Limits not to be exceeded if humanity wants to survive in a safe ecosystem
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. 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."
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. 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. 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.
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). 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. Freshwater change was concluded to be the 6th transgressed planetary boundary in 2023. ;templatestyles src="Template:TOC limit/styles.css" / ;
Framework overview and principles.
The basic idea of the Planetary Boundaries framework is that maintaining the observed resilience of the Earth system in the Holocene is a precondition for humanity's pursuit of long-term social and economic development. The Planetary Boundaries framework contributes to an understanding of global sustainability because it brings a planetary scale and a long timeframe into focus.
The framework described nine "planetary life support systems" essential for maintaining a "desired Holocene state", and attempted to quantify how far seven of these systems had been pushed already. Boundaries were defined to help define a "safe space for human development", which was an improvement on approaches aiming at minimizing human impacts on the planet.
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." The framework consists of nine global change processes. In 2009, two boundaries were already crossed, while others were in imminent danger of being crossed. Later estimates indicated that three of these boundaries—climate change, biodiversity loss, and the biogeochemical flow boundary—appear to have been crossed.
The scientists outlined how breaching the boundaries increases the threat of functional disruption, even collapse, in Earth's biophysical systems in ways that could be catastrophic for human wellbeing. While they highlighted scientific uncertainty, they indicated that breaching boundaries could "trigger feedbacks that may result in crossing thresholds that drastically reduce the ability to return within safe levels". The boundaries were "rough, first estimates only, surrounded by large uncertainties and knowledge gaps" which interact in complex ways that are not yet well understood.
The planetary boundaries framework lays the groundwork for a shifting approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, toward the estimation of the safe space for human development. Planetary boundaries demarcate, as it were, the "planetary playing field" for humanity if major human-induced environmental change on a global scale is to be avoided.
Authors.
The authors of this framework was a group of Earth System and environmental scientists in 2009 led by Johan Rockström from the Stockholm Resilience Centre and Will Steffen from the Australian National University. They collaborated with 26 leading academics, including Nobel laureate Paul Crutzen, Goddard Institute for Space Studies climate scientist James Hansen, oceanographer Katherine Richardson, geographer Diana Liverman and the German Chancellor's chief climate adviser Hans Joachim Schellnhuber.
Most of the contributing scientists were involved in strategy-setting for the Earth System Science Partnership, the precursor to the international global change research network Future Earth. The group wanted to define a "safe operating space for humanity" for the wider scientific community, as a precondition for sustainable development.
Nine boundaries.
Thresholds and tipping points.
The 2009 study identified nine planetary boundaries and, drawing on current scientific understanding, the researchers proposed quantifications for seven of them. These are:
For one process in the planetary boundaries framework, the scientists have not specified a global boundary quantification:
The quantification of individual planetary boundaries is based on the observed dynamics of the interacting Earth system processes included in the framework. The control variables were chosen because together they provide an effective way to track the human-caused shift away from Holocene conditions.
For some of Earth's dynamic processes, historic data display clear thresholds between comparatively stable conditions. For example, past ice-ages show that during peak glacial conditions, the atmospheric concentration of CO2 was ~180-200 ppm. In interglacial periods (including the Holocene), CO2 concentration has fluctuated around 280 ppm. To know what were like with an atmosphere with over 350 ppm CO2, scientists need to look back about 3 million years. The paleo record of climatic, ecological and biogeochemical changes shows that the Earth system has experienced tipping points, when a very small increment for a control variable (like CO2) triggers a larger, possibly catastrophic, change in the response variable (global warming) through feedbacks in the natural Earth System itself.
For several of the processes in the planetary boundaries framework, it is difficult to locate individual points that mark the threshold shift away from Holocene-like conditions. This is because the Earth system is complex and the scientific evidence base is still partial and fragmented. Instead, the planetary boundaries framework identifies many Earth system thresholds at multiple scales that will be influenced by increases in the control variables. Examples include shifts in monsoon behavior linked to the aerosol loading and freshwater use planetary boundaries.
"Safe operating spaces".
The planetary boundaries framework proposes a range of values for its control variables. This range is supposed to span the threshold between a 'safe operating space' where Holocene-like dynamics can be maintained and a highly uncertain, poorly predictable world where Earth system changes likely increase risks to societies. The "boundary" is defined as the lower end of that range. If the boundaries are persistently crossed, the world goes further into a danger zone.
It is difficult to restore a 'safe operating space' for humanity that is described by the planetary boundary concept. Even if past biophysical changes could be mitigated, the predominant paradigms of social and economic development appear largely indifferent to the looming possibilities of large scale environmental disasters triggered by human actions. Legal boundaries can help keep human activities in check, but are only as effective as the political will to make and enforce them.
Interaction among boundaries.
Understanding the Earth system is fundamentally about understanding interactions among environmental change processes. The planetary boundaries are defined with reference to dynamic conditions of the Earth system, but scientific discussions about how different planetary boundaries relate to each other are often philosophically and analytically muddled. Clearer definitions of the basic concepts and terms might help give clarity.
There are many many interactions among the processes in the planetary boundaries framework. While these interactions can create both stabilizing and destabilizing feedbacks in the Earth system, the authors suggested that a transgressed planetary boundary will reduce the safe operating space for other processes in the framework rather than expand it from the proposed boundary levels. They give the example that the land use boundary could "shift downward" if the freshwater boundary is breached, causing lands to become arid and unavailable for agriculture. At a regional level, water resources may decline in Asia if deforestation continues in the Amazon. That way of framing the interactions shifts from the framework's biophysical definition of boundaries based on Holocene-like conditions to an anthropocentric definition (demand for agricultural land). Despite this conceptual slippage, considerations of known Earth system interactions across scales suggest the need for "extreme caution in approaching or transgressing any individual planetary boundaries."
Another example has to do with coral reefs and marine ecosystems: In 2009, researchers showed that, since 1990, calcification in the reefs of the Great Barrier that they examined decreased at a rate unprecedented over the last 400 years (14% in less than 20 years). Their evidence suggests that the increasing temperature stress and the declining ocean saturation state of aragonite is making it difficult for reef corals to deposit calcium carbonate. Multiple stressors, such as increased nutrient loads and fishing pressure, moves corals into less desirable ecosystem states. Ocean acidification will significantly change the distribution and abundance of a whole range of marine life, particularly species "that build skeletons, shells, and tests of biogenic calcium carbonate. Increasing temperatures, surface UV radiation levels and ocean acidity all stress marine biota, and the combination of these stresses may well cause perturbations in the abundance and diversity of marine biological systems that go well beyond the effects of a single stressor acting alone."
Proposed new or expanded boundaries since 2012.
In 2012, Steven Running suggested a tenth boundary, the annual net global primary production of all terrestrial plants, as an easily determinable measure integrating many variables that will give "a clear signal about the health of ecosystems".
In 2015, a second paper was published in "Science" to update the Planetary Boundaries concept. The update concluded four boundaries had now been transgressed: climate, biodiversity, land use and biogeochemical cycles. The 2015 paper emphasized interactions of the nine boundaries and identified climate change and loss of biodiversity integrity as 'core boundaries' of central importance to the framework because the interactions of climate and the biosphere are what scientifically defines Earth system conditions.
In 2017, some authors argued that marine systems are underrepresented in the framework. Their proposed remedy was to include the seabed as a component of the earth surface change boundary. They also wrote that the framework should account for "changes in vertical mixing and ocean circulation patterns".
Subsequent work on planetary boundaries begins to relate these thresholds at the regional scale.
Debate and further research per boundary.
Climate change.
A 2018 study calls into question the adequacy of efforts to limit warming to 2 °C above pre-industrial temperatures, as set out in the Paris Agreement. The scientists raise the possibility that even if greenhouse gas emissions are substantially reduced to limit warming to 2 °C, that might exceed the "threshold" at which self-reinforcing climate feedbacks add additional warming until the climate system stabilizes in a hothouse climate state. This would make parts of the world uninhabitable for people, raise sea levels by up to , and raise temperatures by to levels that are higher than any interglacial period in the past 1.2 million years.
Change in biosphere integrity.
According to the biologist Cristián Samper, a "boundary that expresses the probability of families of species disappearing over time would better reflect our potential impacts on the future