Carbon (C) is the 6th element on the periodic table, and also happens to be the 6th most abundant element in the universe. Carbon is unique because of its four valence electrons which make it very versatile and allow it to bond with many other elements including itself; with over 10 million known compounds containing carbon.
Carbon and its different compounds play a large role in the world we live in. Carbon by itself is important in the world, as it bonds with itself to form diamonds, graphite and other forms of carbon. Carbon by itself isn't necessarily an issue, however its inclusion in other molecules is where interesting things start to happen. Although there are over 10 million compounds of carbon, energy science is primarily concerned about a select few:
Hydrocarbons and CO2 are very closely linked, as will be explored in the next section.
Hydrocarbons are molecules which contain carbon and hydrogen. They make up the fuels the world uses to get most of its primary energy - around 95% - and when we use them to get this energy, one of the products that is made is CO2. Hydrocarbons are the primary constituent of fossil fuels, namely natural gas, petroleum, and coal, which is where a large amount of the world's energy comes from. Fossil fuels provide most of the world's electricity, along with providing by far the most dominant transportation fuels like gasoline and diesel.
The process of hydrocarbon combustion is how energy is harnessed from fossil fuels. A hydrocarbon reacts with molecular oxygen, producing carbon dioxide and water vapour, as seen in Figure 2. Different types of hydrocarbons produce different amounts of carbon dioxide, with those found in coal producing the most.
The burning of these hydrocarbons produces carbon dioxide which is a greenhouse gas; meaning that it absorbs and re-emits infrared radiation back to the Earth as seen in Figure 3, producing a climate change effect. Although greenhouse gases are necessary for the habitable temperatures on the Earth (click here for more info), the increase of these gases in the atmosphere contribute to global warming. The management of CO2 emissions from the combustion of fossil fuels is a central technological, economic, and political concern.
Methane (CH4), carbon monoxide and black carbon are all present in the atmosphere, and each have varying effects on the planet. The amount of each of these in the atmosphere has been increasing rapidly: the amount of carbon dioxide in the atmosphere has increased by about 35% in the industrial era, and methane by about 250%. These levels had been relatively stable for around 10 thousand years prior to this. The climate forcing effect of methane is about 1/3 that of carbon dioxide, so it is considered the second most important greenhouse gas. This increase is known to be due to human activities, primarily the combustion of fossil fuels and removal of forests.
Both carbon dioxide and methane, along with all other forms, are natural parts of the carbon cycle, which is the composite of all forms of carbon on the planet found anywhere from the ocean to the upper layers of the atmosphere.
The natural carbon cycle is kept very nearly in balance; animals and plants emit CO2 to the atmosphere through respiration, while plants absorb it through photosynthesis. The ocean also cycles CO2 with the atmosphere, done in an almost perfect balance. This cycle occurs rapidly, as a typical molecule of CO2 spends only about 5 years in the atmosphere.
Humans are affecting this natural cycle, mostly by the burning of fossil fuels, manufacturing of cement, and removal of forestry, as seen in red in Figure 4 below. The result is a net increase of carbon in the atmosphere and oceans, which is what causes the warming effect along with ocean acidification.
The effects of both of these can have disastrous effects on the carbon cycle, as they can work to amplify the effects of added carbon into the atmosphere. For example forest fires caused by global warming effects could return much of the CO2 that plant-life is expected to rid of from the atmosphere. Acidification of the oceans from too much absorbed CO2 can harm marine life, which play important roles in the ocean carbon cycle.
Mentioned earlier is that a typical carbon dioxide molecule will only remain in the atmosphere for around 5 years. This seems promising, however it doesn't tell the full story. If humans were to stop their effect on the carbon cycle instantly, it would take about 100 years for half of the carbon that humans have put in the atmosphere to sink into the deep ocean or land. However as less and less human-induced carbon is present in the cycle, it begins to take longer for the cycle to rid of it. Only after thousands of years will this carbon be nearly totally rid of, and at that point the effects of it might already be too late to reverse.