What natural phenomenon can lead to the expansion of deserts?

The correct answer is desertification, as it specifically refers to the process through which fertile land becomes increasingly arid and eroded, ultimately leading to the expansion of deserts. This phenomenon typically occurs in semi-arid regions due to various factors such as climate change, unsustainable land management practices, deforestation, and overgrazing. Over time, desertification results in the degradation of the land's ability to support vegetation, contributing to the spread of desert conditions.

While urbanization, deforestation, and overgrazing can contribute to environmental degradation, their relationship to desert expansion is more indirect or specific to particular contexts compared to desertification. Urbanization generally relates to the growth of cities rather than the direct transformation of landscapes into desert-like conditions. Deforestation may locally affect land use and biodiversity but does not encompass the broader spectrum of land degradation characteristic of desertification. Overgrazing, while a significant issue in some areas, is often a contributing factor to desertification itself rather than a standalone process leading to the expansion of deserts. Thus, desertification captures the essence of how these changes ultimately lead to larger desert areas.