There are natural and human caused carbon dioxide (CO2) sources. The natural sources are much larger than human sources. However, over geologic time (billions of years), the natural sources have managed to balance themselves out so that life as we like it can survive. Maintaining this balance is part of the carbon cycle. Changing the CO2 balance
is a big deal.
Natural CO2 sources include plants, animals, the oceans, and volcanoes. Human caused sources are primarily some form of combustion from fires to furnace flues.
Animals create CO2 through respiration (breathing) and plants use CO2 during photosynthesis. Plants concentrate carbon from the CO2. This is the carbon released by humans burning the plants later on. Plants have over time also been converted to fossil fuels, which people burn a lot of, and which pollute the air considerably more.
The oceans are huge carbon dioxide sinks. CO2 is constantly being exchanged from the surface of the ocean to the air above and then from the air above back into the ocean. The amount depends on how much CO2 is already in the water, water temperature, mixing and so on. Oceans are becoming packed with excess CO2 and more acidic as a result.
Volcanoes were the first of the carbon dioxide sources as part of the carbon cycle billons of years ago. Carbon rich rocks and sediments plunging into the hot mantle released carbon dioxide gas through volcanoes. This is still happening. The amount is estimated at around 110 million tons per year, which is only a small fraction (0.4%) of what people put into the air.
The largest human carbon dioxide sources have one thing in common: the burning of carbon based fuels. When a carbon based fuel is burned, one of the main compounds released is carbon dioxide gas. Carbon based fuels may be wood, paper, plant matter or another type of hydrocarbon. These fuels are often termed fossil fuels.
To simplify classification, there are three main CO2 sources in the carbon cycle: vegetation/land, oceans, and humans. The values of these sources are shown in the chart below.
As you can see, the largest CO2 source is a combination of vegetation and land. This includes volcanoes, plants, and animals. Plants and animals add 440 billion metric tons, while at the same time the plants remove 450 billion MT every year. This leaves a net CO2 imbalance of around 10 billion tons annually.
The oceans are constantly emitting and adsorbing CO2; they are a huge CO2 sink. The oceans can also remove around 6 billion more MT than they adsorb every year. It has been suggested that we put more CO2 into the oceans. Recent experiments with iron filings have tried to induce the oceans to suck up more CO2. The oceans refused.
Finally, we have the humans. As you can see from the numbers, the oceans and vegetation would have a net negative balance if not for the human input of CO2. People however are changing the balance to a net positive of around 14 billion MT or so this year. The amount is going up.
This positive imbalance is the result of human carbon dioxide sources and a very likely cause of recent climate change.
Cars put some 350 million tons of CO2 into the air every year.
