Carbon dioxide (CO2) is a trace gas in Earth's atmosphere and a member of the family of "greenhouse gases." These gases, led by water vapor, receive their name because they help insulate the surface of the planet from the cold vacuum of outer space, enabling the existence of the planetary biosphere itself.
CO2, like the other members of the group, qualifies as a greenhouse gas because of the way in which it absorbs infrared radiation and keeps it in the atmosphere.
Earth's Atmosphere
The Earth's atmosphere has gone through several iterations, the first version developing within several million years of the planet's early formation and consisting mainly of vaporized silicon particles and then hydrogen and helium.
Over a longer period of hundreds of millions of years, the Earth began to cool while volcanoes pumped out massive quantities of water vapor, CO2, sulfur and other gases.
From Atmosphere to Biosphere
The sun's intense ultraviolet (UV) radiation promoted the partial conversion of atmospheric water vapor into ozone, which gradually accumulated to block enough UV radiation to make the surface safe for the development of life forms.
Early life forms, like much life today, used CO2 as their energy source, utilizing the sun's light to help them extract energy by combining CO2 and water. The high atmospheric levels of CO2, which had helped keep the planet humid and above freezing, began to drop as oxygen concentrations rose. The CO2 in the atmosphere had incubated the origins of life.
Greenhouse Gas Blend
The airborne combination of water vapor, CO2, methane and other gases helped moderate the Earth's surface temperature, preventing the sun's energy from escaping into space every night and helping to shield the planet from wild climate fluctuations. In addition to serving as the main food source of plant life, the greenhouse gases helped turn the Earth into an actual greenhouse; a warm environment shielded from the harshness of outer space, able to nurture the growth of incipient life.
How Greenhouse Gases Work
Greenhouse gases are largely transparent to incoming solar radiation streaming from the sun towards the surface, but absorb greater amounts of infrared radiation reflected back by the much cooler surface of the Earth.
While the main components of the atmosphere, molecular nitrogen and oxygen, are transparent to this heat radiation, the greenhouse gases first absorb it before re-emitting it in all directions. This re-emission sends some of the surface's lost heat back down toward the surface, maintaining a moderate temperature equilibrium and protecting the Earth's surface and atmosphere from losing their warmth to the cold vacuum of outer space.
Water Vapor, CO2 and Trace Gases
With an airborne concentration of 0.4 percent, water vapor (H2O) is the most abundant of the atmospheric greenhouse gases, boasting over 10 times higher concentration than the next most abundant gas, CO2. Present in smaller quantities are also methane (CH4), ozone (O3) and several others.
Human activity, the natural biosphere, ocean circulation and volcanoes are the four main sources of atmospheric CO2.
Greenhouse Gases and Climate
While greenhouse gases have helped make Earth habitable and life-friendly, the role of atmospheric CO2, and particularly of that fraction of CO2 produced by humans, in affecting global climate remains unclear and strongly debated. Most predictions regarding planetary climate and CO2 are based on software programs and computer models, instead of observable data. Paleoclimate research suggests that past periods of low temperature have existed concurrently with high atmospheric CO2 concentrations. The role of CO2 in planetary climate, and its interaction with solar radiation and ocean cycles, continues to be the subject of intensive research.
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