Carbon dioxide is a common gas that comes from primarily natural sources, when a carbon atom combines with an oxygen molecule. It makes up a small, but important part of our atmosphere, which is primarily nitrogen and oxygen. Though only a small amount of the atmosphere is made of CO2, this is still a huge amount in weight and volume. It is reasonable to wonder what the major sources are.
The greatest amount of carbon dioxide is locked up in plants, rocks, and the oceans. It should not be surprising that these each contribute more CO2 emissions than any other sources.
The people who firmly believe that man is the biggest culprit may not take it happily, but the biggest source of CO2 emissions is volcanic eruptions. There is a huge amount of carbon dioxide locked up in rocks. As the rocks melt, they give up the gas, and this is expelled during the eruption. Often, the larger the eruption, the more carbon dioxide is released along with other gases, such as hydrogen sulfide.
At any given time, according to agencies such as the USGS, there are about 13-17 volcanoes erupting somewhere on Earth. This means that yearly, volcanoes spew out hundreds or even thousands of times more carbon dioxide than man is capable of producing, even if he tried. Man is actually an insignificant producer of CO2, though he is prideful enough to think he is a major player.
Thankfully, ocean water has a great propensity for absorbing this gas, and as ice melts, as it has done for the past 11,000 years, it means that the oceans can take in a great deal more CO2. Many of the volcanoes occur in the ocean, so it has a good chance to absorb a lot of this gas. Above the surface, though, the gas is vented into the atmosphere.
Next in line for emissions is the decomposition of plant life. This can be in the form of natural death and decay, forest fire, or even use and consumption. Plants contain a great deal of carbon dioxide and carbon. These are released as the plant dies and decomposes. (Oregon State agricultural extension service, Albany, Oregon)
According to Steve (last name withheld on request), retired thermal imaging specialist, the amount of CO2 released is staggering. One major forest fire can release nearly as much carbon dioxide as a moderate volcanic eruption. That is enormous compared to other sources of emissions, excluding volcanic eruption. (US Forest Service; western fire suppression center, Boise, ID)
The next biggest emitter of carbon dioxide is probably the ocean. It absorbs a great deal of the gas, however, the colder it is, the more it can hold. The bottom of the ocean contains water that is below the freezing point, but salinity and pressure prevent it from freezing. Contained CO2 tends to stay there for a long time.
However, in some places, like the Gulf of Mexico and in the Caribbean, surface waters get relatively hot, releasing carbon dioxide in the process. Colder polar waters offset this, because the gas is absorbed again, however this is still a major source of emissions.
A person may wonder where man and other animals fit into all of this. After all, animals breathe in oxygen and breath out carbon dioxide. Their bodies also contain CO2 and carbon, which is released when they die and decompose. Man also burns fossil fuels, which does release CO2 as a byproduct. However, animals including man don't produce nearly as much carbon dioxide as the major producers with the possible exception of the death and decomposition of animals.
Note that exact figures for the amount of CO2 released through the use of fossil fuels, is hard to come by. The figures tend to range from high to low, depending on sources, though not approaching that produced by volcanic eruption, by comparing the numbers to those given by the USGS and volcano researchers. The latter figures are available from the USGS website, and through the US national park system.
The largest emitters of carbon dioxide are volcanic eruptions, forest and wild fires, and natural decomposition of plants and animals. This is a good thing, since there is a relatively stable and finite amount of both oxygen and carbon on this planet. If it weren't for carbon dioxide, the earth could well be a frozen ball in space, and life, as we know it, would probably not be able to survive.
US Geological Survey
Oregon State University, Oceanography department
National Geographic Explorer
Volcanoes National Park