As I understand it, the plant life preceded the animal life, and the plants used carbon dioxide in the process of photosynthesis. Oxygen was created in the process, as a waste product, which gave us much of the present oxygen content in the atmosphere today. I agree, with all the pollutants man and his machines produce, combined with deforestation, it is a small wonder the carbon dioxide levels aren't accelerating faster than they are.
There are more people and polluters, and fewer trees. The earth's surface used to be covered in plant life where there is now concrete jungle. Over time, this is going to catch up with us. As we cut down more and more trees, we cannot just keep planting new ones. A little sapling does not compare to a mighty redwood.
A misconception I see in these answers is the link that so many are making between fossil fuels and CO2. The reality is that combustion of ANY hydrocarbon, alcohol, or just about any other organic molecule will produce CO2 as a product of the combustion. So burning wood, or ethanol or natural gas or ANYTING else that is considered as an alternative to fossil fuels will still produce the CO2. The increase in CO2 in the atmosphere is settled science. So is the correlation this has with the rising atmosperic temperature. So is the correlation with human activity since we started burning stuff on a large scale. The massive PR effort (that we see in this country coming from the monied interests that profit from our current mistreatment of the atmosphere and our environment) to cloud this issue and raise doubts about the solid science is NOT HELPING!
No one will dispute that the amount of atmospheric Carbon Dioxide has been increasing, and for reasons stated in all the responses. However, even before the world Industrialized, the Earth was experiencing a warming trend. The real question is if current human activity has accelerated the process, and if that will cause irreparable environmental damage.
Since the Industrial Revolution, carbon dioxide emissions have been steadily rising. This is due to the fact that first, people used coal to run factories and machinery. Coal is a fossil fuel and when burned, releases a great deal of carbon dioxide. Later on, oil and gas replaced coal to supply most of our energy needs. Again, these are fossil fuels and release carbon dioxide during combustion. All of the automobiles burn gasoline and power plants are using coal. The fact that there are more people than ever on Earth adds to the carbon dioxide levels due to cellular respiration. All of the animals we raise for food add to it as well. The trees which naturally remove CO2 from the atmosphere have been steadily cut down to make room for farms, and land for housing. Carbon dioxide levels have been increasing due to all of these reasons and is a complex issue.
I would have to agree with the above posts. Most importantly, I would agree that the loss/lack of plant life, which transforms carbon dioxide to oxygen, is one of the most important factors. On top of that, the burning of fossil fuels, as previously stated, accounts for the increased levels as well.
Clearly the amount of carbon dioxide that is being released into the atmosphere comes largely as a result of the ways in which we are burning more and more fossil fuels as a planet. This has been fuelled (no pun intended) in recent years by the ways in which formerly underdeveloped countries such as China and India are rapidly catching up in terms of industrialisation and burning more and more fossil fuels themselves so as to catch up with other developed countries.
The second answer is obviously superb. I have little to add, except to say that the whole issue of the build-up of carbon dioxide in the atmosphere is related to fears of "global warming," since carbon dioxide is a "greenhouse gas" that traps heat inside the earth's atmosphere. I have read that scientists are experimenting with ways to trap carbon dioxide and sequester it beneath the ground. Below is a link to a fairly full discussion of this issue:
Most scientists believe that the increasing levels of carbon dioxide in the atmosphere come from the interplay of multiple factors. Loss of plant cover on the land areas is the most visible issue, but is generally considered to be the smallest of the problems, and the easiest to address.
Another issue is human-caused changes to the oceans. Keep in mind that the Earth is a water planet; there is twice as much sea surface as dry land surface. The plants of the sea, particularly the tiny floating phytoplankton, perform about 70% of the Earth's photosynthesis. Hence the sea is responsible for producing most of our oxygen, and trapping most of the carbon dioxide. Humans have been using the oceans as a sewer for many centuries, and as our population and technology levels rise, the oceans have begun to show negative effects from all the accumulated pollution. As the phytoplankton die off, the ocean absorbs less carbon dioxide. This effect seems to have begun, and no one knows how bad it will get or how quickly it will proceed.
The third issue, which is generally considered to be the biggest carbon dioxide contributor, is the burning of fossil fuels. Scientists believe that in the past the Earth was much warmer, with higher levels of carbon dioxide in the atmosphere. When the plants and animals that eventually became coal and petroleum were buried in the earth, that removed some of the carbon dioxide from global circulation. Now we are digging and drilling into the earth to find those fossil fuels, and burning them, which releases that long-stored carbon dioxide into the atmosphere once again. In just two centuries we have burned up fossil fuels that took millennia to create. The fear is that, if we put that ancient carbon dioxide back into the air, we will return the planet's climate to the way it was when the fossil fuels began to be created.