Growing scale raises concerns about the stability of ecosystems and resulting risk of harm to human civilization.
Planetary Boundaries
Planetary boundaries is a framework that seeks to characterize several metrics that define a “safe operating space” for human civilization. The framework was introduced by Rockström et al. (2009) and substantially updated by Steffen et al. (2015) and Richardson et al. (2023). Nine planetary boundaries have been identified and quantified by Richardson et al. (2023).
Planetary boundaries identified and quantified in Rockström et al. (2009), Steffen et al. (2015), and Richardson et al. (2023). Image from the Stockholm Resilience Center.
The boundaries are chosen so as to replicate the ecological conditions that prevailed over most of the Holocene, the present geologic epoch. Richardson et al. (2023) fear that too great a transgression of the boundaries creates a risk of a climate change (defined more broadly than greenhouse gas-induced global warming) that will result in serious and irreversible harm to human civilization.
There is limited characterization of the interaction between the different metrics, but Richardson et al. (2023) argue that the transgression of one boundary will affect the risk gradients of the other boundaries. We will now briefly consider the nine boundaries.
Biosphere Integrity
Richardson et al. (2023) define two metrics related to biosphere integrity. The first is the rate of species extinction, for which the boundary is set at 10 extinctions per million species-years (10 E/MSY). In other words, on average no more than one of 100,000 species should go extinct every year. Barnosky et al. (2011), analyzing the apparent disappearance of species from fossil records, estimate an average extinction rate of 1.8 E/MSY over the past million years among mammals, higher than the 0.1 to 1.0 E/MSY that was found in earlier studies.
By comparison, Barnosky et al. (2011) observe extinction rates as high as 693 E/MSY over one year periods in the last 1000 years, and the rate was 9 E/MSY during the Pleistocene megafauna extinction event. Ceballos et al. (2015), based on analysis of the International Union of Conservation of Nature database of species, estimate the extinction rate of the past century as up to 200 E/MSY.
The second metric related to biosphere integrity is the human-appropriated share of net primary productivity (HANPP), replacing the Biodiversity Intactness Index of Steffen et al. (2015) that was found to be difficult to relate to Earth systems functions and difficult to quantify in an objective manner. HANPP includes the net primary productivity that is unrealized due to land use change. The boundary for HANPP is set at 10% of the preindustrial Holocene NPP, thereby not accounting for the growth of NPP that is attributed to carbon fertilization.
Krausmann et al. (2013) find that HANPP increased from 13% to 25% from 1910 to 2005, while population and economic output increased by factors of 4 and 17 respectively over the same time. Using a slightly different metric, Richardson et al. (2023) infer that HANPP was 23.5% in 2020.
Climate Change
Richardson et al. (2023) set a boundary of 350 parts per million of atmospheric carbon dioxide concentration and a radiative forcing of 1 watt per square meter, meaning the level additional energy that reaches the surface of the Earth relative to preindustrial (1750) norms as a result of elevated greenhouse gases. As of 2022, these metrics were at 417 ppm and 2.93 W/m² respectively. According to Lindsey (2024), preindustrial Holocene CO₂ concentration was typically 280 ppm or less.
Novel Entities
Novel entities refer to anthropogenic introductions into the biosphere, and Richardson et al. (2023) define them to include chemicals and substances without natural analogues, anthropogenic radioactive materials, and modifications of evolution, e.g. genetically modified organisms.
Following Persson et al. (2022), Richardson et al. (2023) does not regard any environmental release of novel entities to be safe unless they are thoroughly tested, such as under the European Union’s Registration, Evaluation, Authorisation and Restriction of Chemicals. Under the subset of entities that are covered by REACH, Persson et al. (2022) report that 20% of the chemicals that have been in use for at least 10 years have been tested to the satisfaction of the regulation.
Stratospheric Ozone Depletion
Richardson et al. (2023) assess the boundary for stratospheric ozone to be at least 276 Dobson units, 5% below preindustrial levels. They find the current value to be 284 DU. A Dobson unit, often used to measure trace gases in the atmosphere such as ozone, is a level of concentration such that if all the gas was purely concentrated on the surface at standard temperature and pressure, it would have a thickness of 10 micrometers (Goddard Space Flight Center). Nair et al. (2015), using ground- and space-based detectors, find that the ozone layer is recovering.
Freshwater Change
Richardson et al. (2023) consider two metrics related to freshwater: one is for blue water, which is freshwater contained in lakes, rivers, and aquifers, and the other is for green water, which is water stored in soil. For both types of water, the metrics are computed as follows. The world is divided into grid cells that are 30 arc-minutes on a side, and for each cell, a deviation is defined as a cell that is outside of its 95% variability range, whether it is too wet or too dry. The boundaries are that no more than 10% of cells should experience deviation for blue water, and no more than 11% should experience deviations for green water. Porka et al. (2024) assess these deviations presently to be 18.2% and 15.8% respectively.
The picture above shows a substantial worsening of the world’s freshwater metrics since the 2015 assessment (Steffen et al. (2015)). However, this worsening is due primarily to a change in the choice of metric rather than a worsening of actual conditions.
Atmospheric Aerosol Loading
Aerosols are fine particles suspended in the air, and they have natural and anthropogenic sources. Wang et al. (2021), for instance, find that anthropogenic sources contribute 29.6% of aerosols above the Arctic ocean and 33.4% of aerosols above middle and low latitudes.
Richardson et al. (2023) develop two metrics related to aerosol loading. The first is aerosol optical depth (AOD), the portion of direct sunlight that is blocked from reaching the surface of the Earth as a result of aerosols. The boundary for AOD is set at 0.25. The second metric is the interhemispheric difference in AOD. This metric is relevant since aerosol loading is known to play a significant role in monsoon patterns. The boundary is that the interhemispheric difference should be less than 0.1.
Vogel et al. (2022) find the mean global AOD to be 0.14, though in some regions AOD exceeds the boundary of 0.25. Zanis et al. (2020) find the interhemispheric difference to be 0.076 ± 0.006.
Ocean Acidification
As the oceans absorb excessive carbon dioxide from the atmosphere, they form carbonic acid, lowering the pH level and becoming more acidic (University of New Hampshire (2023)). A common metric for ocean acidification is Ω_arag, the concentration of aragonite ions in the ocean (National Oceanic and Atmospheric Administration (2015)). Richardson et al. (2023) set the boundary for Ω_arag to be at least 80% of the preindustrial level, which is 3.44. Jiang et al. (2015) estimate Ω_arag = 2.8, around 81% of the preindustrial value.
Land System Change
Richardson et al. (2023) use a measure of forest cover to set the boundary for land use change. Three biomes of interest are tropical forest, temperate forest, and boreal forest. Each of these should cover 85%, 50%, and 85% of their Holocene potential respectively for a weighted average of 75%. Using data from the European Union’s Copernicus Climate Change Service (EU Copernicus Climate Change Service), Richardson et al. (2023) find that current forest cover is about 60%.
Biogeochemical Flows
Biogeochemical flows refer to the alteration of the flows of elements in the environment. Richardson et al. (2023) focus on nitrogen and phosphorus. The global boundary for phosphorus is a sustained flow of 11 million tons per year of phosphorus from freshwater to the oceans. Carpenter and Bennett (2011) find the flow to be 22 million tons per year. An additional phosphorus boundary of 6.2 million tons per year of application of fertilizers to edible soils is set, and several studies cited find that the actual application is between 17.5 and 32.5 million tons per year.
A boundary of 62 million tons of anthropogenically fixed nitrogen per year is also set, and the actual value has been estimated at 190 million tons per year (Food and Agriculture Organization of the United Nations).
Shortcomings of Planetary Boundaries
A key virtue of Richardson et al. (2023), and of the preceding papers that establish the planetary boundaries framework, is that they integrate several disparate ecological topics into a single framework. It is useful for attaining a big picture of the current world ecological situation. However, the paper suffers from several weaknesses.
Blomqvist, Nordhaus, and Shellenberger (2012) respond to Rockström et al. (2009), the original planetary boundaries paper. Applying their critiques to the more recent Richardson et al. (2023), Blomqvist, Nordnaus, and Shellenberger (2012) observe that of the nine planetary boundaries, only three–climate change, ocean acidification, and stratospheric ozone depletion–can be said to be truly global in scope. The other impacts are regional in scope, in that, for instance, excessive nitrogen and phosphorus flows, deforestation, or high aerosol optical depth in one world region have little bearing on the values of these parameters in another world region. Thus the aggregation of all world values of these metrics into a single parameter is meaningless.
The highly critical Montoya, Donohue, and Pimm (2018) asserts that the planetary boundaries framework posits tipping points, whose values are not known with any precision, but which represent a threshold of serious harm. Rockström, Richardson, and Steffan (2017) deny that this is a proper characterization of the framework. However, the concept of planetary boundaries is coherent only if one posits that a change in environmental conditions might result in sharply nonlinear feedbacks, a phenomenon that would differ from a “tipping point” in semantics only.
That said, Richardson et al. (2023) do not provide any evidence for the existence of tipping points or even indicate what they might be. The precautionary principle is invoked to justify this omission.
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