Since the Industrial Revolution, human sources of carbon dioxide emissions have been growing. Human activities such as the burning of oil, coal and gas, as well as deforestation are the primary causes of the increased carbon dioxide concentrations in the atmosphere.
This is when the natural environment is converted into areas for human use such as agricultural land or settlements. In 2011, land use changes contributed 3.3 billion tonnes of carbon dioxide emissions worldwide. From 1850 to 2000, land use and land use change released an estimated 396-690 billion tonnes of carbon dioxide to the atmosphere, or about 28-40% of total human activity carbon dioxide emissions.
Forests are carbon stores and act as CO₂ sinks when they increase in area and density, for example in 2004, forests sequestered 10.6% of the CO₂ released in USA by the combustion of fossil fuels, urban trees sequestered another 1.5%. But to further reduce US CO₂ emissions by 7.1% would require planting an area the size of Texas (8% of the area of Brazil) with trees.
There are many industrial processes that produce significant amounts of carbon dioxide emissions as a by-product of chemical reactions needed in their production process. Industrial processes contributed 1.7 billion tonnes of carbon dioxide emissions in 2011.
So if we compare the amount of carbon dioxide emissions produced by man compared to the natural sources of carbon dioxide, we can see the levels are much smaller. That may be the case, but the increase is significant as you will see in the next section.