The biggest rage for the last decade or so has been to blame carbon dioxide for global warming, global cooling and an assortment of other worldwide perceived or actual difficulties. This totally ignores the importance of CO2 and the true percentage of the gas in our atmosphere. There are few better ways to bring out how overblown the evils of carbon dioxide are than to take a hypothetical trip and to consider what might happen if the atmospheric CO2 levels were cut in half. Much of this amounts to a best guess, of course, since there aren't many mechanisms that are known that could cause such a drop of any atmospheric gas, so this is something that can only be conjectured.
Current CO2 percentages
In order to look at what a drop in carbon dioxide would mean, it is necessary to first take a look at the actual percentage of Earth's atmospheric gases are comprised of CO2. The most abundant atmospheric gas on Earth is Nitrogen, which makes up about 78.08 percent of the atmosphere. Next comes oxygen at 20.95 percent. Argon accounts for an additional 0.93 percent. Just with those three gases, nearly 99 percent of the gaseous components of the atmosphere of Earth have been accounted for. Carbon dioxide makes up only about .036 percent of the atmosphere, or 36 thousandths of a percent. This is obviously not a very large part of the atmosphere and certainly not as great as one might think, listening to all the hype. If half of that was taken away, that would only be 18 thousandths of a percent, but the impact would be catastrophic.
Carbon dioxide, water vapor and methane are all greenhouse gases, and are probably the most important, as well. That is, they allow the sun's rays to pass through to the ground and heat up the Earth, but prevent the heat from escaping right back out into space. Even as small as the percentage of CO2 is, if the amount in the atmosphere was cut in half, it is likely that the global temperatures would drop to the point they are in Antarctica, all over the world. It is hard to imagine wintertime temperatures in Bermuda or Hawaii at minus 100 F. Summer temperatures might reach above the freezing point of water, barely, but most plant life would begin to die out, and with it, the animals that rely on the plants. The earth would become a frozen ball orbiting the sun.
This effect could be hastened because most plants require carbon dioxide in order to live and for photosynthesis to occur. A huge drop in CO2 would mean that many plants simply would not be able to exist, as people know them. Fewer plants, in turn, would mean a drop in water vapor and breathable oxygen released into the atmosphere, since both are byproducts of photosynthesis. This could force global temperatures even lower.
It has been all but proven that in the distant past, the amount of atmospheric CO2 was much higher than it is right now. The result was a lush but steamy planet covered by growing things, and one of those periods was called the Carboniferous. It was from this period of time that many of the fossil fuels came, such as coal, because of the large number of plants that were growing. There is also evidence that toward the end of the Carboniferous, atmospheric oxygen levels were also much higher than today, perhaps because of the luxuriant plant growth. And yet, core samples show that the much greater amount of carbon dioxide was probably not more than a couple tenths of a percent, and the percentage apparently dropped toward the end of the period. This means that the increase in CO2 of one to three thousandths of a percent some people have claimed to have occurred in the past century probably isn't much of a variance. Still, it shows the opposite of what would happen if the carbon dioxide amounts were cut in half.
Basically, if the carbon dioxide levels in the atmosphere were cut in half, it is likely that plant life on the planet would soon perish, rapidly followed by animal life, and the world would become frozen solid. It isn't a pretty picture. How long it would take is difficult to say, because as the plants and animals died, carbon should be released, which could bond with oxygen in the atmosphere to boost CO2 levels. Volcanoes would almost certainly continue to produce an enormous amount of CO2, as well. The Earth tends to be a self-regulating system. Still, though 18 thousandths of a percent may not sound like much, if the change was sudden, the impact would be enormous. Thankfully, it isn't very likely to occur any time soon.