Human activities contribute carbon dioxide (CO2) to the atmosphere, which is a greenhouse gas that traps heat radiating from Earth toward space. The following are some of the human activities that increase CO2 levels in the atmosphere:

• Burning fossil fuels: This is the largest source of CO2 emissions from human activities. Coal releases twice as much CO2 as petroleum when burned, and natural gas releases the least amount of CO2.

• Transportation: Locomotion is the second-largest source of CO2 emissions in the US. International travel is also a factor. For example, a study of cruise ships found that vacationing at sea emits 12 times more CO2 than vacationing on land.

• Deforestation: Trees absorb and remove CO2 emissions from the air. Cutting them down stops that process. Although deforestation is a minor source of increased CO2 in the US, loss of forestlands accounts for 20% of global CO2 emissions.

• Cement production: Cement production is a significant source of CO2 emissions because it requires high temperatures to produce.

• Industrial processes: Industrial activities that our modern civilization depends upon have raised atmospheric CO2 levels by nearly 50% since 1750.

• Land use changes: Deforestation and other land use changes have also released carbon from the biosphere (living world) where it normally resides for decades to centuries.

• Agriculture: Agricultural activities such as livestock farming and rice cultivation produce methane, which is another greenhouse gas that contributes to climate change.

These human activities have significantly disturbed the natural carbon cycle by extracting long-buried fossil fuels and burning them for energy, thus releasing CO2 to the atmosphere. The additional CO2 from human activities has disturbed the balance of the carbon cycle because the natural processes that could restore the balance are too slow compared to the rates at which human activities are adding CO2 to the atmosphere.