The carbon cycle is important because all living things are made of carbon in one way or another. The carbon cycle is how carbon is exchanged throughout the earth: between the atmosphere, oceans, ecosystem and geosphere. If the carbon cycle fails, then life begins to break down, causing life to end.
Carbon in the form of CO2 is present in the atmosphere in a very small amount, about 0.04 percent, but it has a big impact on sustainable life on the planet. The CO2 traps radiation in the atmosphere and acts like a warm blanket around the planet, trapping heat in and keeping the surface from freezing.
The way that humans live, using fossil fuels and other practices that release CO2 into the air, contributes to the amount of CO2 in the atmosphere. As this CO2 builds, the atmospheric blanket continues to grow, adding to the amount of heat trapped in the atmosphere, which raises the temperature on the surface of the planet. This is a major contributor to what is being called global warming or global climate change. If humans leave the excess CO2 in the atmosphere, it takes several thousands of years to work its way out of the air through natural mechanisms.