Not many people dispute the belief that the global climate is changing, though there is some disagreement about whether there is a warming trend, cooling trend or if the climate is simply shifting. Along with this belief, many theories have been put forth to explain the changes. One of those explanations has been that CO2 is the ultimate cause. There are solid reasons for discounting this, however.
Carbon dioxide, or CO2, is a common compound contained in the atmosphere, in rocks and in living organisms. Without it, life on this planet, as we know it, couldn't exist. Most plants require CO2 to breathe and plants are the base of the food chain. Still, atmospheric carbon dioxide is a greenhouse gas, meaning that it traps energy from the sun rather than letting it escape right back into space. One of the kinds of energy it traps in is infrared radiation, which causes heating.
It is the greenhouse properties of carbon dioxide that are most often cited as the cause of global climate change. There is little doubt that it is efficient at trapping heat close to the surface of the earth. Several researchers have even postulated that without the gas and subsequent greenhouse effect, our world would be a solid, frozen ball. However, this doesn't necessarily signal or cause climate change.
Atmospheric levels in history and prehistory
There isn't a way to say definitively what the level of CO2 has been throughout the lifetime of the earth. The reason is in part that through most of history, there has been nobody to measure the levels. It should also be noted that even the study of global climate is relatively new, starting barely a century or so ago, though the climate in some areas was well known.
Instead of direct observation, scientists often turn to rock samples and ice cores. Neither is conclusive, but they give a lot of insight into periods of time when carbon was both more abundant and less abundant than it is now, in the atmosphere. The closer to the present time the samples are, the more accurate they tend to be.
Looking at the data
The cores that have been taken indicate that there have been periods when CO2 levels were higher than they are today and when global temperatures appear to have also been higher. An example is the carboniferous period. At other times, the carbon dioxide levels have been higher than today while the temperatures were lower, such as the period just before the last ice age. The opposite has also been shown, with low gas levels during periods of both hot and cold global climates.
In fact, just before and just after what is called the little ice age, in the 1700s, the amount of CO2 in the air was nearly identical. Carbon dioxide levels apparently didn't trigger the about 70 year period of global cooling, nor did they end it. This would strongly refute the claims that CO2 has much to do with climate change.
Sources of CO2
The non-relation is further supported by looking at the main sources of carbon dioxide emissions. A moderate volcanic eruption produces 10 times more CO2 than man can purposely produce in a year, according to the US Geological Survey. There are an average of 11-13 volcanic eruptions occurring somewhere on Earth at any given time. However, this is an average, so there are periods when there is a lot more volcanic activity, and other times when there isn't as much. If CO2 was a major player in global climate change, it would stand to reason that during times when there were above average volcanic eruptions, the climate should change more rapidly. This isn't borne out.
The earth has been around for about 4.5 billion years. Climate, as we think of it, hasn't been around quite as long; perhaps only 4 billion years. As far as we know, the global climate has been changing since this planet has had a climate. For that matter, climate is dynamic rather than static, so it must change. The gas composition of the atmosphere has fluctuated over that time, and yet, no one compound has been identified as leading to a climate change.
Ice ages have come and gone. Most of the time, the earth hasn't even had ice caps, though the climate continues to change. The composition of the atmosphere has also changed. However, there just isn't anything to show that CO2 plays much of a part in the ongoing change. It does play an important role in allowing life to exist here, but not so much in regard to changing the climate.