Which is an effect of lower levels of solar radiation striking Earth?

Lower levels of solar radiation striking Earth lead to decreased rates of photosynthesis, which is a crucial process for plants and other photosynthetic organisms. Photosynthesis relies on sunlight to convert carbon dioxide and water into glucose and oxygen. When solar radiation is reduced, the energy available to drive this process diminishes, resulting in less glucose production. This can have cascading effects on the entire food web, as primary producers form the foundation for nearly all ecosystems.

The other options do not directly relate to the impact of reduced solar radiation in the same way. For instance, increased incidences of skin cancer are more closely associated with higher levels of UV radiation rather than lower. Increased phytoplankton activity typically occurs in more favorable conditions with adequate light, not reduced light, and decreased carbon dioxide levels are influenced by a variety of factors, including photosynthesis but not solely related to changes in solar radiation. Therefore, the connection between lower solar radiation and decreased rates of photosynthesis clearly illustrates the impact on the Earth's ecosystems.