Which planetary boundary measures the extinction rate of species?

The measure that specifically addresses the extinction rate of species is the rate of biodiversity loss. This planetary boundary focuses on the decline in both the abundance and variety of species within ecosystems as a result of human activities such as habitat destruction, pollution, and climate change. Maintaining a healthy level of biodiversity is crucial for ecosystem stability, resilience, and the provision of essential services like pollination, nutrient cycling, and water purification.

In contrast, the other options pertain to very different environmental issues. Climate change deals with the increase in global temperatures and its effects on weather patterns and ecosystems. Ocean acidification relates to the decreasing pH levels of ocean water due to increased carbon dioxide absorption, which impacts marine life, particularly organisms with calcium carbonate structures. Stratospheric ozone depletion involves the reduction of ozone in the stratosphere, which protects life on Earth from harmful ultraviolet radiation. Each of these options is critical to environmental sustainability, but they do not specifically measure the extinction rate of species like the rate of biodiversity loss does.