Final answer:
The increase of carbon dioxide in Earth's atmosphere is primarily due to human activities such as the burning of fossil fuels and deforestation. These activities reduce the natural absorption of CO₂ through photosynthesis and add significant amounts of CO₂ through burning and land clearing.
Step-by-step explanation:
The activity that has increased the amount of carbon dioxide in Earth's atmosphere is the burning of fossil fuels, such as gasoline, coal, and natural gas. Fossil fuel burning accounts for a significant portion of the recent increase in atmospheric CO₂ levels. Additionally, deforestation, which includes the cutting of forests and clearing of land, contributes to this increase. When trees and plants are removed, not only is there less photosynthesis occurring to absorb CO₂, but the act of clearing often involves burning, which releases carbon dioxide that was previously stored in plants back into the atmosphere.
The increase in atmospheric carbon dioxide over time is a well-documented phenomenon, with levels now exceeding those found at any point in the last 800,000 years, and possibly as far back as 20 million years. Scientists attribute this drastic rise to human activities, and the effects are far-reaching, contributing to climate change and impacting sea levels and biodiversity. It is vital that societies shift towards non-carbon energy sources like hydro, solar, wind, and nuclear to mitigate these negative effects.