Identifying Chemicals That Are Planetary Boundary Threats

Policy Analysis pubs.acs.org/est

Identifying Chemicals That Are Planetary Boundary Threats Matthew MacLeod,† Magnus Breitholtz,† Ian T. Cousins,† Cynthia A. de Wit,† Linn M. Persson,‡ Christina Rudén,† and Michael S. McLachlan*,† †

Department of Applied Environmental Science (ITM), Stockholm University, 10691 Stockholm, Sweden Stockholm Environment Institute, Linnégatan 87D, Box 24218, 10451 Stockholm, Sweden

‡

ABSTRACT: Rockström et al. proposed a set of planetary boundaries that delimit a “safe operating space for humanity”. Many of the planetary boundaries that have so far been identified are determined by chemical agents. Other chemical pollution-related planetary boundaries likely exist, but are currently unknown. A chemical poses an unknown planetary boundary threat if it simultaneously fulfills three conditions: (1) it has an unknown disruptive effect on a vital Earth system process; (2) the disruptive effect is not discovered until it is a problem at the global scale, and (3) the effect is not readily reversible. In this paper, we outline scenarios in which chemicals could fulfill each of the three conditions, then use the scenarios as the basis to define chemical profiles that fit each scenario. The chemical profiles are defined in terms of the nature of the effect of the chemical and the nature of exposure of the environment to the chemical. Prioritization of chemicals in commerce against some of the profiles appears feasible, but there are considerable uncertainties and scientific challenges that must be addressed. Most challenging is prioritizing chemicals for their potential to have a currently unknown effect on a vital Earth system process. We conclude that the most effective strategy currently available to identify chemicals that are planetary boundary threats is prioritization against profiles defined in terms of environmental exposure combined with monitoring and study of the biogeochemical processes that underlie vital Earth system processes to identify currently unknown disruptive effects.

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INTRODUCTION Rockströ m et al.1,2 introduced the concept of planetary boundaries that delimit a “safe operating space for humanity”. The concept emerged from a recognition that human activities can lead to impacts at a planetary scale that threaten the vital Earth system processes that allow humanity to thrive. Vital Earth system processes were defined by Rockströ m et al. as “biophysical processes of the Earth System that determine the self-regulating capacity of the planet”. One of the nine planetary boundaries identified by Rockström et al. is “chemical pollution”, and four other planetary boundaries they identified are also governed by chemical agents. We recently postulated that “chemical pollution” is not a single category in the planetary boundary framework, but rather a placeholder for currently unrecognized planetary boundaries governed by chemical agents.3 We dubbed chemical-related problems that could destabilize the Earth system and that we are currently ignorant about chemical pollution planetary boundary threats. Chemical pollution may pose unacceptable risks to human health or the environment on local, regional, and global scales, and society attempts to mitigate known risks and respond to new ones with continually evolving national and international regulations and guidelines. However, confronting currently unknown planetary boundary threats from chemical pollution is an important societal task that remains unaddressed. By highlighting the importance of vital Earth system processes as effect end points © 2014 American Chemical Society

for chemical pollutants, the planetary boundary concept addresses a dimension of chemicals management that is neglected in current chemicals policy. In a previous paper,3 we identified three conditions that must be simultaneously met for chemical pollution to pose a planetary boundary threat (Table 1). The first condition (Condition 1) is that the chemical pollution has a disruptive effect on a vital Earth system process. Importantly, we must recognize and accept that we are currently ignorant of this effect, as we are likewise currently ignorant of the planetary boundary. The second condition (Condition 2) is that the disruptive effect is not discovered until it is, or inevitably will become, a problem at the planetary scale. If the disruptive effect is discovered before it is a problem at the planetary scale, then it is not a planetary boundary threat since it would be possible to take action to control the chemical pollution before the planetary boundary is exceeded. The third condition (Condition 3) is that the effects of the pollution in the environment cannot be readily reversed. If they can be readily reversed, then exceedance of the planetary boundary could be corrected, and the problem does not fit our definition of a planetary boundary threat. In summary, chemical Received: Revised: Accepted: Published: 11057

April 17, 2014 August 28, 2014 September 2, 2014 September 2, 2014 dx.doi.org/10.1021/es501893m | Environ. Sci. Technol. 2014, 48, 11057−11063

Environmental Science & Technology

Policy Analysis

Table 1. Three Conditions That Must Be Simultaneously Fulfilled for Chemical Pollution to Pose a Planetary Boundary Threat, Scenarios for Chemicals to Fulfill Each Condition, And Profiles of Chemicals That Fulfill Each Scenario Condition

Scenario

Chemical Profile

C1 unknown disruptive effect on a vital Earth system process

C1-1 unknown disruptive effect on a vital Earth system process

C1-1 unknown, but less likely for chemicals that have been used and emitted for many years if production/emissions and environmental concentrations are not increasing.

C2 disruptive effect is not discovered until it is, or inevitably will become, a problem at a planetary scale

C2-1 rapid development of nearly homogeneous pollution levels at the global scalea C2-2 rapid global distribution of effects (independent of exposure) C2-3 time delay between exposure and effects C2-4 effects only observable on a global scale

C2-1 emissions dispersed globally combined with moderate mobility, or volatile and persistent chemicals that are rapidly distributed in the atmosphere.

C3-1 poor reversibility of exposurea

C3-1 emissions from a technology that society is highly dependent on or from multiple sources that are costly to eliminate, or the chemical is persistent in the exposure environment. C3-2 effects that are permanent or cause a regime shift to a new stable state.

C3 disruptive effect is poorly reversible

C3-2 poor reversibility of effects

C2-2 unknown, but an example would be a chemical that causes genetic changes in organisms followed by distribution and proliferation of the affected organisms. C2-3 unknown, but an example would be a subtle effect on long-lived organisms, or a transgenerational epigenetic effect. C2-4 unknown, but could arise as a result of global exposure to persistent chemicals distributed in the oceans or persistent and semivolatile chemicals distributed by a combination of atmospheric and oceanic transport.

a

The chemical agent causing disruptive effects may be a persistent, immobile compound formed from the transformation of a mobile compound, which in turn could be the transformation product of an immobile compound. Therefore, transformation products must be considered when chemicals are evaluated against these chemical profiles.

pollution is a planetary boundary threat if it impacts a vital Earth system process, if this impact is not discovered until the planetary boundary is exceeded, and if the effects that result from the exceedance cannot be readily reversed. Confronting chemical pollution planetary boundary threats requires that the causative agents be identified so that action can be taken to control pollution before the planetary boundary is exceeded. If society is rational and capable of responding to threats, then once the potential for a disruptive effect on a vital Earth system process is identified action will be taken to prevent exceedance of the planetary boundary. Experience with the planetary boundaries that have so far been identified indicates that society will not always act quickly and decisively to deal with known planetary boundary issues, but this failure to act is a separate problem from the challenge of identifying unknown planetary boundary threats. In this paper, we develop a set of profiles of chemicals that are potential planetary boundary threats. Using the three conditions for chemical pollution to pose a planetary boundary threat as a starting point, we define chemical pollution scenarios that lead to the fulfillment of each of the three conditions. Then, for each scenario we define chemical profiles in terms of the currently unknown effect of the chemical on a vital Earth system process, environmental fate properties, and characteristics of use and emission. Finally, we discuss the prospects for using the profiles to prioritize chemicals according to their potential to be chemical pollution planetary boundary threats. Scenarios for Chemical Pollution to Fulfill the Conditions to Pose a Planetary Boundary Threat. Our point of departure to develop profiles for chemicals that are potential planetary boundary threats is to identify scenarios in which a chemical could fulfill each of the three conditions for being a planetary boundary threat. A useful point of reference is a scenario that has characterized much of our experience with chemical pollutants and that has strongly influenced current chemical regulation. In this “reference scenario”, illustrated in Figure 1, society introduces a chemical into commerce without fully understanding its effects on the environment, and concentrations in “local” environments where the chemical is released begin to rise (solid green line and primary axis in Figure

Figure 1. Reference scenario in which pollution occurs in areas with local chemical emissions, unacceptable local effects occur, and there is effective chemical management after a response time ΔtRESP.

1). Eventually, the concentration in the environment crosses a critical threshold, and effects are observed in regions where the chemical is released locally (dashed green line and secondary axis in Figure 1). If the effects are unacceptable, society responds by curtailing emissions of the chemical, but this action requires a certain response time, ΔtRESP, during which concentrations continue to rise and effects become more prevalent. After ΔtRESP, concentrations begin to decline in response to society’s actions, and the prevalence of the effect of the chemical in the affected areas disappears. In the reference scenario illustrated in Figure 1 the chemical pollution is hardly dispersed into the global environment, and chemical concentrations fall rapidly after society acts to curtail emissions. A related scenario reflects effective management of persistent organic pollutants (POPs) that are identified under international regulations (Figure 2). The POPs scenario assumes that the potential for unacceptable effects at the global scale as a result of long-range transport is recognized such that the rise in concentrations can be halted before the critical threshold concentration is exceeded on the global scale, but in this case 11058

dx.doi.org/10.1021/es501893m | Environ. Sci. Technol. 2014, 48, 11057−11063


Policy Analysis

None of the following scenarios alone describes a planetary boundary threat since that requires fulfilling at least one scenario for each of Condition 1, Condition 2, and Condition 3. Scenarios Fulfilling Condition 1: Unknown Disruptive Effect on a Vital Earth System Process. In the reference scenario, society begins to use chemicals and they enter the environment as pollutants, then previously unknown disruptive effects are discovered at the local scale. Condition 1 is satisfied when the unknown effect disrupts a vital Earth system process. If we recognize and accept our prior ignorance of the effect, then it is apparent that classifying possible disruptive effects into scenarios is not possible. Therefore, there is only one relevant scenario for Condition 1: Scenario C1-1: Chemical Pollution Has an Unknown Disruptive Effect on a Vital Earth System Process. Scenarios Fulfilling Condition 2: Disruptive Effect Is Not Discovered until It Is, Or Inevitably Will Become, A Problem at a Planetary Scale. In the reference scenario, the unwanted effect of chemical pollution is discovered at the local scale and remains spatially contained. Condition 2 is satisfied if the disruptive effect is not discovered until it occurs, or inevitably will occur, on the scale of the entire planet. There are at least four scenarios that could lead to this condition being satisfied:

Figure 2. A scenario for effective management of a persistent organic pollutant under current international regulations.

concentrations in the environment do not respond quickly to society’s actions. Below, we explore scenarios that deviate from those illustrated in Figures 1 and 2 such that one of the three conditions for chemical pollution to pose a planetary boundary threat is fulfilled.

Figure 3. Scenarios for chemical pollution to fulfill Conditions 2 and 3 for being a planetary boundary threat. A, Concentrations are nearly homogeneous on a global scale (C2-1) and exposure is poorly reversible (C3-1). B, Effects are rapidly distributed globally (C2-2) and are poorly reversible (C3-2). C, Time delay between exposure and effects (C2-3). D, Effects are only manifested at the global scale (C2-4). Another variation of Panel B is conceivable that combines C2-2 with poorly reversible exposure (C3-1), but rapid global distribution of effects independently of the global distribution of concentrations implies that the effect is poorly reversible. 11059



Policy Analysis

Scenario C2-1: Concentrations Are Nearly Homogeneous on a Global Scale. The entire planet can be exposed to chemical pollution that is rapidly distributed in the global environment. In this case the difference between concentrations of the chemical at the local scale and at the global scale is small enough that an effect can occur at the global scale without society having sufficient time to take action to ensure the critical concentration is not exceeded at the global scale (Figure 3A). Scenario C2-2: Effects Are Rapidly Distributed Globally. Although the pollution is largely constrained in space, once effects occur on the local or regional scale they are rapidly distributed around the planet. In analogy to Scenario C2-1, the response time of society is longer than the time needed for the effect to be distributed around the planet, causing a global scale problem (Figure 3B). Scenario C2-3: There Is a Time Delay between Exposure and Effects. A time delay in the manifestation of effects at the local scale, ΔtDELAY, combined with the time delay for society to respond to the effect results in global concentrations exceeding the critical threshold before chemical management actions can arrest the increase in environmental concentrations (Figure 3C). Scenario C2-4: Effects Are Only Observed on a Global Scale. The adverse effect is not manifested until the chemical is distributed at a planetary scale. Even though there is a strong gradient in chemical concentration in the environment the disruptive effect is not manifested in more highly contaminated local areas near sources until the concentrations on a planetary scale exceed the threshold required to disrupt the vital Earth system process (Figure 3D). Scenarios Fulfilling Condition 3: Disruptive Effect Is Poorly Reversible. In the reference scenario society decides to take action to reduce the concentrations of chemical pollution in the environment and, after a response time ΔtRESP, the prevalence of the effect associated with the chemical pollution declines in concert with declining concentrations. Condition 3 is satisfied when the disruptive effect is poorly reversible. There are at least two scenarios that could lead to Condition 3 being satisfied: Scenario C3-1: Exposure to the Chemical Pollution Is Poorly Reversible. The emissions of the chemical cannot be readily reduced, or the environmental contamination is not rapidly reversible. Exposure to the chemical pollution may be poorly reversible if society is highly dependent on a technology that releases the chemical, if the emissions come from long-lived products that cannot be rapidly removed from society, or if the chemical has high environmental persistence (Figure 3A). Scenario C3-2: The Effects of the Chemical Pollution Are Poorly Reversible. The disruptive effect itself is persistent even after levels of the chemical pollution have been reduced. Effects may be poorly reversible if the pollution has caused a regime shift, that is, a change to a new, stable state. If a regime shift occurs, reducing pollutant levels will not mitigate the effect (Figure 3B). Chemical Profiles. Chemicals must have a certain profile to fulfill each of the scenarios described above. The profile may be defined by properties of the substance, or by the way the substance is used. Chemical profiles that fulfill each of the scenarios are summarized in Table 1, and below we give examples of chemicals or combinations of chemical properties and use patterns that match the profiles. Note that chemicals named as examples do not necessarily represent planetary boundary threats since at least one scenario from each of the three conditions must be fulfilled for a chemical to pose a planetary boundary threat. As noted in the footnote of Table 1,

transformation products must be considered when chemicals are evaluated against some of the chemical profiles. Profile C1-1: Unknown Disruptive Effect on a Vital Earth System Process. Confronting our ignorance requires us to recognize that we cannot exclude any chemicals from potentially fulfilling Scenario C1-1. Therefore, the most precautionary approach is to assume that all chemicals that are in the environment as a result of human activity fulfill this scenario. However, there is also a range of other possible approaches to prioritize chemicals according to their likelihood to fulfill the scenario that apply different levels of precaution. For instance, one might assign a lower priority to chemicals that have been in use for a long period of time without having disrupted vital Earth system processes, provided that levels in the environment are not increasing. Further lowering the level of precaution, low priority might also be assigned to chemicals that have structures that are similar to chemicals that have been nondisruptive, provided that their anticipated concentrations in the environment are not higher. Profile C2-1: Nearly Homogeneous Environmental Exposure at the Global Scale. Chemicals that are volatile and persistent in the atmosphere, such as carbon dioxide, become nearly well-mixed in the troposphere of the northern and southern hemisphere from a single emission source within months, and globally in just a few years.4 However, chemicals are unlikely to be emitted from a single source. It is more likely that emissions occur from many sources distributed on the major continents. This emission pattern, combined with moderate mobility in the environment can lead to a situation in which there are not strong gradients in contamination levels in the environment, and Condition 2 could be satisfied. Examples of chemicals that fit this profile are lighter (mono- to tetrachlorinated) PCBs and hexachlorobenzene (HCB). A combination of their widespread historical use and mobility in the atmosphere has led to atmospheric levels of lighter PCBs and HCB being similar across the northern hemisphere.5 Levels of HCB and lighter PCBs in global background surface soils are also similar when normalized to soil organic content.6 Profile C2-2: Rapid Global Distribution of Effects. The nature of the disruptive effect caused by the chemical is a defining characteristic of the chemical profile for this scenario, and by definition the effect is unknown. However, fulfilling this scenario requires that the effect be local or regional in nature, but with the potential to become a global scale problem due to mobility. In addition to mobility, the effect must be self-amplifying in order to expand its sphere of influence from the local/regional to the global scale. Organisms that have been modified by chemicals are candidates for such effects. The development of antibiotic resistant bacteria as a result of exposure to chemical pollution is an illustrative example of a chemical-induced effect that initially may have a local or regional impact but could become a global scale problem.7,8 Current use practices for antibiotics have promoted antibiotic resistance that is now considered a global health concern.8 Since antibiotics are released to the environment via wastewater (e.g., during manufacturing and following use), the development of antibiotic resistance in the environment is also of concern.9 Resistance traits can be exchanged between bacteria, and global trade and travel allows resistant bacteria to spread rapidly,8 implying that the disruptive effects could be selfamplifying. Profile C2-3: Time Delay between Exposure and Effects. As for Profile C2-2, the chemical profile for Scenario C2-3 is closely linked to the unknown effect of the chemical on a vital Earth 11060



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chemicals that fit the profile of having poor reversibility of environmental contamination due to high environmental persistence are the aforementioned, extremely persistent fluorinated surfactants (e.g., PFOS and PFOA) which will remain in circulation in the environment for centuries to come. Profile C3-2: Effects Are Poorly Reversible. Effects can be poorly reversible if they cause permanent damage, or if the vital Earth system process responds very slowly to the change in emissions. Effects can also be poorly reversible if they result in a shift to a new stable regime. In the case of a regime shift, reducing contaminant levels would not result in a return to the original regime. Two well-known chemically induced cases of regime shifts are (i) greenhouse gas-driven global warming causing loss of sea and land ice, and possibly disrupting ocean currents driven by thermohaline circulation and (ii) eutrophication, which is the ecosystem response in a water body to the addition of excessive nutrients, especially nitrates and phosphates. Freshening of the water in the Arctic Ocean due to increased riverine inputs and melting ice is expected to irreversibly affect ocean circulation16 and cause regime shifts in ecosystems,17 whereas eutrophication has been shown to irreversibly alter ecosystem structure.18 As we are, by definition, ignorant of the nature of the effects in a planetary boundary threat context, further refining the chemical profile for this scenario is difficult. Research Priorities to Identify Chemical Pollution Planetary Boundary Threats. All of the three conditions must be simultaneously met for a chemical to be a planetary boundary threat. Hence, one approach to screening for potential planetary boundary threats would be to evaluate chemicals against the chemical profiles developed for the different scenarios that fulfill the three conditions. If a chemical fits one (or more) chemical profile for each condition, then it would have the potential to pose a chemical planetary boundary threat. Chemicals that fit one or more profiles but cannot be conclusively categorized as fitting a profile for each of the three conditions would be candidates for more detailed study, monitoring, and for precautionary substitution or restrictions on use. Before we can begin prioritizing chemicals as planetary boundary threats, criteria must be defined for the chemical profiles. The chemical profiles are defined in terms of the nature of the effect of the chemical on a vital Earth system process and the nature of the exposure of the environment to the chemical. Given that the effect of the chemical is, per definition, unknown, it is more difficult to screen chemicals against the chemical profiles that are defined in terms of the effect than against those defined in terms of environmental exposure. Clearly the most challenging chemical profile to evaluate chemicals against is Profile C1-1, since it is solely defined in terms of the unknown effect of the chemical on a vital Earth system process. While one could take the most precautionary approach and classify all chemicals introduced to the environment by anthropogenic activities as fulfilling C1-1, experience indicates that only a very small fraction actually have disruptive effects on a vital Earth system process. Hence, developing criteria for prioritizing chemicals for their potential to disrupt a vital Earth system process is an important challenge in the context of chemical pollution planetary boundary threats. For instance, chemicals might be assigned a low potential of fulfilling Profile C1-1 when society has long experience with the chemical, concentrations in the environment are not increasing, and no disruptive effects have been observed. The prioritization might then be extended to substances with similar chemical structures

system process, but the requirement that the manifestation of the effects must be delayed compared to exposure introduces constraints. The time delay of the effect is linked to the environmental transport and fate properties of the chemical since the chemical must transition from being a local or regional pollutant to being a global pollutant during the delay phase. Short delays in the manifestation of effects (i.e., of the order of several years) are unlikely to be sufficient to allow a chemical that is not already highly mobile in the environment (and thus already fulfills scenario C2-1 (nearly homogeneous environmental exposure on a global scale)) to make this transition. It is more likely that a time delay of the order of a decade or more is required. Chemicals capable of making the transition from regional-scale to global-scale pollutants on the time-scale of decades are persistent and semivolatile chemicals that are distributed in the atmosphere but partition reversibly to water, soil and vegetation, and persistent water-soluble substances that are transported in the global oceans.10 One example of delayed effects that occur on the time scale of decades is reproductive impairment in long-lived organisms that are only manifested in the second or third generation of offspring. Profile C2-4: Effects Are Only Manifested on a Global Scale. This profile differs from profile C2-3 in that the effects occur with little or no time delay, but they are not manifested as an unwanted effect until they are global in scale. Thus, Profile C2-4 is not dependent on the time scale for global distribution of the chemical, and time scales of a century or longer are conceivable. Chemicals that combine high persistence with some degree of mobility in the atmosphere or oceans fit this profile. For example, perfluorinated alkyl acids (e.g., perfluorooctanesulfonic acid, PFOS and perfluorooctanoic acid, PFOA) are persistent, strong surfactants that are becoming globally distributed in the world’s oceans11 following more than 60 years of global emissions.12 If fluorinated surfactants have an effect on a vital Earth system processes, the nature of the effect is currently not known. However, it is known that organic surfactants present in the oceans can be transferred onto sea spray aerosols, significantly increasing the concentration of cloud condensation nuclei (CCN) and thus cloud albedo.13 An increase in cloud albedo leads to more efficient absorption of solar radiation, and contributes to global warming. Hypothetically, if fluorinated surfactants contribute to increases in global CCN concentrations, the effect would only manifest itself once fluorinated surfactants are distributed on a global scale. Profile C3-1: Environmental Exposure Is Poorly Reversible. The chemical profile that fulfills this scenario can be determined by use characteristics of the chemical, or by intrinsic properties. Environmental exposure will be poorly reversible if emissions of the chemical cannot be readily reduced because society is dependent on technological services that cause emissions of the chemical, or because it is released from products that have a long usage lifetime. Two examples illustrate these two variations of poor reversibility of emissions. The first is carbon dioxide formed by the combustion of fossil fuels. Society is highly dependent on the combustion of fossil fuels, and this technological dependency has so far made reducing emissions very difficult. Polychlorinated biphenyls (PCBs) in sealants and other materials common in buildings are an example of a chemical in products with a long usage lifetime. In this case the poor reversibility of PCB emissions results from the multitude of diffuse sources and the high cost of controlling each of them.14,15 Environmental exposure is also poorly reversible for chemicals that have high overall environmental persistence. Examples of 11061



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against C2-1 and C3-1 will be trivial. On the contrary, the development of efficient and robust procedures is certain to present interesting research challenges. Experience with the existing hazard assessment indicators long-range transport potential (LRTP) and chemical persistence (P) will inform this process, but they do not generally address all aspects of C2-1 (e.g., the influence of spatial distribution of emissions on homogeneous environmental exposure) or C3-1 (e.g., the reversibility of emissions) so new methodologies will be required. Furthermore, there are difficult issues that environmental managers must confront. For instance, what is the speed of reversibility of chemical pollution that society would require in order to feel confident in its ability to mitigate a planetary boundary issue? Confronting the challenge of chemical planetary boundary threats should not be confused with or detract from chemicals management motivated by protection of human and ecosystem health at the local and regional scale. There are some common considerations, but the challenge posed by planetary boundary threats from chemical pollution is unique. Although there are considerable uncertainties, prioritizing chemicals in commerce against some of the chemical profiles defined in this paper appears achievable from the perspective of our existing experience with chemical assessment and regulation. For other chemical profiles a more substantial research effort may be required to make them operational. Given the high uncertainties associated with evaluating chemicals against Profile C1-1, monitoring and studying the biogeochemical processes that underlie vital Earth system processes is probably the most effective strategy to identify chemicals with unknown disruptive effects. Chemicals that fit one or more of the profiles for C2 and C3 should receive high priority for exploratory research about potential effects on vital Earth system processes, and should be candidates for precautionary substitution or phase out, especially when concentrations in the environment are increasing.

using read-across techniques, provided they are also not increasing in concentration in the environment. The applicability of such approaches could be increased by expanding the number of chemicals in commerce that are monitored in the environment. However, such approaches are not sufficient. We must recognize the possibility that any prioritization of chemicals against Profile C1-1 could be rendered invalid by the discovery of a currently unknown disruptive effect on a vital Earth system process. The history of identification of the chemical pollutionrelated planetary boundaries originally presented by Rockström et al. provides some guidance about what is needed to identify unknown effects of chemicals on vital Earth system processes. In 1985, a totally unexpected ozone hole was discovered over Antarctica,19 and the hole was eventually shown to be caused by CFC-catalyzed ozone depletion within polar stratospheric clouds.20 Another example is emissions of carbon dioxide from fossil fuel burning, which were long thought to not pose a threat, but are now recognized to be causing climate change and ocean acidification.21 In both these cases the chemical agents causing an unknown planetary boundary threat were only identified after a disruption of a vital Earth system process was discovered. History thus suggests that understanding the function of key Earth system processes and monitoring their status are vital activities to confront the challenge posed by chemical planetary boundary threats. Vigilance and an improved understanding of processes vital to the Earth system will also contribute to a reduction in the response time of society, ΔtRESP, thereby reducing the likelihood of scenarios C2-1, C2-2, and C2-3 occurring (Figure 3). Profiles C2-2, C2-3, C2-4, and C3-2 all include characteristics of the unknown effect on an Earth system process. However, in each case there are restrictions on the nature of the effect or the nature of the exposure that causes the effect that can be exploited to prioritize chemicals. As outlined above, for Profile C2-2 (rapid global distribution of effects), only a subset of possible disruptive effects will be mobile. The unique challenge for evaluating chemicals against C2-2 will be identifying possible scenarios for mobile effects. Similarly, only a subset of disruptive effects will be poorly reversible, and thus fulfill Profile C3-2. Above, we identified two different effect classes: irreversible effects, or a regime shift. There may be some possibility to assess the likelihood that a chemical will cause either irreversible effects or regime shifts, but research will be required to elucidate this. Both Profile C2-3 (time delay between exposure and effects) and Profile C2-4 (effect only manifests at global scale) require that global exposure to the chemical pollution occurs over long time scales. This requirement provides a basis to prioritize chemicals based on a combination of persistence, mobility and geographic extent of emissions. Profiles C2-1 and C3-1 are defined only in terms of the nature of environmental exposure to chemical pollution, and criteria and models can likely be developed to screen chemicals against these profiles. For instance, Profile C2-1 (nearly homogeneous environmental exposure on a global scale) can be assessed if there is an understanding of how the chemical will be emitted to the environment, and of its mobility and persistence. Likewise, Profile C3-1 (poor reversibility of exposure) is related to the persistence of the chemical in the environment, and chemical fate and transport models developed in recent years provide a conceptual basis for assessing reversibility of chemical contamination, provided reliable and representative data about degradation rates in air, water and soils can be obtained.22,23 This is not to say that defining methodologies to evaluate chemicals

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46 8 674 7228; fax: +46 8 674 7638; e-mail: michael. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by faculty funding from the Department of Applied Environmental Science, Stockholm University, Sweden. Astrid Safron (www.astridsafron.eu) created the TOC Art for this paper.

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REFERENCES

(1) Rockström, J.; Steffen, W.; Noone, K.; Persson, A.; Chapin, F. S.; Lambin, E. F.; Lenton, T. M.; Scheffer, M.; Folke, C.; Schellnhuber, H. J.; et al. Planetary boundaries: Exploring the safe operating space for humanity. Supplementary Information. Ecol. Soc. 2009, 41. (2) Rockström, J.; Steffen, W.; Noone, K.; Persson, A.; Chapin, F. S.; Lambin, E. F.; Lenton, T. M.; Scheffer, M.; Folke, C.; Schellnhuber, H. J.; et al. A safe operating space for humanity. Nature 2009, 461, 472− 475. (3) Persson, L. M.; Breitholtz, M.; Cousins, I. T.; de Wit, C. A.; MacLeod, M.; McLachlan, M. S. Confronting unknown planetary boundary threats from chemical pollution. Environ. Sci. Technol. 2013, 47, 12619−12622. (4) Jacob, D. J. Introduction to Atmospheric Chemistry; Princeton University Press: Princeton, NJ, 1999. 11062



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