Justification for the planetary boundary selection

What makes a planetary boundary process a planetary boundary process?  Well, the key criteria that have to be fulfilled is that it’s an environmental process that is part of regulating the ability of the Earth system to remain in our current desired state, the Holocene equilibrium that has enabled human development of the past 10 000 years.

Together with scientists across the world we plunged into this challenge over several years of enquiry to try and identify what are all the environmental processes that qualify to this criterion of being absolutely fundamental in regulating the resilience and stability of our desired state of the planet? And the result is nine planetary boundary processes.

And I can tell you that this enquiry was extremely challenging, and we turned every stone of evidence to see what are the processes that could qualify to play this role? And we were actually ourselves surprised that there were only nine processes rather than 20 or 30 processes.

And this was put out, and it’s been put out for scrutiny for several years, and there’s so far not been any scientific suggestion of adding a tenth or eleventh process, or taking away one of the nine. So we’re today quite confident that if humanity can manage these nine processes within safe boundaries we have a very high likelihood of enabling a prosperous future for humanity on a stable planet.

Among these nine processes we have different types of planetary boundary processes, some of which have scientific evidence of planetary scale tipping points, some of them which do not have evidence of planetary scale tipping points, but which under the hood of the Earth system regulate the stability of those who have global scale tipping points, or that they have themselves sub-planetary scale tipping points at the ecosystem or biome scale, which if they cross tipping points at enough places in the world simultaneously could cause an impact at the planetary scale.

And this is why we define them as different categories of planetary scale and slow variables, which do not have evidence of global scale tipping points. And scientific evidence exploring the paleo record of how oceans have developed in the geological history of Earth indicates that oceans have large scale planetary level tipping points related to acidification, which makes oceans qualify as a planetary scale boundary process.

The planetary boundary process is an environmental process that is fundamental in regulating the ability of planet Earth to remain in the Holocene-like state. And the scientific enquiry here has then been exploring and deepening our understanding of what are the different processes that determine, for example, the ability for the climate to stay stable; for our polar regions to stay as they are today for the ability of our forests and oceans and land areas to continue to produce both food, air quality, and fresh water as it has been doing for the past 10 000 years. And in doing that and sharing that analysis with international leading scientists across the world the result is nine planetary boundary processes.

Now what is fundamentally important to recognize is that to be and to fulfill the criterion of a planetary boundary process does not require that that process is associated with a planetary scale tipping point. The key is what are the processes that regulate the ability of the entire planet to stay in the Holocene? But among the nine processes that we’ve identified, three of them have evidence of planetary scale tipping points, as shown in this graph.

That is the climate system. Clearly we know that in the past history of the Earth system the climate system has been pushing the entire planet in and out of glacial and interglacial periods, for example.

Ocean acidification is another such process where we see paleoscientific evidence that the entire ocean can go from anoxic to oxic events, so basically oxygen-free or oxygen-rich states, that the ocean can actually flip between different stable states.

And clearly the stratospheric ozone layer, which is the protective layer in the upper atmosphere, which protects the entire biosphere from harmful radiation from the Sun.

But then we have, perhaps more surprisingly, six processes that science now believes qualify as planetary boundaries but which don’t have planetary scale tipping points. And they fulfill the criterion for two reasons. One is that they play a fundamental role in regulating whether or not the large-scale processes potentially could cross a tipping point. So for example land use systems, fresh water use, and biodiversity, which are fundamental in providing the capacity of land areas to be carbon sinks, and if that ability is not there climate system would very rapidly cross a tipping point. And another example is the ability of land areas to sequester and keep nutrient flows intact as a way of regulating the amount of different pollutants in the air.

So among the processes which have this ability of operating under the hood of the Earth system and regulate its ability to stay in the Holocene we’ve identified four, which are biosphere processes forming part of planetary boundaries.

One is the interference, or the way we manage the large biogeochemical flows of nitrogen and phosphorus, which together with carbon are the big cycles in the world.

Atmospheric aerosol loading, which is the amount of soot and pollutants in the air, which in turn regulates the stability of the large rainfall systems, for example in tropical regions, such as the monsoon.

Global fresh water use, which is one a significant greenhouse gas, as water vapor, but also the fundamental role of water as the bloodstream of the entire biosphere, regulating the amount of biomass which in turn regulates the amount of carbon in the entire Earth system.

Land use change, which is the fundamental fabric for all living species on Earth.

And biodiversity, the genetic diversity from animals and vegetation and trees, forests, which overall determine the ability of the biosphere to cope with and adapt to changing conditions on Earth.

Now finally we also identify that there is most likely one final ninth planetary boundary, which we originally defined as chemical pollution, and increasingly talk of as new entities.

This is the recognition from increasing evidence, despite its complexity, that the cocktail of chemical accumulation in the biosphere could potentially cause major shifts in for example the genetic composition of species on Earth, which could be a tipping point in terms of life conditions on Earth.

So overall therefore nine planetary boundaries: three of which have evidence of large scale tipping points; climate, stratospheric ozone layer, and ocean acidification; four boundaries which operate a little bit more at the smaller scale but regulating the Earth system: biodiversity, land, water, nutrients, and fresh water; and two boundaries which are very heavily anthropogenically caused: both air pollution, which we call aerosol loading, and chemical pollution.

Now are there tipping points among these that operate below planetary scale? And the answer is yes, so that’s the second criterion why even those that do not have planetary scale tipping points qualify as planetary boundaries.

For example, biodiversity loss is increasingly shown to be involved in tipping points at ecosystem scale. And in the Anthropocene we see the risk that we can have tipping points occurring in so many places in the world that they aggregate into becoming a planetary concern.

So it’s not necessarily so that a system has to have one tipping point at the planetary scale, you can have multiple tipping points, and if they occur in enough places simultaneously they actually add up to a potential influence and impact factor at the Earth system as a whole. So there you have it in a nutshell where the planetary boundaries originate from.