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The amount of carbon on planet Earth by definition remains pretty much the same. Man has been burning fossil fuels, which puts carbon into the atmosphere. Where did the carbon in the fossil fuels come from? It mostly came from plants and bacteria that got buried underground due to geological processes. Over millions of years natural processes turned the plants and bacteria into fossil fuels. Where did the plants and bacteria get their carbon from? They got it from the atmosphere. The carbon that we are now putting into the atmosphere originally came from the atmosphere.
To better understand this, we have to understand the complete history of atmospheric carbon dioxide on planet Earth. The original earth atmosphere was an amazing 43% carbon dioxide compared with the roughly .04% that we have now. That original atmosphere had so much pressure that it could crush a man flat. About 2.5 billion years ago, cyanobacteria began using photosynthesis to convert carbon dioxide into free oxygen, which lead to the creation of our oxygen rich "third atmosphere" 2.3 billion years ago. At that time the carbon dioxide levels were about 7,000 parts per million, but it went into a somewhat steady but uneven decline because geological processes would sequester carbon underground. The decline was uneven because as part of the "carbon dioxide cycle", sometimes geological processes like volcanoes would cause massive amounts of carbon dioxide to be released back into the atmosphere.
Thirty million years ago during the Oligocene Epoch, the average temperature of the earth was about 7 degrees Celsius warmer than it is now. There was no ice on the poles, but the amount of carbon dioxide in the atmosphere was in rapid decline during this epoch. About 23 million years ago, at the beginning of the Neogene period, ice began to form on the poles. About ten million years ago, a series of intermittent ice ages began that continue to this day. I found one source that said that we are still technically in an ice age because we still have ice at the poles.
These ice ages helped create human evolution. The ice ages caused Africa to dry up which lead to some deforestation. This forced some arboreal (tree dwelling) apes to venture onto land. About 7 million years ago, the first apes that could comfortably walk upright appeared. They had evolved a new type of pelvis that allowed upright locomotion, which is about three times more efficient when trying to cross land.
The first tool making ape that resembled modern humans, Homo habilis, arose 2.5 million years ago. It would be soon followed by Homo erectus, and then about 200,000 years ago, modern humans, Homo sapiens would arise. However, Homo sapiens almost died out. About 50,000 years ago an ice age in Europe had caused Africa to almost completely dry up. The total human population had dropped to 7,000 individuals living on the southern coast of Africa. During this period humans learned how to fish, make new tools, and create permanent dwellings. When the ice age abated, these humans with their new tools spread out to rest of the world at a pace of about a mile per year. This was the beginning of the Upper Paleolithic (Late Stone Age) period.
More ice ages would follow, and during each ice age human population would decline. It is no coincidence that all of human civilization (i.e. agriculture, use of metals) would arise during a "brief" warm period between two ice ages starting about 10,000 years ago. I have heard that no matter what we do, we will enter a new ice age in about 10,000 years from now, but I have also heard speculation that the next ice age will be delayed by global warming. This actually should be our goal, since humans have always declined during the ice ages and always prospered during the intermittent warm periods.
During the geological time period of the earth, the amount of carbon dioxide in the atmosphere has been on an uneven decline and mostly disappeared. Atmospheric carbon dioxide is necessary for plant growth, and I have read that we were running dangerously low on atmospheric carbon dioxide, about 00.02%, before mankind at least temporarily reversed the trend. I just read a wikipedia article that said that atmospheric carbon dioxide will eventually get so low that all plants and animal will die off. What mankind has done is put carbon dioxide back into the atmosphere that was previously there, thus possibly delaying the next ice age. Currently the amount of carbon dioxide in the atmosphere is about 00.04%.
Carbon dioxide by itself cannot cause significant global warming. There are diminished returns. Carbon dioxide has to double again to produce the same effect as the last doubling. The effect is not linear but logarithmic. What the alarmists are worried about, and they could be correct, is positive feedback. The warming of the earth causes more water vapor to enter the atmosphere, and water vapor is a much stronger greenhouse gas than carbon dioxide, thus causing more warming. If this were true, however, the last warming period around the year 2000 should caused a continuous positive feedback, a runaway greenhouse, which didn't happen. Instead temperatures went into a major decline and hit a really big low point in the year 2007.
The skeptics believe that increased cloud cover reflects sunlight back into space thus causing a negative feedback. The skeptics are not "global warming deniers", which is a pejorative phrase used by global warming theorists to make the skeptics sound like holocaust deniers. These skeptics actually believe in global warming. At least, the legitimate skeptical scientists do. They just think that global warming is happening at a rate slower than predicted by the theorists. I can point you to an article that shows that the positive feedback models have been contradicted by the actual temperature data, which in reality has been closer to the negative feedback models.
The worst case scenario is that the polar ice caps will melt. If that happens we will lose some coastlines and all of Florida due to sea level rise. However, according to what I just read, it will take 5,000 years for the polar ice caps to melt. In other words, these are processes that take a very long time to happen. In this century we are only looking at modest temperature increases. In the meantime, humans are very adaptable. We are only five to ten years away from creating the first workable prototypes of nuclear fusion. It might take 25 years for this to be practical, but at that point if we wanted to get rid of fossil fuels altogether, we could. I think that we will also see advances in solar power, which is already happening, and battery technology to store the energy created by solar. In other words, we have it within our means to avoid any possible disasters that might be coming.