Carbon dioxide (CO2) probably the most important of the greenhouse gases released by human activity, because its greenhouse impacts are large and because human activities generate so much of it.

Carbon dioxide is a very "natural" ingredient in the atmosphere - so natural that we only quite recently began to think of human-induced carbon dioxide as a "pollutant". Carbon dioxide can be a good thing, but the key question is, "How much is too much of a good thing?"

What seems "natural" to humans today can be quite different from what is "natural" from Earth's larger perspective, because we have been around for a very part of Earth's 4.6-billion-year geological history.

Probably for a long time carbon dioxide was the dominant active gas in the early Earth's atmosphere. Today CO2 makes up only about 0.03% of the atmosphere, and the highest estimates are that it could rise to 0.09% by the year 2100 as a result of human activities. Around 4.5 billion years ago, some scientists think it may have made up as much as 80% of Earth's atmosphere, diminishing slowly down to 30% or20% over the next 2.5 billion years or so. Free Oxygen was almost non-existent in this early atmosphere, and indeed poisonous to most of the anaerobic life forms that existed.

Human life as we know it today would have been impossible in such a CO2-rich atmosphere. Fortunately, most of this carbon dioxide was removed from the atmosphere later in Earth's history when sea-dwelling life, the earliest algae, evolved the process of photosynthesis. In photosynthesis, plants use light energy from the Sun to turn carbon dioxide and water into sugar and oxygen. Eventually, algae - and more highly evolved organisms, like plankton, plants, and trees - died and locked up most of this carbon in the forms of carbonate minerals, oil shale, coal, and petroleum in Earth's crust. What was left in the atmosphere is the oxygen we breathe today.

Atmospheric carbon dioxide comes from many sources - most of them natural - but is usually brought into balance with "sinks" that drain carbon out of the atmosphere.

One of the biggest "sources" is the exchange of gas between the atmosphere and the ocean surface. This exchange is actually a finely balanced two-way process, but the amounts of carbon dioxide involved are tremendous.

Carbon dioxide can be dissolved in water (the process that makes sparkling water). It is constantly being dissolved in the water on the surface of the oceans, and the sea surface is constantly releasing carbon dioxide back into the atmosphere. These processes are almost entirely physical and chemical. Most important of all is the magnitude of these carbon flows relative to other processes, because small changes in the delicate balance (or in out estimates of them) could have a large impact.

Equally important are the biological processes that cycle carbon dioxide to and from the atmosphere, plants "breath" in carbon dioxide through the process of photosynthesis. But plants, animals, and other organisms also "breathe" out carbon dioxide. One way they do this is when they "burn" oxygen in the metabolic processes known as respiration. Humans and other land mammals, for example, breathe in oxygen to sustain life and breathe out carbon dioxide as a waste product.

When plants and animals die, the organic carbon compounds they have stored become part of the soil. Nature composts this "detritus" of life much as gardeners do, by breaking it down through many kinds of chemical decomposition and microbial action.

So the amount of carbon dioxide taken out of the atmosphere every year by plants is almost perfectly balanced by amount of carbon dioxide put back into the atmosphere by respiration and decay. The magnitude of this cyclic flow of carbon is also important to realize, because small disturbances in the balance can have large implications.

By comparison, the amount of carbon dioxide added directly to the atmosphere as a result of human activities seems at first inconsequential. But these superficially small amounts do matter, because the natural parts of the carbon cycle (the air-sea exchange and the biological processes) have long been in good balance, at least on the time scales of immediate relevance to humans. Industrial and agricultural activities seem to have significantly tipped the balance of the carbon cycle.

Many kinds of scientific measurements have shown that the concentration of carbon dioxide in the atmosphere has been increasing over the past several centuries. During this time the human population increased geometrically, the steam engine was put to industrial use, the gasoline-powered automobile came into use across the globe, and farmer-settlers cleared native vegetation from vast expanses of he Americas, Australia, and parts of Asia.

During this same period, the atmospheric concentration of carbon dioxide has increased from its pre-industrial (1750) level by about 25 percent. That's enough to make a potentially significant difference, if climate is as sensitive to greenhouse gases as many scientists suspect it is.

The close relationship between carbon dioxide concentrations and estimated global mean temperature is striking. Whether the relationship is a causal one, however is still uncertain. It is tempting to conclude that fluctuations in CO2 cause temperature changes - but it could be the other way round.

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour