What Is CO2
Carbon dioxide (CO2) is the one gas considered the biggest culprit in causing potential man-made greenhouse climate warming. Knowing where CO2 comes from is a good way to get a handle on climate change.
Carbon dioxide is one of the main heat-trapping gases that contribute to the “greenhouse effect” that warms Earth. It is also one of the most abundant and one of those whose concentration human activities have changed most radically.
CO2 occurs naturally in the atmosphere in small amounts — making up about 0.03% of the atmosphere, or 300 parts per million. CO2, water vapor, and other greenhouse gases are enough to warm Earth by about 60°F above what it would otherwise be. That “natural” greenhouse effect makes life as we know it possible. But the extra “unnatural” warming caused by human emissions of CO2 could make life unpleasant in a lot of places.
The “natural” CO2 in the atmosphere comes mostly from the “respiration” of living things. Every time a person breathes out, he or she exhales CO2 which the lungs have removed from the bloodstream, where it is the waste product left when cells burn carbohydrates to make energy. The microbes that break down the dead plant matter in soil release CO2 as they digest it. The biggest source is plant respiration.
Much of this CO2 input from respiration is offset by photosynthesis. In photosynthesis, the biochemical process made possible by chlorophyll, plants breathe in CO2 and combine it with the water they take up from their roots to produce carbohydrates. Eventually, those carbohydrates are metabolized again to produce CO2. It’s a cycle. Most of the time, CO2 going into the atmosphere from respiration is closely balanced by CO2 taken out by photosynthesis — and the “natural” concentration of CO2 in the atmosphere changes little.
A similar process happens with the oceans. Living things in the sea breathe out CO2, which dissolves in the water. Algae, and other sea-dwelling plants, take in CO2 to build their bodies, and they in turn are eaten by other creatures. These creatures store the carbon in the form of calcium carbonate in their shells, and when they die, the shells drift down to become part of the bottom sediments and are eventually incorporated into Earth’s crust. The ocean exchanges huge amounts of carbon dioxide with the atmosphere at the air-sea boundary — but all these processes are believed to be largely in balance most of the time, yielding little natural net change in atmospheric CO2.
Enter human culture — a latecomer on the planet which changed everything. People cut down the trees which were removing carbon dioxide from the atmosphere so they could plant farms. People dug up coal and pumped oil which had stored the carbon locked up underground by plants millions of years ago. And as the wood, coal, and oil are burned to make our civilized economies hum, carbon dioxide is released back into the atmosphere. The “extra” CO2 from the fires of civilization, while not much compared to natural carbon flows, is enough to change the concentration of CO2 in the atmosphere. In 1750, before the industrial revolution, it was about 280 parts per million. Today it is about 367 parts per million. That is a 31% increase.
Can we break down where the human CO2 contribution comes from? Of the total 1,480 million metric tons of carbon (mmtc) emitted from the burning of fossil fuels in the United States, the biggest amount (627 mmtc) comes from petroleum, with most of that coming from gasoline (293 mmtc) and diesel/heating oil (144 mmtc). The other large contributor is the combustion of coal (533 mmtc), most of which is burned by electric utilities.
What countries do the fossil fuel emissions come from? Well, that 1480 mmtc emitted by the United States is a big fraction of the 6,036 mmtc emitted worldwide. Most of the current emissions, it turns out, come from industrialized Western nations. The fossil fuel emissions from all of Africa (219 mmtc) or all of Central and South America (146 mmtc) are tiny compared to what the United States emits.
Of similar importance is the carbon left in the atmosphere when people cut down forests and modify soils. This is harder to quantify, but is thought to be about 20 percent of total human emissions. The slash-and-burn agriculture which is eating away at the Amazon rain forest, for example, not only destroys CO2-removing trees, but releases CO2 back to the atmosphere when trees are burned