Planetary boundaries

Planetary boundaries are a framework to describe limits to the impacts of human activities on the Earth system. Beyond these limits, the environment may not be able to self-regulate anymore. This would mean the Earth system would leave the period of stability of the Holocene, in which human society developed.^[2][3][4] The framework is based on scientific evidence that human actions, especially those of industrialized societies since the Industrial Revolution, have become the main driver of global environmental change. According to the framework, "transgressing one or more planetary boundaries may be deleterious or even catastrophic due to the risk of crossing thresholds that will trigger non-linear, abrupt environmental change within continental-scale to planetary-scale systems."^[2]

The normative component of the framework is that human societies have been able to thrive under the comparatively stable climatic and ecological conditions of the Holocene. To the extent that these Earth system process boundaries have not been crossed, they mark the "safe zone" for human societies on the planet.^[3] Proponents of the planetary boundary framework propose returning to this environmental and climatic system; as opposed to human science and technology deliberately creating a more beneficial climate. The concept doesn't address how humans have massively altered ecological conditions to better suit themselves. The climatic and ecological Holocene this framework considers as a "safe zone" doesn't involve massive industrial farming. So this framework begs a reassessment of how to feed modern populations.

The concept has since become influential in the international community (e.g. United Nations Conference on Sustainable Development), including governments at all levels, international organizations, civil society and the scientific community.^[5] The framework consists of nine global change processes. In 2009, according to Rockström and others, three boundaries were already crossed (biodiversity loss, climate change and nitrogen cycle), while others were in imminent danger of being crossed.^[6]

In 2015, several of the scientists in the original group published an update, bringing in new co-authors and new model-based analysis. According to this update, four of the boundaries were crossed: climate change, loss of biosphere integrity, land-system change, altered biogeochemical cycles (phosphorus and nitrogen).^[7] The scientists also changed the name of the boundary "Loss of biodiversity" to "Change in biosphere integrity" to emphasize that not only the number of species but also the functioning of the biosphere as a whole is important for Earth system stability. Similarly, the "Chemical pollution" boundary was renamed to "Introduction of novel entities", widening the scope to consider different kinds of human-generated materials that disrupt Earth system processes.

In 2022, based on the available literature, the introduction of novel entities was concluded to be the 5th transgressed planetary boundary.^[8] Freshwater change was concluded to be the 6th transgressed planetary boundary in 2023.^[1]