Which human activity is a major contributor to increased CO2 levels?

The major contributor to increased CO2 levels among the provided choices is the burning of fossil fuels. This activity releases significant amounts of carbon dioxide into the atmosphere, primarily due to the combustion of coal, oil, and natural gas for energy production, transportation, and industrial processes. When fossil fuels are burned, carbon that has been stored in these fuels over millions of years is released back into the atmosphere as CO2, contributing to the greenhouse effect and climate change.

In contrast, the other activities listed tend to have a neutral or positive impact on CO2 levels. Afforestation, for example, involves planting trees which absorb CO2, thus helping to mitigate climate change. Recycling reduces the need for new raw materials and minimizes energy consumption in production processes, which can also lead to lower emissions. Organic farming focuses on sustainable agricultural practices that often reduce the carbon footprint compared to conventional farming methods. Therefore, burning fossil fuels stands out as the primary human activity that significantly increases atmospheric CO2 levels.