The nine planetary boundaries are a framework mapping the safe limits for humanity to avoid the effects of unacceptable human-induced environmental change. Unacceptable environmental change is the risks humanity faces in the transition of the planet from the Holocene, the desired planetary state developed over the past 10 000 years, to the Anthropocene, the Earth’s current state originating with the industrial revolution.

Alarmingly, the article proposes that the planetary boundaries are already exceeded for climate change, biodiversity loss and the global nitrogen cycle. Seven of the boundaries are quantified and measurable, with the exception of chemical pollution and atmospheric aerosol loading due to spatial variability and estimating impacts. Importantly, the boundaries are a tentative guide and there continues to be knowledge gaps that require further research into Earth System and resilience science.

The planetary boundaries must be understood as interconnected and interdependent parts within the whole Earth System. The Earth System is a self-regulating set of the biophysical boundaries which sustain humanity. A transgression in one boundary will result in consequences for the others. For example, at the regional scale, deforestation in the Amazon in a changing climate regime may reduce water resource availability in Asia.

The nine planetary boundaries include:

Climate change

The approach to defining the climate change boundary is based on a scientific understanding of what is required to avoid the crossing of critical thresholds that separate qualitatively different climate system states using both atmospheric CO2 concentration and radiative forcing as global-scale control variables.

Ocean acidification

Ocean acidification poses a challenge to marine biodiversity and the ability of oceans to continue to function as a sink of CO2 (currently removing roughly 25% of human emissions).

Stratospheric ozone depletion

Stratospheric ozone filters ultraviolet radiation from the sun. The appearance of the Antarctic ozone hole was a textbook example of a threshold in the Earth System being crossed.

Biogeochemical nitrogen and phosphorous cycles

Local to regional-scale anthropogenic interference with the nitrogen cycle and phosphorus flows has induced abrupt shifts in lakes and marine ecosystems. Human induced influxes of nitrogen and phosphorus can push aquatic and marine systems across thresholds.

Biodiversity loss

Biodiversity changes can have pervasive effects on Earth System functioning and interact with several other planetary boundaries. For example, loss of biodiversity can increase the vulnerability of terrestrial and aquatic ecosystems to changes in climate and ocean acidity, thus reducing the safe boundary levels for these processes.

Global freshwater use

Global manipulations of the freshwater cycle affect biodiversity, food, and health security and ecological functioning, such as provision of habitats for fish recruitment, carbon sequestration, and climate regulation, undermining the resilience of terrestrial and aquatic ecosystems.

Land system change

Land system change, driven primarily by agricultural expansion and intensification, contributes to global environmental change, with the risk of undermining human well-being and long-term sustainability.

Atmospheric aerosol loading

Atmospheric aerosol loading is an anthropogenic global change process and a potential planetary boundary for two  reasons, first, the influence of aerosols on the climate system and secondly, their adverse effects on human health at a regional and global scale.

Chemical pollution

Primary types of chemical pollution include radioactive compounds, heavy metals, and a wide range of organic compounds of human origin. Chemical pollution adversely affects human and ecosystem health, which has most clearly been observed at local and regional scales but is now evident at the global scale.

The planetary boundaries approach rests on three branches of scientific inquiry. First, the scale of human action in relation to the capacity of the Earth to sustain it, second, draws from research on the essential processes of the Earth, as captured by the nine boundaries and third, is the framework of resilience. Examples for each of the nine planetary boundaries are explored highlighting their distinct function while demonstrating their effects and dependence on other boundaries. For instance, atmospheric CO2 concentration, classified under climate change, poses a challenge to marine biodiversity and the ability of the ocean digest CO2 increasing ocean acidity. Marine biodiversity is compromised, organisms are sensitive to changes in oceanic CO2 levels especially organisms with shells and skeletal structures.

The planetary boundaries approach offers a unique contribution to research by focusing on the biophysical processes of the Earth System that determine the self-regulating capacity of the planet. Understanding the boundaries will identify the ways in which humanity can maintain itself within the limits to secure a sustainable future.

The article identifies the need for further research to address the knowledge gaps in analysing the risks and uncertainties in the thresholds of the Earth System. Only through understanding the interconnectedness of the nine planetary boundaries will humanity know how to manage the Earth’s vulnerabilities.

1. Sustainable development is reliant on humanity to respect the limits of the nine planetary boundaries. 
2. The article makes the ethical case for the importance of environmental stewardship by defining the safe limits of human development in the face of unacceptable human-induced environmental change.
3. The article supports the economic case towards sustainable finance by offering a framework to conceive and comprehend the ecological impacts of alarming anthropocentric development.
4. The Earth System is defined as the integrated biophysical and socioeconomic processes and interactions among the atmosphere, hydrosphere, cryosphere, biosphere, geosphere, and anthroposphere in both spatial and temporal scales, which determine the environmental state of the planet within its current position in the universe. Therefore, humans and their activities are fully part of the Earth System, interacting with other components.
5. The planetary boundaries are already exceeded for climate change, biodiversity loss and the global nitrogen cycle.
6. Observations demonstrating anthropocentric climate change include a rapid retreat of summer sea ice in the Arctic Ocean, retreat of mountain glaciers around the world, loss of mass from the Greenland and West Antarctic ice sheets, increased rate of sea-level rise in the last 10-15 years, a 4° latitude pole-ward shift of subtropical regions, increased bleaching in coral reefs and a rise in the number of floods.
7. Despite the evidence of an acceleration of anthropogenic pressures on the biophysical processes of the Earth System, current governance and management paradigms are often oblivious to or lack a mandate to act upon these planetary risks.
8. The Montreal Protocol is an example of international success in reversing the trend with regard to the stratospheric ozone boundary.
9. As seen from Fig. 6, the estimates indicate that humanity is approaching, moreover at a rapid pace, the boundaries for freshwater use and land-system change.
10. See Appendix 1 for supplementary information including detailed descriptions of the climate change boundary, additional descriptions of the interactions between boundaries, the method for identifying and defining planetary boundaries and additional references supplying further information.