Which of the following is NOT an example of a proposed planetary boundary?

The proposed planetary boundaries framework identifies critical thresholds in environmental processes that, if crossed, could lead to catastrophic environmental changes. The boundaries are designed to maintain a safe operating space for humanity with respect to the Earth system.

Freshwater use, ocean acidification, and stratospheric ozone depletion are all recognized planetary boundaries that scientists have identified as essential to monitor for maintaining ecological stability. Freshwater use addresses the consumption of freshwater resources, ocean acidification refers to the decrease in the pH of the oceans due to increased carbon dioxide absorption, and stratospheric ozone depletion highlights the thinning of the ozone layer that protects life on Earth from harmful ultraviolet radiation.

Loss of coral reefs, while a critical environmental issue, is not formally recognized as one of the planetary boundaries. While it reflects significant ecological concerns such as biodiversity loss, climate change impacts, and marine health, it does not fall into the predefined categories established within the planetary boundaries framework. Recognizing this helps to clarify the focus on global-scale environmental processes rather than specific ecosystem components.