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Carbon Dioxide - CO2
CO2 (carbon dioxide) is a gas. If you burn carbon you get carbon dioxide. If you burn chemicals containing carbon, you get products including carbon dioxide. It's a gas that's heavier than air, non-toxic in the small percentages it occurs in atmospheric air. Carbon Dioxide (CO2) is one of the most talked-about substances in the early 21st Century, possibly because of the changing levels of it in the earth's atmosphere and the consequent effects this might have on the environment.
It's not a pollutant; it's natural stuff that's supposed to be in the atmosphere, in the right quantities. In the early earth, thousands of millions of years ago, there was a huge amount more CO2 in the atmosphere. As life evolved it converted a lot of the CO2 into carbon-bearing solids and as time has gone on the amount of CO2 has reduced considerably, up until a few hundred years ago that is. Combustion of some of the carbon-bearing solids (coal, for example), has started to push the balance back the other way, so there's more CO2 than would be expected.
The problem is that the more CO2 there is in the atmosphere, the hotter the planetary temperature. That's because of the greenhouse effect, where the greenhouse gases including CO2 in the atmosphere let light and heat from the sun in, but don't let the heat out so easily, like the panes of glass on a greenhouse. On the neighbouring planet Venus, there's an enormously high amount of CO2 in the atmosphere and it is too hot, much hotter than would be explained by the fact that Venus is closer to the sun.
Earth has a better balance, and it's good if it stays in balance rather than going too far towards the "too hot" or "too cold" conditions which are plausible. There's a strong link between CO2 and planetary temperature because of the greenhouse effect, and so there is a tendency for people to worry, panic, feel guilty, or adopt an odd belief-set because of the CO2 or a misunderstanding of the CO2. Looking at the geological history the correlation is there to be seen. What's less obvious is whether CO2 causes heating or whether heating causes CO2 , and so the issue of who's to blame for climate change is popularly discussed, often missing some of the more important issues about good management of resources, good planning, and having a reasonable approach. Humanity may cause climate change, but climate change happens anyway, and has been happening for a long time before there were any people on the planet.
If there gets to be too much CO2 , there will be global warming, sea level rise, and a shuffling of the local climates in the world, thus leaving the great cities and other human developments meteorologically in the wrong place. It would be equally bad if there gets to be not enough CO2 , with its associated global cooling, sea level reduction, freezing over of temperate zones, and, as with the global warming, a shuffling of the local climates in the world, thus leaving the great cities and other human developments meteorologically in the wrong place.
"Reducing human impact on the environment" is a popular idea, but if an ice age were to be imminent, human impact to prevent it might be a good idea! However, current evidence suggests the global climate is warming up, so it might be better if things are done towards cooling it down.
Reducing greenhouse gases is only one technique, and even then there are some greenhouse gases which have a bigger effect than CO2; Methane, for example. Hence, burning fuel which would otherwise have rotted (eg burning gambling machines to save money) saves energy, gets rid of waste, and reduces greenhouse gases even despite the fact it produces CO2.
There are some ideas for carbon sequestration, but these have to actually make sense! It's no good trying to turn CO2 into solid dry ice and burying it, because that won't work! It's about as daft as burning oil to make electricity to power an electric car. However, in contrast, supposing there was some way to encourage coral reefs which turn carbon dioxide into calcium carbonate, that really would reduce atmospheric CO2 ! (1m2 of coral can produce 10g of calcium carbonate per day, representing 2.4 litres of CO2 gas removed from the atmosphere)
When thinking about the practicality of schemes to do with CO2 in the environment, it's a good question to consider where the carbon in question came from. In the subject of alternative fuels, if you grow fuel and burn it, the CO2 has been borrowed from the atmosphere and then returned to it, so it doesn't alter the total amount in the long run. This is in contrast to burning fossil fuels, where it's a one-way process.
Carbon itself is a solid, and when combined with oxygen it produces carbon dioxide gas, (and if the combustion is incomplete there's some carbon monoxide too, which is poisonous), and energy. The carbon dioxide gas is one of the lowest energy states of carbon, so if you want to turn it back into carbon you need to put in as much energy as was released by burning it (like going up and down hill). Plans involving turning CO2 back into carbon require the effective "un-burning" to release the oxygen and leaving the carbon, and the consequent energy input.
Air is about 78% nitrogen, 21% oxygen, 1% argon, and 0.03%-0.04% carbon dioxide. If you are breathing air in a confined space, the concentration of CO2 increases, and that's what causes the feeling of being short of air to breathe. That's different to the life sustaining aspect, which is how much oxygen there is. As a result of this, death by hypoxia in a pure nitrogen atmosphere is painless, whereas death by suffocation related methods is distressing.
Carbon dioxide gas can be frozen at -78.5 degrees C, and it goes from being a gas to being a solid without becoming a liquid (unless at pressure). Solid carbon dioxide is DRY ICE, curious white stuff that produces mysterious fog effects which have interesting theatrical applications. With such things as dry ice on stage, and CO2 fire extinguishers, there is no global environmental problem with releasing carbon dioxide into the atmosphere, because it was made from the atmosphere quite recently, and it's simply being returned. Bottles of lemonade and other fizzy drinks contain CO2 which is released into the atmosphere, but the drinks bottling plants are getting their CO2 from the atmosphere to put in the drinks in the first place, so again, no "carbon footprint" issues.
There is more about carbon dioxide on the page about who's to blame for climate change