Not surprisingly, the planetary boundaries framework has also triggered scientific and broader debates, where some criticisms have been presented.
Critique is the basis of scientific progress and is welcomed in scientific endeavours such as the one we are engaged in. Nevetheless, several of the criticisms advanced in these discussions have been based on some key misconceptions.
Here, we address these.
The planetary boundaries framework (hereby called PB) has been criticised for not being well adapted to policy. It is important to stress that the planetary boundaries research is first and foremost designed to advance Earth System science.
However, the fact that it has already attracted considerable attention in the policy sector suggests that it could indeed become a useful policy tool with further development. Our assessment of why the PB framework resonates with governments, businesses and NGOs is the growing awareness of risks facing world development with continued increase in human pressures on a finite Earth system.
The conceptual framework for the planetary boundaries research is based on identifying biophysical boundaries that are intrinsic to the operation of Earth as a system. They are not policy based adaptations with assessments on land needs per capita, land productivity assumptions, or amount of land required to provide energy/food needs of a certain size of world population. In that sense, the concept is void of policy assumptions. In other words, we welcome policy related discussions, but only beyond the biophysical boundaries we are trying to identify. These biophysical boundaries, once identified, potentially provide a framework for policy decision-making.
The original scientific questions we posed were:
1. What environmental processes regulate the stability of the Earth system?
2. Do these processes have well-defined thresholds at global or regional levels, or do they contribute significantly to the resilience of the Earth System?
3. What boundary positions do they have?
It was a scientific effort to identify the ample evidence that Earth not only is a coupled self-regulating system, but also a system with finite limits.
We sought to identify boundary positions beyond which we cannot exclude non-linear changes in one or several sub-systems on Earth. It is up to societies to choose where the boundary position is placed. We chose to place it at the lower end of the uncertainty range in science as a measure of applying a precautionary principle (e.g., for climate change at 350 ppm (CO2)). One could also take a more risk prone approach, opting for the higher end of our analysis of uncertainty, in this case at 550 ppm (CO2). This is a social choice, but the range is based on an Earth System analysis.
Critics have suggested an additional normative dimension, that the biosphere is the basis for human wellbeing. We argue that this is no longer a normative issue for argument, but rather a fact based on empirical evidence.
Welcome to the Anthropocene
The PB research concludes, based on paleo-climatic evidence, that the environmental conditions during the Holocene is the only state that we know for certain can support the modern world we live in. It may be incorrectly perceived as a normative statement, but it is above all a robust and evidence-based conclusion which is difficult to refute:
1. Human civilizations only started to develop after the onset of the Holocene.
2. The Holocene is an unusually long and relatively stable inter-glacial period in the late Quaternary, providing a predictable and relatively low risk geophysical environment for the biosphere, which in turn provides the basis for human prosperity.
3. Within the Holocene environment we have propelled ourselves to a world economy hosting seven billion people committed to nine billion by 2050. Before the beginning of the Holocene, human numbers were much lower and we existed in hunter-gatherer societies only.
Assessing the boundaries
We note that even in the recent critical discussions over whether or not there are tipping points in the Earth System, most agree that there is strong scientific evidence of tipping points in the climate system, in the stratospheric ozone layer, for ocean chemistry (i.e., acidity) and for phosphorus. These are four out of our nine boundary processes.
Since the publication of the original Nature paper, Steve Carpenter and Elena Bennett have shown very clearly, based on ample evidence, that we have both a marine and a terrestrial planetary boundary, due to tipping points in freshwater ecosystems (Carpenter, S.R. and Bennett, E.M. (2011) Reconsideration of the planetary boundary for phosphorus. Environmental Research Letters 6: doi:10.1088/1748-9326/6/1/014009).
So, what about the remaining boundaries? Let's start with biodiversity. It's an interesting coincidence that almost at the same time as the recent Breakthrough Institute report was released, significant scientific support was added to a planetary boundary for biodiversity through two Nature articles published in June (Barnosky et al. doi:10.1038/nature11018; and Cardinale et al., doi:10.1038/nature11148).
Indeed, the Barnosky paper goes even further than we did: it demonstrates the risk for a planetary scale state shift if the human influence on biological systems (biodiversity and ecosystems) is pushed too far. The Cardinale paper neatly summarizes why biodiversity matters for human well-being and the risks associated with losing it.
Our own research acknowledged the difficulty of setting a planetary boundary on how far humanity can afford to lose biodiversity before triggering non-linear changes in ecosystem functioning, with flow-on effects for societies, but there is enough evidence to demonstrate the critical role biodiversity plays for ecosystem resilience, i.e., the ability of ecosystems to stay in a desired environmental state .
On land use change, we assessed how much of the Earth's land cover we can change for anthropogenic purposes before risking major shifts in ecosystem functioning (habitats for biodiversity; carbon sequestration; water flows through landscapes; moisture feedback from terrestrial ecosystems, etc).
We made it very clear that land itself is not associated with a global tipping point but rather contributes to, as a slow variable, the resilience of terrestrial ecosystems. This in turn is coupled with other boundaries such as water, biodiversity, nitrogen, phosphorus and climate.
There is ample evidence of how land use change has turned productive landscapes to degraded (much less productive) areas or forests to savannas and steppes. They are all examples of regional scale tipping points, and some of them have global consequences via atmospheric teleconnections (e.g., substantial conversion of the Amazon Basin forests to savannas or grasslands).
On water, there is scientific evidence of water induced tipping points at larger system scales. The Aral Sea is one example. Water has a regulating function for regional climate systems and is crucial for the stability of ecosystems. Needless to say, it is a sine-qua-non for all food production and carbon sequestration in landscapes. Unsustainable water use can push farming systems into degraded states.
We have always stressed the fact that many of the PB definitions are tentative. However, they all depart from a consistent, common approach of identifying non-linear change/tipping points that can have dramatic impacts for humans.
We concluded, as most critics should be aware of, that water, land, biodiversity loss, nitrogen and phosphorus all constitute "slow variables" in the Earth System. We never claimed there were "planetary tipping points" for these slow variables, but rather evidence of tipping points at local and regional scales that add up to a global concern if they occur at the same time in multiple places on Earth (thereby causing local social problems and triggering feedbacks affecting regional to global scale processes, such as the hydrological cycle or the climate system).
There is much agreement
Despite the criticisms that have been raised, the discussions demonstrate that there seems to be a shared view that biophysical thresholds do exist and that resource constraints are a challenge for prosperity in the world.
There also seems to be agreement on the scale challenge, because if operationalised, planetary boundaries need to translate to the relevant scale where both the environmental and governance processes occur (which was also explicitly acknowledged in the original Nature and Ecology and Society papers on Planetary Boundaries).
The governance implications of the planetary boundaries concept is a research challenge in its own right. This is why the original framework cannot simply be taken off the shelf and translated directly to operational policy. What it can do already at this stage, however, is to be used as a framework to guide sustainable development goals in the Anthropocene.
On behalf of Rockström et al. (2009),
Lead author and Executive Director, Stockholm Resilience Centre