A number of processes influence greenhouse gas concentrations. Some, such as tectonic activities (tectonics), operate at timescales of millions of years, whereas others, such as vegetation, soil, wetland, and ocean sources and sinks, operate at timescales of hundreds to thousands of years. Human activities—especially fossil-fuel (fossil fuel) combustion since the Industrial Revolution—are responsible for steady increases in atmospheric concentrations of various greenhouse gases, especially carbon dioxide, methane, ozone, and chlorofluorocarbons (chlorofluorocarbon) (CFCs).

The effect of each greenhouse gas on Earth's climate depends on its chemical nature and its relative concentration in Earth's atmosphere. Some gases have a high capacity for absorbing infrared radiation or occur in significant quantities, whereas others have considerably lower capacities for absorption or occur only in trace amounts. Radiative forcing, as defined by the Intergovernmental Panel on Climate Change (IPCC), is a measure of the influence a given greenhouse gas or other climatic factor (such as solar irradiance or albedo) has on the amount of downward-directed radiant energy impinging upon Earth's surface. To understand the relative influence of each greenhouse gas, so-called forcing values (which are displayed in watts (watt) per square metre) calculated for the time period between 1750 and the present day are given below.

A number of oceanic processes also act as carbon sinks. One such process, called the “solubility pump,” involves the descent of surface sea water containing dissolved CO₂. Another process, the “biological pump,” involves the uptake of dissolved CO₂ by marine vegetation and phytoplankton (small, free-floating, photosynthetic organisms) living in the upper ocean or by other marine organisms that use CO₂ to build skeletons and other structures made of calcium carbonate (CaCO₃). As these organisms expire and fall to the ocean floor, the carbon they contain is transported downward and eventually buried at depth. A long-term balance between these natural sources and sinks leads to the background, or natural, level of CO₂ in the atmosphere.

The natural background level of carbon dioxide varies on timescales of millions of years due to slow changes in outgassing through volcanic activity. For example, roughly 100 million years ago, during the Cretaceous Period, CO₂ concentrations appear to have been several times higher than today (perhaps close to 2,000 ppm). Over the past 700,000 years, CO₂ concentrations have varied over a far smaller range (between roughly 180 and 300 ppm) in association with the same Earth orbital effects linked to the coming and going of the ice ages (ice age) of the Pleistocene Epoch. By the early 21st century, CO₂ levels reached 384 ppm, which is approximately 37 percent above the natural background level of roughly 280 ppm that existed at the beginning of the Industrial Revolution. According to ice core measurements, this level (384 ppm) is believed to be the highest in at least 650,000 years.

Radiative forcing caused by carbon dioxide varies in an approximately logarithmic fashion with the concentration of that gas in the atmosphere. The logarithmic relationship occurs as the result of a saturation effect wherein it becomes increasingly difficult, as CO₂ concentrations increase, for additional CO₂ molecules to further influence the “infrared window” (a certain narrow band of wavelengths in the infrared region that is not absorbed by atmospheric gases). The logarithmic relationship predicts that the surface warming potential will rise by roughly the same amount for each doubling of CO₂ concentration. At current rates of fossil-fuel use, a doubling of CO₂ concentrations over preindustrial levels is expected to take place by the middle of the 21st century (when CO₂ concentrations are projected to reach 560 ppm). A doubling of CO₂ concentrations would represent an increase of roughly 4 watts per square metre of radiative forcing. Given typical estimates of “climate sensitivity” in the absence of any offsetting factors, this energy increase would lead to a warming of 2 to 5 °C (3.6 to 9 °F) over preindustrial times. The total radiative forcing by anthropogenic CO₂ emissions since the beginning of the industrial age is approximately 1.66 watts per square metre.

Natural sources of methane include tropical and northern wetlands (wetland), methane-oxidizing bacteria that feed on organic material consumed by termites (termite), volcanoes, seepage vents of the seafloor in regions rich with organic sediment, and methane hydrates trapped along the continental shelves (continental shelf) of the oceans and in polar permafrost. The primary natural sink for methane is the atmosphere itself, as methane reacts readily with the hydroxyl radical (OH^-) within the troposphere to form CO₂ and water vapour (H₂O). When CH₄ reaches the stratosphere, it is destroyed. Another natural sink is soil, where methane is oxidized (oxidation–reduction reaction) by bacteria.

As with CO₂, human activity is increasing the CH₄ concentration faster than it can be offset by natural sinks. Anthropogenic sources currently account for approximately 70 percent of total annual emissions, leading to substantial increases in concentration over time. The major anthropogenic sources of atmospheric CH₄ are rice cultivation, livestock farming, the burning of coal and natural gas, the combustion of biomass, and the decomposition of organic matter in landfills. Future trends are particularly difficult to anticipate. This is in part due to an incomplete understanding of the climate feedbacks associated with CH₄ emissions. In addition, as human populations grow, it is difficult to predict how possible changes in livestock raising, rice cultivation, and energy utilization will influence CH₄ emissions.

It is believed that a sudden increase in the concentration of methane in the atmosphere was responsible for a warming event that raised average global temperatures by 4–8 °C (7.2–14.4 °F) over a few thousand years during the so-called Paleocene-Eocene Thermal Maximum, or PETM. This episode took place roughly 55 million years ago, and the rise in CH₄ appears to have been related to a massive volcanic eruption that interacted with methane-containing flood deposits. As a result, large amounts of gaseous CH₄ were injected into the atmosphere. It is difficult to know precisely how high these concentrations were or how long they persisted. At very high concentrations, residence times of CH₄ in the atmosphere can become much greater than the nominal 10-year residence time that applies today. Nevertheless, it is likely that these concentrations reached several ppm during the PETM.

Methane concentrations have also varied over a smaller range (between roughly 350 and 800 ppb) in association with the Pleistocene ice age cycles. Preindustrial levels of CH₄ in the atmosphere were approximately 700 ppb, whereas early 21st-century levels exceeded 1,770 ppb. (These concentrations are well above the natural levels observed for at least the past 650,000 years.) The net radiative forcing by anthropogenic CH₄ emissions is approximately 0.5 watt per square metre—or roughly one-third the radiative forcing of CO₂.

Additional trace gases produced by industrial activity that have greenhouse properties include nitrous oxide (N₂O) and fluorinated gases (halocarbons (halocarbon)), the latter including sulfur hexafluoride, hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). Nitrous oxide is responsible for 0.16 watt per square metre radiative forcing, while fluorinated gases are collectively responsible for 0.34 watt per square metre. 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.