Beyond Safe Operating Space: Finding Chemical FootprintingFeasibleLeo Posthuma,*,† Anders Bjørn,‡ Michiel C. Zijp,†,∥ Morten Birkved,‡ Miriam L. Diamond,§

Michael Z. Hauschild,‡ Mark A. J. Huijbregts,∥ Christian Mulder,† and Dik Van de Meent†,∥

†RIVM, Centre for Sustainability, Environment and Health, P.O. Box 1, 3720BA Bilthoven, The Netherlands‡DTU Management Engineering, Quantitative Sustainability Assessment, Technical University of Denmark, 2800 Kgs. Lyngby,Denmark§Dept Earth Sciences, 22 Russell Street, University of Toronto, Toronto, M5S 3B1, Canada∥Dept Environmental Science, Institute for Water and Wetland Research, Radboud University, 6525AJ Nijmegen, The Netherlands

■ OVERSHOOT?

Environmental overshoot occurs when human demands exceed thebiosphere’s regenerative capacities.1 Earth Overshoot Day (EOD)marks the day that humanity’s footprint exhausts the Earth’sannual regenerative capacity. The EOD of 2013, on August 20th,was memorable for the f irst author as it fell on his mother’s 89thbirthday. Each EOD, falling earlier every year, conf ronts us withurgent environmental problems, some of which are poorly def ined.One such example is chemical pollution, which threatens theEarth’s capacities. Rockstrom et al.2 listed chemical pollution as animportant but yet undef ined boundary in their selection ofplanetary boundaries delineating the “safe operating space forhumanity”. Can we use the well-known concept of “ecologicalfootprints” to express a chemical pollution boundary aimed atpreventing the overshoot of the Earth’s capacity to assimilateenvironmental pollution? Current literature is replete with ideas onthis, and shows the benef its of trans-disciplinary collaborations.Borrowing our subtitle f rom Don Mackay’s seminal paper thatintroduced fugacity-based modeling for quantif ying the environ-mental distribution of chemicals,3 we now see the development ofchemical footprinting that is feasible, relevant, and necessary for

expressing the overshoot of the Earth’s capacity. With widespread“chemical overshoot” leading to adverse ef fects of pollution, weargue for implementing a solution-focused assessment paradigm:Chemical footprinting helps identify scenarios that allow us toavoid “chemical overshoot” beyond the Earth’s safe operatingspace.

■ CHEMICAL FOOTPRINTS

The signs of chemical overshoot have been obvious for decades.Published in 1962, Rachel Carson’s Silent Spring described withdeadly accuracy the transgressions of chemical thresholds.Carson showed that pesticide emissions were insufficientlydiluted to be safe. Three decades later, the ecological footprintconcept was introduced,4 arguing that cities appropriate goodsand services from a land footprint far exceeding that of theirmunicipal boundaries. The ecological footprint allowed forconceptualizing, and later quantifying, the extent to which citiesand citizens overshoot their resource boundaries. Borrowingfrom the concept of ecological footprints and expanding onMackay’s principles, several research teams have proposed newmethods for estimating chemical footprints to situateanthropogenic pollution vis-a-vis the Earth’s safe boundaries.They argue for defining the chemical footprint as a dilutionrequirement necessary to maintain safe concentrations,rejuvenating the adage that “the solution to pollution is dilution”.Chemical footprinting uses chemical emissions and fugacity-based modeling, together with ecotoxicological data andmixture modeling to predict chemical impacts in relation toboundaries established to prevent ecological damage. Chemicalfootprints are expressed as ratios of required to available watervolumes within defined geographic areas: ratios below andabove unity reflect safe versus unsafe dilution space for emittedchemicals. Empirical evaluation of the chemical footprints usingcase studies show that the footprints capture observed trends.We propose that chemical footprinting can be used to assess the“safe operating space” needed to dilute chemical pollution.

Received: April 29, 2014Published: May 21, 2014

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■ ECOLOGICAL THRESHOLDS AND BOUNDARIES OFCHEMICAL POLLUTION?

The literature on the safe operating space with respect tochemical pollution discusses safe boundaries (B) and naturalthresholds (T) beyond which ecosystems collapse (Figure 1).Stressor interactions and other numerous unknowns imply thatB must be lower than T to remain safe, but both are difficult todefine. Mulder et al.5 recently quantified concrete thresholdsand real vulnerabilities for 18 near-pristine rivers using food-web modeling, from which we derive several insights (Figure1). One: different locations harbor different food webs. Two:food-web resilience to chemicals differs, implying web-specificand trait-mediated T’s. Three: when T’s are exceeded, distinctcollapse trajectories imply different vulnerabilities (V). Four:effects caused if a stressor acts randomly across the food webare different from those occurring when only a single trophiclevel is affected. For example, herbicides cause differentresponses than neurotoxic or narcotic compounds, as reflectedin the shifted T’s and V’s. Ecological Analyses Help to LocateNatural Thresholds and Allow for Establishing Safe PollutionBoundaries.

■ CHALLENGES REMAIN!Ecological analyses solve some, but not all challenges. Manyunanswered questions remain. How can we use ecologicalstudies to derive predictive thresholds related to collapse? Howcan we locate ecological thresholds of chemical pollution thataccount for multiple stressors? What distance should separate Band T to remain safe and is this distance ecosystem specific?Next, ecology teaches that some species have key ecosystemfunctions, making us aware of cascading effects. Does this implythat we should derive chemical footprints for key species likepollinators? And because we are dealing with a globalphenomenon, how do we aggregate local information into aglobal safe operating space, acknowledging that both chemicalexposures and environmental thresholds vary geographically?Other challenges lie ahead as well. While methods of chemicalfootprinting have been proposed, data are always needed tomake them operational. However, data on emissions, environ-

mental characteristics and fate processes, and chemical-specific,as well as mixture ecotoxicity are often insufficient, particularlyin poorly studied regions. Environmental science has reached alevel of maturity that enables expression and quantif ication ofchemical footprints. However, relating footprints to planetaryboundaries needs data or, more importantly, ways to surmountdata gaps and needs interdisciplinary bridging between chemicaland ecological concepts.

■ PROCEED WHILE DEVELOPINGSilent Spring Has Not Yet Led to an Impact-Free

“Summer”. To help us reach such a “Summer”, we suggestfootprinting be used to convey the urgency of, and act onenvironmental overshoot. The footprint as an indicator isreadily understood and used, as seen by its expansion from landto carbon and water. Building on this concept, the chemicalfootprint employs well-developed environmental science andrelated tools, to express the proximity of an ecosystem toestablished boundaries. A chemical footprint summarizescomplex information that can be used by citizens, educatorsand regulators to foster the sustainable use of chemicals andthereby steer us away f rom overshooting the PlanetaryBoundary for chemical pollution.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: leo.posthuma@rivm.nl.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWork funded by RIVM, DTU, UT and RU and EU-grant603437 ("SOLUTIONS").

■ REFERENCES(1) Wackernagel, M.; Schulz, N. B.; Deumling, D.; Linares, A. C.;Jenkins, M.; et al. Tracking the ecological overshoot of the humaneconomy. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 9266−9271.

Figure 1. Emissions result in footprints for different systems. Ecological studies (upper left) are needed to derive thresholds (T) and vulnerabilities(V) from disintegration trajectories. Boundaries (B) are defined to stay within the “safe operating space”. Background inspired by ref 2 and adapted bypermission from Macmillan Publishers Ltd.: Nature.

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(2) Rockstrom, J.; Steffen, W.; Noone, K.; Persson, Å.; Chapin, F. S.,III; et al. A safe operating space for humanity. Nature 2009, 461, 472−475.(3) Mackay, D. Finding fugacity feasible. Environ. Sci. Technol. 1979,13, 1218−1223.(4) Rees, W. E. Ecological footprints and appropriated carryingcapacity: what urban economics leaves out. Environ. Urbanization1992, 4, 121−130.(5) Mulder, C.; Boit, A.; Mori, S.; Vonk, J. A.; Dyer, S. D.; et al.Distributional (in)congruence of biodiversity-ecosystem functioning.Adv. Ecological Res. 2012, 46, 1−88.

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