3.3. Planetary boundaries

Planetary boundaries is probably the most discussed conceptual framework for the Earth system in public and political discussions (apart from climate change). Since the end of the last glacial period - about 12 000 years ago - the Earth's climate has remained very stable (this period is called the Holocene) and human civilisations have developed under these stable conditions. However, human impacts on the climate and on the Earth system in general have intensified steadily, especially since the industrial revolution, threatening to change the way the Earth system works.

A team of researchers led by Johan Rockström sought to identify Earth system processes and associated thresholds that, when exceeded, could lead to a change in the Earth system away from the stable conditions that prevailed during the Holocene and were favourable to human societies. In a paper published in 2009, the team identified nine processes and associated thresholds: climate change, chemical pollution, upper atmospheric ozone depletion, atmospheric aerosol loading, ocean acidification, nitrogen and phosphorus cycling, freshwater use, change in land use, and biodiversity loss.

In 2015, the same team of researchers, this time led by Will Steffen, refined the previous analysis. In their newer analysis, the researchers emphasised that two processes, climate change and biosphere integrity (previously referred to as biodiversity loss), are "core processes" that are linked not only to each other but to all the other studied processes. While exceeding the thresholds associated with other processes may cause serious harm to human well-being and increase the risk of exceeding the thresholds of the "core processes", the other processes alone are unlikely to lead to large-scale change in the whole Earth system. However, the climate system and the biosphere integrity are Earth-wide systems within which all other systems operate, and major changes in these systems are likely to shift the Earth system away from the conditions that prevailed during the Holocene.

Figure. Planetary boundaries according to Steffen et al. (2015). Green represents the safe range for human activity, yellow represents increased risk and red represents high risk. Credit: J. Lokrantz/Azote Images. 

Planetary boundaries thus describe scientists' view of "safety limits", the crossing of which increases the risk of changes that threaten human well-being. Planetary boundaries do not imply a tipping point in the system, but the greater the crossing of a safety limit, the greater the risks of crossing potential tipping points or other adverse changes. Next, we will take a very brief look at Steffen et al's 2015 analysis of planetary boundaries.

The safety limit for climate change is set at 350 parts per million (ppm) of carbon dioxide or, more generally, a 1 W/m² increase in anthropogenic radiative forcing relative to pre-industrial times. Both safety limits have already been exceeded, as reflected in global warming and the increase in associated phenomena such as extreme heat waves.

There are no established measures of biosphere integrity, but the researchers proposed two interim measures: the extinction rate (E/MSY; how many species per million become extinct per year) and the biodiversity intactness index (BII), which describes the impact of human activities on the abundance of populations of organisms. There are considerable uncertainties and difficulties in estimating both indicators, as well as the associated safety margins. The rate of population extinctions exceeds a threshold considered safe (below 10 E/MSY), but there is as yet no comprehensive global estimate of BII.

The lowest safety limit for upper-atmosphere ozone depletion (275 Dobson units) is only reached in Antarctica in the southern hemisphere spring, and the ozone layer is recovering as a result of reductions in emissions of ozone-depleting chemicals.

Ocean acidification is closely linked to atmospheric carbon dioxide levels and has increased by about 30% over the last two centuries. The safety limit is set in relation to the degree of saturation of aragonite, the material of the shells of calcareous aquatic organisms (at least 80% relative to pre-industrial times). The saturation level of aragonite is now about 84% relative to pre-industrial times.

The nitrogen and phosphorus cycles are part of the Earth's biogeochemical cycles, which scientists believe should be considered more broadly. The use of fertilisers in intensive agriculture has increased nitrogen and phosphorus run-off into water bodies, causing eutrophication and oxygen depletion, especially in freshwater ecosystems but also in shallow marine areas. Set safety limits are exceeded in central North America, western Europe, northern India and north-eastern China. 

As a safety limit for change in land use, the researchers chose the area of forest in relation to the original forest area. Different safety margins were set for different regions because the role of forests in regulating the planet's climate varies from one part of the world to another. Tropical forests are particularly important, with a safety limit of 85% of the original forest area. Temperate zones were set at 50% and northern coniferous forests at 85%. The set safeguard limits are far exceeded in Central Africa and South-East Asia. The loss of northern coniferous forests also exceeds the safety margin.

The freshwater use safety limits are set on a catchment basis (taking into account seasonal variations in flow) so that water-dependent ecosystems remain in good condition. These safety limits have been far exceeded, particularly on the west coast of North America, in the Mediterranean, in the Middle East, India and north-east China, but also in many other places where rivers have been dammed and water is used in large quantities.

Atmospheric aerosols (fine particles) from biomass and fossil fuel combustion, among other sources, cause health problems for millions of people, but planetary safety limits relate to the effects of fine particles on (regional) precipitation. On the Indian subcontinent, the amount of fine particles in the air is so high that it dims the sunlight reaching the earth by 10-15% and can reduce monsoon rainfall.

Novel entities (formerly chemical pollution) refers to any form of matter or modified organisms that have potentially harmful effects on the Earth's geophysical or biological systems. Typical examples include new synthetic chemicals, genetically engineered organisms and microplastics. Particular attention should be given to entities that (1) are persistent, (2) can spread widely, and (3) can affect important systems on Earth. The best example of the risks associated with such substances is probably chlorofluorocarbons (CFCs), which were originally considered harmless but which threatened to destroy the ozone layer protecting the Earth from ultraviolet radiation. The risks posed by these new entities are so poorly understood that it has not been possible to set a safety limit. 

It is important to note that the concept of planetary boundaries is concerned only with an Earth system that is safe for humans and human societies. The value of other forms of life is in this framework thus seen as important only in relation to human well-being, not as valuable in themselves. This demonstrates that in the natural sciences, very significant value assumptions are often made without much justification or questioning. 

It is also noteworthy that planetary boundaries do not highlight the interactions of the processes underlying the boundaries, even though the processes are in fact closely interlinked. Lade and colleagues (2020) studied these interactions and found that processes tend to be mutually reinforcing. For example, land-use change - mainly clearing of forests for agricultural production - accelerates climate change and ocean acidification by increasing greenhouse gas emissions, reduces biosphere integrity by destroying habitats for organisms, and causes eutrophication of water bodies by increasing fertilizer use. On the other hand, the linkages between processes also allow for actions that simultaneously uphold multiple planetary boundaries. For example, Lade and partners highlight the reduction of meat consumption, which would allow a reduction in agricultural activity and thus have a positive impact on multiple planetary boundaries.

It is therefore essential to note that the processes of the Earth system, as represented by planetary boundaries, are all connected not only to human activities but also to each other. Research shows that these processes are in general mutually reinforcing, which means that the 'safe limits' for human activity are in fact tighter than individual planetary boundaries would suggest: staying within safe limits requires staying well below the safe limit for a number of these processes.

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