What is a major effect of stratospheric ozone layer depletion?

Stratospheric ozone layer depletion has significant implications for human health, particularly in increasing the incidence of non-melanoma skin cancers. The ozone layer serves as a protective shield by absorbing the majority of the sun's harmful ultraviolet (UV) radiation. When the ozone layer is depleted, more UV radiation reaches the Earth’s surface. This increase in UV exposure is directly linked to a rise in skin cancer cases, including non-melanoma types such as basal cell carcinoma and squamous cell carcinoma, as well as other health issues like cataracts and immune system suppression.

The other options address potential biological or environmental effects that are not primarily associated with ozone layer depletion. For instance, higher mean temperatures in the Northern Hemisphere could be influenced by a variety of factors, including climate change, rather than being a direct consequence of ozone depletion. Similarly, an increase in phytoplankton populations and the rate of photosynthesis in plants are typically linked to other environmental conditions, such as nutrient availability and CO2 levels, rather than the direct effects of increased UV radiation due to ozone depletion. Therefore, the connection between stratospheric ozone layer depletion and the increased incidence of non-melanoma skin cancer is a well-established and significant public health concern.