Final answer:
Human activities are the main cause of recent increases in atmospheric carbon dioxide and methane levels, predominantly due to burning fossil fuels and deforestation for carbon dioxide, and agriculture, landfills, and natural gas fields for methane.
Step-by-step explanation:
The recent spikes in atmospheric carbon dioxide and methane concentrations are primarily due to human activities, such as the burning of fossil fuels, deforestation, and agriculture. When fossil fuels like coal, gasoline, and natural gas are burned, significant amounts of carbon dioxide are released. Deforestation leads to a decrease in the number of trees that absorb carbon dioxide through photosynthesis and often involves burning, which releases even more carbon dioxide into the atmosphere.
Methane is another potent greenhouse gas that has increased in concentration due to human activities. Methane emissions come from various sources, such as the digestive processes of cattle, decay of organic material in landfills, and from natural gas fields. The melting of methane clathrates, as ocean temperatures rise, also contributes to higher levels of methane in the atmosphere. This results in a positive feedback loop that can accelerate global warming.
These human-contributed emissions dwarf the effects of natural processes such as volcanic activity and account for the majority of the observed increase in these gases since the industrial era.