Some authors have identified the opportunity to use the DPSIR framework to determine a causal relationship between human activity and the Planetary Boundaries [70, 72, 74]. Two of the national adaptations of the PBs use a methodology based on the DPSIR framework [70, 72]. However, neither study applied this approach across all of the PBs. Nor did either propose a set of indicators that were uniformly of the Pressure category.

The Planetary Boundaries are presented as distinct control variables with explicit limits. This is by design to make them easily communicable [12]. In reality, there is a high level of interconnectivity between the PBs. For example, almost every PB affects biosphere integrity. Exceeding one PB affects our ability to remain within others.

For the PQs to be a robust translation of the PBs, this interconnectivity must be carried over to the PQs. It would not be suitable to translate each PB to a PQ without consideration of all of the other PBs and PQs. To manage this, the method used was to first translate each of the PBs into a list of critical pressures based on the scientific literature (see Additional file 1: Table S1), and then from this list, PQs were developed.

There are many pressures which only have minor contributions towards the PBs, so an exclusion protocol was applied for pressures which contribute less than 1% towards current global impacts. Excluding minor impacts is common practise in environmental assessment protocols as a means to simplify the process with minimal effect on the results [75]. In total, thirty-two critical pressure were found. These were then analysed to determine which of the pressures could be grouped, and to find appropriate Pressure indicators to assess these with. The result was ten Pressure indicators which formed the basis of the PQ development.

Each of the PQ indicators found corresponds to one or more of the critical pressures and therefore one or more PB(s). The PQ limits were thus determined by assessing each of the corresponding PBs and selecting the most stringent limit.

The translation of the PBs to pressures and then to PQs is shown in Fig. 5. The direct relationship between the PBs and PQs is shown in Fig. 6. Two of the Planetary Boundaries have previously been identified as “core boundaries” for their high level of interconnectivity – Climate Change and Biosphere Integrity [12]. Each of these correspond to more than half of the Planetary Quotas (see Fig. 6).

Fig. 5 Translating Planetary Boundaries to Critical Pressures and then to Planetary Quotas Full size image

Fig. 6 The relationship between the Planetary Boundaries and Planetary Quotas Full size image

The PQs are summarised in Table 3. The scientific basis for each PQ is described briefly below. More detailed descriptions are included in the Supplementary Information where needed.

Table 3 The Planetary Quotas Full size table

A quota for carbon dioxide emissions

Carbon dioxide (CO 2 ) emissions is a critical pressure which affects several of the PBs (see Additional file 1). The PB that translates to the most stringent PQ for CO 2 is the PB for the concentration of CO 2 in the atmosphere of ≤350 ppm. The concentration of CO 2 in the atmosphere is currently ≥400 ppm, i.e., this PB has been exceeded. No other pressures were grouped with CO 2 for this PQ because the only way to meet the PB for CO 2 concentration is through the uptake of CO 2 from the atmosphere. The indicator selected for this PQ is thus net carbon dioxide emissions; net because to return to 350 ppm will require uptake of CO 2 from the atmosphere.

There are several pathways for rapid decarbonisation in the literature e.g., [76,77,78,79]. However, only one of these shows the concentration of CO 2 in the atmosphere as returning to 350 ppm within this century [76]. This pathway entails rapid reductions in CO 2 emissions of 15% per annum starting no later than 2020 followed by net CO 2 uptake from 2030 to 2080, and net zero emissions thereafter. The proposed uptake of CO 2 is approximately constant at 7.3Gt/yr. from 2050 to 2080. Thus, the limit is set as net carbon dioxide emissions ≤ − 7.3Gt/yr (see Additional file 1 for further detail).

All of the PQs should be reassessed over time. This is particularly so for the PQ for CO 2 emissions. If 15% reductions do not start by 2020 this PQ will need to be amended at this time. Any delay will mean substantially higher reductions will be required.

A quota for methane and nitrous oxide emissions

Methane and nitrous oxide emissions (hence forth referred to as Me-NO) are the only two long-lived greenhouse gases in the list of critical pressures (Additional file 2) which can have positive limits whilst respecting the PBs. As such, these pressures have been grouped under the PQ for Me-NO. It is common practice to assess impacts of greenhouse gases (GHGs) in terms of the amount of CO 2 emissions that would result in the same amount of global warming – equivalent CO 2 (CO 2 e), this is the unit that has been selected here.

The PB most affected by Me-NO emissions is the PB for radiative forcing, i.e., a change in radiative forcing since preindustrial forcing ≤ ±1 W/m2. However, there are too many different factors which influence radiative forcing (e.g., greenhouse gas emissions, albedo (Earth’s reflectivity), and aerosol emissions) to use this PB to derive specific limits for Me-NO.

The IPCC has identified several emissions pathways for the future. Even the most stringent of these, RCP2.6, does not meet the PB for radiative forcing this century. However, the 2100 targets for Me-NO under RCP2.6 have been derived on the basis of optimal food production with minimal emissions and minimal land use. It can be shown that these targets are sufficient to respect the PB for radiative forcing (see Additional file 1).

Nitrous oxide is also an ozone depleting substance so the limits for nitrous oxide emissions must also be considered in the context of the PB for ozone depletion. It can be shown that the RCP2.6 2100 target is unlikely to prevent the PB for ozone depletion from being respected (see Additional file 1).

Thus, the RCP2.6 2100 targets have been used as the basis of the PQ for Me-NO. The limit is gross Me-NO emissions ≤ 5.4GtCO 2 e/yr.

A quota for forestland

There are several critical pressures which relate to land use and land-use change (see Additional file 1). Forestland is of particular significance however, because it plays an integral role in the carbon, water, and nitrogen cycles. Forests also provide habitat for over 80% of terrestrial species [80]. Forestland function cannot be offset by other land types. As such, there are two PQs pertaining to land use, the PQ for forestland (discussed here) and the PQ for biodiversity which addresses land use more broadly, but with specific consideration for the impacts of land use on biodiversity (see section “A Quota for Water”).

The decarbonisation pathway used to determine the limit for CO 2 emissions [76] (see section “A Quota for Carbon Dioxide Emissions”) and the PB limit for land use of global forest land ≥75% of original forest area [12] both suggest that approximately 0.9Gha of reforestation will be needed by the end of this century (see Additional file 1). Applying this linearly over the remainder of the century gives a PQ for forestland of deforestation ≤ -11Mha/yr.

A quota for ozone depleting substances

The hole in the ozone layer is an example of how significant damage from human activity can be, and of how effective global action can be in restoring planetary health. The Montreal Protocol is a universally ratified agreement to phase out the production and use of ozone depleting substances. It has been predicted that if the Protocol is respected, i.e., that Montreal gases are phased out, the Planetary Boundary for ozone depletion will be too [5]. Not all ozone depleting substances are included under the Protocol, the notable exception being nitrous oxide. However, provided the PQ for Me-NO is respected, it is unlikely that nitrous oxide emissions would cause the PB for ozone depletion to be exceeded (see Additional file 1).

Montreal gases have different effects on ozone but can be collectively measured in the unit ozone depleting potential kilograms (ODPkg), which is a measure of the relative impact on the ozone of different gases compared to a benchmark substance. The PQ for ozone is set at Montreal gas emissions ≈ 0 ODP kg (see Additional file 1 for more detail).

A quota for aerosols

Aerosols are small particles suspended in the air. They can be released directly, or form as a result of emissions of precursor gases. There was not previously a Pressure indicator for the collective measurement of aerosols and precursors that could be related to the state of the atmosphere.

Aerosol optical depth (AOD) is an optical measure of the concentration of particles in the air. It is the ratio of incident light either scattered or absorbed by airborne particles in a vertical column of air [81]. An AOD of 1 indicates that no light can pass. An AOD of 0 indicates perfectly clear skies.

Meyer and Ryberg have proposed a new unit, equivalent aerosol optical depth (AODe).Footnote 1 Characterisation factors have previously been proposed to link annual mass of emissions of aerosols and precursors to globally averaged change in AOD. Building on this approach, Meyer and Ryberg used these factors to link emissions from an activity to global average AOD and thus determine the AOD equivalent (AODe). This should not be confused with an estimation of actual change in AOD. Such an estimation would be highly inaccurate because of variations to local conditions and the interactions between different aerosols and precursors. AODe provides a link between emissions, the Pressure indicator, and the resultant optical depth, the State indicator. It is thus an appropriate indicator for the PQ for aerosols.

The World Health Organisation suggests that no level of particulate concentration is safe for human health, suggesting an AODe of zero would be the most appropriate. However, the impacts of aerosols on global warming must also be considered. Aerosols have a net cooling effect in the atmosphere and are believed to have substantially dampened the warming effects experienced so far because of greenhouse gas emissions. Eliminating them entirely could lead to accelerated warming which could be more harmful to humanity than a small amount of particulate concentration remaining in the atmosphere.

The PB for radiative forcing is linked to the PQs for CO 2 , Me-NO, forestland, Montreal gases, and aerosols. Using the previously discussed PQs, and the PB for radiative forcing, a range of acceptable AODe levels can be determined as 0.05 ≤ AODe ≤0.13. The WHO guidelines for maximum particles in the atmosphere can be translated to an upper limit of AODe ≤0.1, which is in line with the PB for aerosols. Thus, the PQ for aerosols is 0.04 ≤ AODe ≤0.1 (see Additional file 1 for additional details).

A quota for nitrogen

Reactive nitrogen is necessary to grow food. However, the overuse of nitrogen can cause algal blooms and create anaerobic dead zones in rivers, lakes, and oceans. The PB for nitrogen is 62TgN/yr. of intentionally fixated nitrogen. This is a Pressure indicator, yet it cannot easily be compared to human activity. Further, it is not the fixation of nitrogen that causes algal blooms. Rather, it is the loss of nitrogen to the environment. Thus, the indicator for the PQ for nitrogen is net nitrogen lost to the environment. This includes virtual nitrogen that has been lost to the environment during the production of food and products, and the nitrogen released in excreta, less any nitrogen recovered, for example through the denitrification of waste water.

The PB for nitrogen was set on the basis of estimates for critical environmental limits for of nitrogen in surface runoff [82]. This basis is also suitable for the PQ indicator. As such, the PQ for nitrogen is net nitrogen lost to the environment ≤ 62 TgN/yr (see Additional file 1 for additional details).

A quota for phosphorus

Like nitrogen, phosphorus is also necessary to grow food but can cause algal blooms if used excessively. The PB for phosphorus is a flow of no more than 11 TgP/yr. from freshwater systems to the ocean. The limit is set at a point where the risk of a global anoxic ocean event is considered low [4].

This indicator is a Pressure indicator but not one that is easily comparable to human activity. A more accessible indicator has been selected for the PQ for phosphorus – net phosphorous released to the environment. It can be assumed that most phosphorus released to the environment will eventually make its way to the oceans. As such, the PQ for phosphorus is set at the same level as the PB for phosphourus i.e., net phosphorous released to the environment ≤ 11 TgP/yr.

A quota for water

Water availability varies significantly across the globe. In some areas it is plentiful. In others, it is very scarce. It is not feasible to transport water over long distances. For this reason, the concept of a global limit for water is debated. However, only a small fraction of total water consumption is direct consumption of local water. The far larger percentage of water consumed is “virtual water”, i.e., water used in the production of goods. Unlike water in its useable form, virtual water is traded globally. Approximately 40% of Europe’s water footprint is imported. We argue that the global distribution of water through trade justifies a global limit for water.

The PB for water consumption is for gross blue water consumption ≤4000 km3. Blue water refers to fresh surface water and groundwater, i.e., the water found in freshwater lakes, rivers and aquifers. Precipitation on land is classified as green water. The authors of the PBs acknowledge that green water is a scarce resource and should be considered within the PBs. However, because of the difficulty in defining a green water boundary they used blue water as a preliminary proxy indicator [5, 12].

Blue water consumption is not a suitable proxy for use in environmental accounting as this would imply that the use of green water, for example rain fed crops, has no impact. On the contrary, human appropriation of green water can result in loss of soil moisture and a decline in moisture feedback of vapour flows [5]. Further, 74% of the global average water footprint of production between 1996 and 2005 was from green water [83].

Gross water consumption is also a poor proxy indicator for environmental accounting purposes as it ignores the end state of the water. Net water consumption and the inclusion of grey water, i.e., the amount of water required to assimilate pollutants in water, gives a more holistic indicator of human appropriation of the water cycle.

The indicator for the PQ for water is therefore net blue, green, and grey water. There is no clearly defined global limit for this indicator. However, on the basis that more than 30% of major groundwater sources are currently being depleted, it is argued by some that we are already at, if not beyond the limit [84]. The PQ for water is thus ≤8500km3 based on the current global water footprint (which includes blue, green, and grey water consumption) [84] (see Additional file 1 for further detail).

Some water accounting experts believe that a weighted water footprint would better account for regionality [85] (see section “The Planetary Quotas in Context” for a discussion on regionality and the weighted water footprint).

A quota for biodiversity

There are five key drivers of biodiversity loss [5, 52, 86,87,88]:

a) climate change – shifting habitat to an extent that it is no longer suitable for the threatened species; b) pollution that affects the health of species; c) overexploitation of species, especially due to fishing and hunting but also overuse of ecosystem services leading to aforementioned habitat loss; d) spread of invasive species or genes outcompeting endogenous species; and e) habitat loss, fragmentation or change, especially due to agriculture, large-scale forestry, and human infrastructure.

Climate change is considered under the PQs for CO 2 , Me-NO, forestland, Montreal gases, and aerosols. Pollution is considered under the PQs for aerosols, water, nitrogen, phosphorus, and novel entities. The remaining three drivers have complex and diverse pathways. A study by the Convention on Biodiversity CBD [89] summarised the primary drivers for over 500 invasive species and found over 40 drivers ranging from purposeful release for measures such as erosion control, and hunting, to escape of pets, contamination of international trade objects, and stowaways on container ships. At this time, no Pressure indicator exists to account for all three of these drivers.

Land-use change is considered by many to be the greatest threat to biodiversity [88, 90,91,92,93,94,95]. For this reason, the use of land-based indicators as a proxy for biodiversity is common practise. The Ecological Footprint is often used as a proxy indicator for biodiversity health on the basis that it is a measure of how much biologically productive land is used by humans. Some level of overexploitation of marine and terrestrial species is taken into account in this metric [86]. The problem with using this indicator is that there is little consensus as to an appropriate limit [46, 86, 91, 96,97,98].

In a UNEP report on life cycle indicators, the need for a scalable indicator to assess the land use related impacts on biodiversity was identified and a new indicator proposed [99]. The indicator is called the percentage disappeared fraction (PDF) of species. This indicator is similar to the Ecological Footprint in that different types of land are weighted in terms of relative impacts. However, it has been specifically developed as a proxy indicator for biodiversity loss. Moreover, the unit can easily be equated to the Planetary Boundary for biosphere integrity – extinction rate – as both are expressed in terms of the percentage of extinct (or disappeared) species. The difference between the PDF and extinction rate is in their determination. Extinction rate is determined through observation – it is an Impact indicator. In contrast, PDF is estimated using land use data – thus a Pressure indicator. The PQ for biodiversity it thus PDF ≤1E-4/yr.

The purpose of the UNEP report was to propose indicators that allow better consistency in the development and communication of green products. This differs from the purpose of the Quotas in that the Quotas are intended to be the basis of a global Planetary Accounting Framework that can be used for any scale of human activity. In the instance of the UNEP report, there is little need to account for positive land transformation. As such, all of the “correction factors” – numbers used to convert land transformation to percentage disappeared fraction – are positive (i.e. they lead to biodiversity loss). Further work will be required to determine correction factors for positive transformation which results in biodiversity gains.

A quota for novel entities

There is no indicator or limit proposed for novel entities in the PB framework. However, they are included as a PB to give an indication of their importance to planetary health. The authors of the PB framework define novel entities as new substances, new forms of existing substances and modified life-forms that have the potential for unwanted geophysical and/or biological effects.

The environmental impacts from novel entities most often occur because of the disposal of these. The release of toxins into waterways. The disposal of waste to landfill. The disposal of plastic into oceans. As such, we propose the indicator net imperishable waste measured in kilograms to account for the wide variety of novel entities.

There is no specific limit proposed in the literature for this metric. However, there is evidence that we are beyond the limit. For example, 83% of tap water samples from 12 nations have been found to be contaminated with plastic [100]; methane from landfills and wastes contributes approximately 23% of global methane emissions [101]; most fish which are high in the food chain now contain high levels of heavy metals such as mercury [102]. The PQ for novel entities is therefore net imperishable waste ≈ 0 kg/yr.

The choice of net rather than gross waste is to allow environmental impact assessment results to show negative imperishable waste disposal. In this way, activities such as landfill mining which result in a net removal could be encouraged. Value could be assigned to such activities to allow for trading of impacts within a global cap. Further work should be undertaken to determine whether a zero limit is sufficient.