling human sources.

Human sources of greenhouse gases tend to concentrate in urban areas where people live and work. Many of these gases come from the burning of coal, wood, oil and other fuels for electricity, transportation, and heat. Carbon dioxide, methane, nitrous oxide, and halocarbons are the four main gases associated with climate change.

While much attention is currently focused on anthropogenic sources of greenhouse gases, there is ample evidence that emissions of these gases from natural sources have also changed over time. Human activities have significant potential to change emissions from these sources, both directly (e.g., decreased methane from wetlands, due to wetland loss from draining and filling) or indirectly through human-induced climate change (e.g., increased methane emissions from wetlands due to rising temperature, or from wildfires that are more frequent and severe).

In order to address greenhouse gas emissions it is important not only to quantify the current magnitude of natural sources, but also to understand how human activities and climate change affect emissions from these sources.