Executive summary:

How do natural processes and human actions affect the carbon cycle on land, in the atmosphere, and in the oceans?

Human activities such as burning fossil fuels, changing land use, and using limestone to make concrete have a trememdous impact on the carbon cycle. All of those activites transfer significant quantities of carbon into the atmosphere. As a result, the amount of CO2 in the air is rising rapidly. The ocean absorbs much of the CO2 that is in the atmosphere, interfering with sea life as well as lowering the ocean's pH.