Carbon is the stuff of life: all living things on Earth depend on it. The human body is approximately 18% carbon. So why is carbon such a problem?
The answer lies in the carbon cycle. Carbon isn't created, but rather endlessly recycled in a variety of forms. Volcanic activity and weathering of exposed rocks release carbon into the atmosphere. There, it becomes available for plants to use as they convert the sun's energy into sugars, which in turn form the building blocks of the plant's physical structure.
Animals eat plants, so getting their fix of carbon in the process. And some animals eat other animals, absorbing "third hand" carbon, making for healthy bodies and sharp teeth.
Eventually, living things die. That's when the world's great decomposers -- bacteria and fungi -- get to work. They break down the shells and bodies of those dead animals and plants, absorbing the vital nutrients they need to survive and expiring much of the locked up carbon back into the atmosphere.
Crucially however, some of that carbon escapes the continual turnover. In the oceans, carbon dioxide absorbed by the surface waters becomes trapped in the bodies of plankton and so enters the marine food chain. When sea creatures die, many fall to the abyssal depths and take their carbon with them, where it will eventually be incorporated into sedimentary rocks or even subducted under the Earth's crust through tectonic plate movements.
On land, soil carbon steadily accummulates as leaf litter and annual vegetation dieback. In time it's buried sufficiently far below the surface to be effectively locked away. Over millions of years these subterranean deposits are transformed into those great stores of fossil fuel -- oil and coal -- upon which we are now so dependent.
Biosphere vs lithosphere
Fossil carbon has been removed from the current carbon cycle. Digging it back up and burning it releases it back into the atmosphere once again, increasing the stock of circulating carbon.
This is the reason for global warming. Atmospheric carbon dioxide levels are rising as a consequence of bringing all this long-"retired" carbon back into the system. As a powerful greenhouse gas, CO2 traps the sun's energy and increases global temperatures. And this leads to climate change.
The "solution" to rapid climate change rests in finding a way to ensure that no further fossil carbon enters the cycle. That could either be through technological changes in the way we use fossil fuels -- things like carbon capture and sequestration -- or it might meaning turning away from fossil fuels altogether.
