2. What is the carbon cycle?

All living things contain carbon. Carbon is also a part of the air (atmosphere), ocean (hydrosphere) and the ground (geosphere). Carbon is constantly being recycled through all these spheres in different forms – this movement is described as the carbon cycle.

In the atmosphere, carbon is always combined with oxygen to form CO2. Plants, trees, algae and some bacteria absorb (or "photosynthesise") CO2 out of the atmosphere, breaking down the carbon and oxygen – using the carbon to grow and releasing the oxygen. Humans and animals breathe in the oxygen and eat carbon-rich plants and animals. In a process called "respiration" they use the plants and meat for energy and breathe out CO2. Animal waste and dead plants are broken down in the soil and over many millions of years some of that plant and animal matter ends up buried deep in the ground and, under the pressure of layers of rock, is turned into highly concentrated carbon or hydrocarbons – fossil fuels (coal, oil and gas).

From the end of our last ice age about 12,000 years ago, the amount of CO2 in the atmosphere was relatively steady at below 280 parts per million (ppm – see Figure 3). Over the last 200 years carbon dioxide has been increasingly produced by human actvites like the burning of trees and vegetation as well as the burning of fossil fuels. These human actvites have affected the carbon cycle by currently adding around 33 billion tonnes (gigatonnes) of CO2 to the atmosphere every year (that is 33,000,000,000 tonnes)3. While levels of CO2 in the atmosphere may have been even higher in ancient times (in the era of the dinosaurs, for example) current levels of CO2 in the Earth's atmosphere have reached their highest recorded levels in over 350,000 years. The high levels of CO2 in the atmosphere are a major concern in inffuencing global climate change.

The WMP studied the injection of one source of manmade or anthropogenic CO2 – from a coal gasification plant in North Dakota – into the Weyburn and Midale oilfields. This manmade CO2 was injected as part of enhanced oil recovery operations, but an additional benefit is that the manmade CO2 has remained permanently underground rather than being released into the atmosphere.

Figure 2. The carbon cycle is a process in which tens of billions of tonnes of carbon move between the atmosphere (air), hydrosphere (water) and geosphere (soil). The numbers in this illustration are billions of tonnes (gigatonnes). Current human activity adds about 9 gigatonnes of carbon to the atmosphere (33 gigatonnes of CO2, which is made up of one-third carbon and two-thirds oxygen). This amount is increasing each year as we burn more and more of the estimated 4000 gigatonnes of oil, gas and other hydrocarbons that contain carbon.