More on the PBs...

Regarding the last entry on the framework of planetary boundaries (PBs) introduced by Will Steffen (2009), which provides a science-based analysis on the risk that current anthropogenic activities will destabilise, in other words, threaten the resilience of the Earth system to any abrupt environmental change, Steffen has provided an updated version of his PBs analysis in his recent paper: Planetary boundaries: Guiding human development on a changing planet (Steffen et. al 2015). The core concept of PB remains, whilst the framework of it has been modified to conceptualise the substantial risks that human perturbations are imposing to the Earth system at both regional and global scale in a more sophisticated way. 

Source: Steffen W. et. al (2015), Planetary boundaries: Cuiding human development on a changing planet 347 (6223)

Comparing to Steffen’s previous analysis, climate change, stratosphere ozone depletion and ocean acidification are the three PBs remain essentially unchanged, which the latter two remain below safety boundary (see Figure 1). He also combines nitrogen cycle and phosphorus cycle into the planetary boundary of biochemical flows, which phosphorus cycle has drastically increased and reached beyond zone of uncertainty, as compared to previous studies. This gives climate change, biosphere integrity and biochemical flows remain beyond zone of uncertainty, with land-system change in the zone of uncertainty. 

As being one of the core PBs apart from climate change, Steffen has replaced ‘rate of biodiversity loss’ in his previous study by ‘biosphere integrity’, which is an aggregate of genetic diversity and functional diversity. Changes in genetic diversity and functional diversity impose different subsequent impacts on the Earth system, however the long-term concept is to achieve biosphere integrity - the combination of both below safety boundary to guarantee biodiversity and stable Earth system. The former ensures the complexity and long-term capacity of the biosphere to resile from abrupt or gradual environmental changes, while the latter manifests the role of biosphere in Earth system functioning (Steffen et. al 2015). Although a  global decline in species richness (genetic diversity) will not result in impaired functioning of the world’s ecosystem (Brook et. al, 2010), partly because of constant introduction of new species to replace native / generic species, it is arguable that complexity and diversity is key to the integrity and stability of the Earth system, which then improves its resilience (Polis, 1998).  

Source: Polis (1998) Nature 395 (6704), 744-45

Figure 2 shows a food-web complexity on a island in the Gulf of California, which illustrates an interwoven interactions between species in wet El nino years. Marine sources and land plants provide the fuel for an estimated 500-1,000 species  (Polis, 1998). In this case, a complex food web helps to moderate population fluctuations and prevents species loss. Take the spiders from this food web as an example. Spiders relies on a range of feeding sources / prey such as scavenging insects, herbivorous insects, detritivorous insects and so on, and in case there is a reduction in herbivorous insects due to a fire that burns down hectares of land plants, they may shift to consume other resources. As such, having many prey does not only help to secure the consumer’s feeding sources, but also preventing consumers from driving any particular prey to extinction, and so enhancing the diversity and stability within the system. 

Although the new analysis from Steffen provides a more sophisticated framework in explaining the nine PBs, it has its limits and analysis has to be taken further. Firstly, data for the atmospheric aerosol loading is regional (focus on the case study of South Asian Monsoon) and so the predicted boundary shall not be reliable enough to representative at a global scale. To take it further, different regions have their own climate and monsoon system, levels of atmospheric aerosol should then have various impacts on different monsoon system. Secondly, the framework only provides a guidance of decision making in societal development without showing any potential results in the Earth system in the long term. Steffen made his framework of planetary boundary based on the Holocene climatic levels and may not be applicable to today’s development. The Earth system may evolve to a new state that is compatible with the changing climate like how they have been adapted to different climates during the interglacial periods. In addition, technologies advancement and innovations may help to improve biosphere resilience and diversity in the face of climate change. As such, the concern is not about how unstable the Earth system would be, but how destructive an unstable Earth system could be in affecting a human civilisation. 

Meanwhile, Steffen's framework provides a valuable contribution to decision making in charting desirable courses for societal and economic development.