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Reconciling Development and Conservation under Coastal Squeeze from Rising Sea Level

Reconciling Development and Conservation under Coastal Squeeze from Rising Sea Level Climate change; planning; priority setting; Among the biggest global challenges for policymakers is the development of adaptation; retreat; managed realignment; land use policies robust to climate change impacts. While diverse fields can in- defend. form adaptation, integrated social-ecological assessment of the multiple adap- Correspondence tation options are rare and cannot be easily applied. Here, we build on past Morena Mills, School of Biological Sciences, The studies by undertaking an integrated fine scale and strategic allocation of sea University of Queensland, Brisbane, Queensland level rise (SLR) adaptation options that can direct policy making. We use mod- 4072, Australia. els of probabilistic SLR inundation, urban growth, and sub- and intertidal Tel: +61-422-229-074; ecosystem migration, to investigate the impacts of different SLR adaptation Fax:+61 7 3365 1655. strategies, and how these can be allocated to best achieve both development E-mail: morena.mills@uq.edu.au and conservation goals. Coastal adaptation will involve trade-offs among de- Received velopment and conservation objectives and these will vary based on the ex- 18 May 2015 tent to which sea levels rise. There will be trade-offs between conservation Accepted objectives regardless of the adaptation options chosen, however, retreat does 28 October 2015 provide opportunities for enabling the expansion of coastal ecosystems inland. Local governments can save billions of dollars and minimize political conflict Editor Richard Zabel between conservation and development goals through integrated strategic spa- tial planning. Our planning approach both informs policy and is transferable doi: 10.1111/conl.12213 to other coastal regions faced with a rising sea. Introduction as mangroves and saltmarsh, and their provision of Developing robust adaptation strategies to sea level ecosystem goods and services (e.g., Arkema et al. 2013). rise (SLR) poses a serious challenge to policy makers Although progress has been made in developing SLR globally (Nicholls & Cazenave 2010), and knowledge adaptation policies, assessments rarely include an in- from diverse fields can be harnessed to inform adaptation tegrated social-ecological assessment of the multiple options. SLR will increase the risk of permanent flooding adaptation options (Fankhauser 1995; Ng & Mendelsohn of low-lying coastal land (Nicholls 2004), resulting in 2005; Nicholls & Tol 2006) and none show how such the forced migration of tens of millions of people this assessments can be applied in practice. century (Nicholls et al. 2011). SLR will also change Resolving the trade-off between development and the distribution of vulnerable coastal ecosystems, such conservation goals is challenging in the context of SLR Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 361 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Planning for coastal squeeze M. Mills et al. as adaptation strategies can mitigate or exacerbate SLR can be strategically combined to provide a compromise impacts (e.g., loss of coastal ecosystems; Nicholls & Tol between development and conservation objectives. The 2006). Driven by the desire to protect existing infras- development and biodiversity conservation objectives tructure, coastal armoring through levees and seawalls were to allow for all coastal ecosystems and urban areas (hereafter called “defend”) has historically been the main to reach their maximum projected extent. response to an encroaching sea. This strategy typically prevents the spread of ecosystems, such as saltmarsh or mangrove inland (Nicholls & Cazenave 2010) resulting Methods in “coastal squeeze.” Coastal squeeze is defined as the loss of intertidal habitat “due to the high water mark We conduct our investigation in the coastal region of being fixed by a defense or structure . . . and the low Moreton Bay Regional Council, Queensland, Australia water mark migrating landwards in response to SLR” (Figure S1); seaward of the metropolis of Brisbane, the (Pontee 2013). Alternately, managed realignment of fastest growing “mature” city in the world (Lasalle 2012), the shore (hereafter called “retreat”) is adopted in some and landward of the coastal embayment of Moreton Bay. regions such as the United Kingdom and Germany Marine and coastal ecosystems within Moreton Bay sup- (Rupp-Armstrong & Nicholls 2007). Deciding which port significant populations of the International Union for strategy to choose is driven by a mix of economic and en- Conservation of Nature Red-listed marine species (e.g., vironmental goals, including the maintenance of coastal dugongs, turtles, and shorebirds), commercial fisheries, wetlands, sustainable flood defenses and the creation of and high biodiversity (Chilvers et al. 2005). Our study new intertidal habitats (Turbott 2006; Rupp-Armstrong & region consisted of the area in Moreton Bay Regional Nicholls 2007). Council with >5% probability of inundation (total area of Scientists from different fields have developed state-of- 193 km ). the-art, spatially explicit models that predict inundation, We combined four spatially explicit models, at a 30 urban growth and the ecosystem dynamics, all of which by 30 m resolution, to quantify the impacts of different can inform SLR adaptation. SLR inundation models coastal adaptation options to inundation by SLR and now include probabilistic estimates of the likelihood of storm surge. We focus on impacts of adaptation on inundation and can be integrated with the probability urban areas and the distribution and abundance of distribution of SLR (Leon et al. 2014; Mills et al. 2014). coastal ecosystems (Figure 1) and consider predicted Meanwhile, models of urban growth and ecosystems changes of the extent of urban areas and migration of migration have been used to inform land-use planning subtidal (seagrass) and intertidal (e.g., mangroves and decisions in the context of SLR for development and tidal marsh) ecosystems by 2100. We estimated changes conservation, respectively (Huong & Pathirana 2011; in the extent of “developed” areas to 2031, which Runting et al. 2013). To date, these modeling approaches includes urban and rural residential land, industrial and have not been integrated to resolve the growing con- commercial land using a cellular automata-based urban flict between development and conservation interests growth model (Text S1, Liu 2012). Future distributions in the coastal zone (Table S1for review of existing of coastal wetlands were modeled using the Sea-Level literature). Affecting Marshes Model (SLAMM, Text S2, Craft et al. We use systematic spatial planning to assess the 2009) and the future distribution of seagrass was based optimal configuration and trade-offs involved in SLR on a local Moreton Bay model (Saunders et al. 2013, Text adaptation, incorporating spatial models of inundation by S3). The probability of coastal inundation due to SLR and SLR, urban growth (Liu 2012), and ecosystem migration storm surge from a 100-year storm was determined using (Figure 1, SI 2, Craft et al. 2009; Saunders et al. 2013). a novel probabilistic approach combining uncertainties The analysis is based on costs of adaptation strategies from errors in mapped elevation (Leon et al. 2014) and their contributions to development and conservation and the probability distribution function for global SLR goals, translated to quantitative area-based objectives. (Johansson et al. 2014; Mills et al. 2014; Figure 2, Text Here, we assess the impacts of SLR on urban growth S4). Future SLR scenarios were based on likely SLR and coastal ecosystems and how the implementation of scenarios (66% probability of occurrence, in line with two adaptation strategies influence these impacts: (1) the Intergovernmental Panel on Climate Change (IPCC) “defend,” where areas subject to flooding are defended standard for communicating uncertainty) and encompass by building levees protecting existing housing develop- the IPCC likely range for SLR for Representative Concen- ment and (2) “retreat,” where areas subject to flooding tration Pathway scenarios 4.5, 6.0, and 8.5 (Stocker et al. are purchased to permit inland migration of coastal 2013; Figure 3). Models predicted 27 km of urban de- ecosystems. We also assess how adaptation strategies velopment (a 51% increase since 2011) and 72–102 km 362 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze Figure 1 Flow chart of the process undertaken to plan for SLR. Initially the spatial data were collected and models were processed independently. Impacts and trade-offs between different SLR adaptation strategies were assessed by combining models using geographic information system and systematic planning software. Finally, recommendations were provided of how policy should be adapted based on the results of this study. of coastal ecosystems within the inundated region, 14% objectives are achieved (Watts et al. 2009). Marxan uses and 37–53% of the area with over 5% probability of a simulated annealing algorithm to identify near-optimal inundation, respectively. spatial configurations of adaptation. The time frame used for the urban growth model Adaptation strategies were assessed individually and (2011–2031) was shorter than that used for the SLR and then in combination, providing a trade-off curve be- coastal ecosystem migration models (2011–2100) because tween the conservation and development benefits. The of uncertainties in predicting urban growth accurately SLR adaptation strategies considered were: (1) Defense: past existing infrastructure plans, which would hinder where reinforced levees were built to defend existing our ability to provide insight into different coastal adap- and projected development. The cost associated with tation policy options. We opted to assess the likelihood defense was estimated to be between AU$4,000–10,000 of potential coastal squeeze using a realistic scenario of per meter of levee (pers. comm. Dr. Ian Teakle, BMT urban growth to 2031 as opposed to a less realistic sce- WBM; SI 6.1), and the length of levee required was nario that ran to 2100 given that the more defined model calculated as that needed to surround the inundated is likely to improve our policy recommendations. Given edge of land parcels. For simplicity, we assumed the levee that the average life of a building is 75 years (Conti 2007), would be built using the most direct routes across the urban development existing in 2031 will be impacted by coastline where development (existing or future) had to and interact with other SLR impacts in 2100. We modeled be defended. We assumed that coastal ecosystems could SLR to 2100 because the impacts of SLR become more ap- no longer exist within land parcels that were defended parent over longer time frames. The mismatch in model by levees. (2) Retreat: this strategy refers to “managed time frames results in a likely underestimate of the poten- realignment” of the shoreline, where the migration of tial impact of SLR on urban development, and of urban ecosystems is facilitated though the purchase of land. For development on ecosystems. simplicity, we assumed that an adaptation strategy would We used the systematic planning software Marxan be selected for groups of land parcels (hereafter termed to allocate the two common SLR adaptation strategies planning units, n = 306) that would be collectively (Titus 1991). Marxan allows users to identify the location flooded because of their hydrological connectivity (based of multiple “zones” (i.e., adaptation strategies) and min- on topography and geographic proximity; Figure S1). imize the cost involved subject to the constraint that all Only a single adaptation strategy, defense or retreat, Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 363 Planning for coastal squeeze M. Mills et al. Figure 2 (A) Probability of inundation modeled in our study region, based on uncertainty of the digital elevation model combined with the probability distribution function representing global mean SLR (Weibull distribution scale 0.95 and shape 2.2). (B) Transect (white line in [c]), across a subsetofthe study region, indicating the different land covers encountered. The uncertainty in the digital elevation model varies across the transect and is influenced by land cover, as shown in (C). In (C), the difference between the LiDAR derived and the geostatistical simulated digital elevation model demonstratesthe uncertainty in information on surface elevation. could be allocated to a given planning unit. The cost (e.g., upper intertidal mangroves) and others almost associated with retreat was the current acquisition cost becoming locally extinct (e.g., sedgelands, Figures S2 and (improved or unimproved) of land parcels (Text S5). S3). Therefore, regardless of the adaptation response, We assumed that development was no longer allowed in as coastal ecosystems are redistributed across space as a these properties (for additional details, see Table S2). response to SLR, trade-offs will occur among conserva- We quantified the costs of using information of the tion objectives. Choices will have to be made between current and potential future spatial distribution of urban allowing for the expansion of ecosystems and actively areas and coastal ecosystems and compared it to that maintaining those ecosystems that are predicted to dis- of applying one adaptation strategy to the whole of appear (e.g., tidal marsh) by preventing the expansion of the coastal region. We also quantified the difference in others (e.g., mangroves). The latter strategy can involve benefits and costs of using a strategic versus random substantial costs and decisions are complicated by the approach to allocate different adaptation strategies across value and conservation status (e.g., saltmarsh is pro- the landscape. tected by the Environment Protection and Biodiversity Conservation Act 1999) of the ecosystems or organisms at risk of being lost. Yet by 2100, if all ecosystems could migrate inland, there would be a net increase in the Results total area of coastal ecosystems of 17–78 km (23–104%) We found that SLR is predicted to exert negative impacts compared to present day. The increase in the area of on urban development, increasing risk on inundation, some coastal ecosystems with widespread coastal retreat and mixed impacts on coastal ecosystems, increasing the can be regarded as an opportunity to increase fisheries total area of available coastal ecosystems but decreasing production, water quality, and coastal protection. the number of ecosystems types. Approximately 18 km The costs and benefits of defend versus retreat contrast (12%) and 10 km (6%) of existing and future urban ar- markedly when strategies are considered individually. A eas in the planning region is more than 50% and 95% strategy of defense across all potentially inundated areas likely to be inundated, respectively. of the study region would require 321 km of levees and While SLR negatively impacts most coastal ecosystems would cost between AU$1.28–3.21 billion (depending on (on an ecosystem by ecosystem basis), there are marked the cost of levees). With defense, the extent of all coastal contrasts in the predicted response of coastal ecosys- ecosystems decreases significantly, ranging from a loss of 2 2 tems to SLR, with some increasing in area dramatically 0.01 km of sedgelands to loss of 71 km of mangrove 364 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze Figure 3 (A) Probability distribution function representing global mean SLR (Weibull distribution scale 0.95 and shape 2.2; Johansson et al. 2014; Mills et al. 2014). The blue shading on the curve represents the trade-off based on “likely” SLR scenarios (66% probability of occurrence, from 0.44 to 1.23 m). (B) Trade-off curve between conservation objectives (average % area of each ecosystem) and development (% area of urban development). Trade-offs are calculated for different SLR scenarios based on the global mean SLR probability distribution function represented in (A). The blue shading on the graph represents the trade-off based on “likely” SLR scenarios (66% probability of occurrence, from 0.44 to 1.23 m). Points represent the configuration of SLR adaptation strategies along the Pareto frontier (e.g., point A to F within the 0.98 m scenario). Point A and F represent scenarios where all land parcels have been selected for defense and retreat, respectively. The black crosses represent random allocation of coastal adaptation strategies (point G is illustrated by the map of ∗ ∗∗ the bottom left of the figure). “ ”and “ ” illustrate the gain in development and conservation objectives, respectively, by moving from a random allocation to a strategic approach to coastal adaptation. The range of costs for each scenario is calculated as the cost of land acquisition plus the cost of building a levee (AU$4,000–10,000 per meter). (Tables S1 and S3, Figure S3). For the adaptation strategy expanding ecosystems by retreat is generally higher than of retreat, coastal ecosystems were predicted to expand that of defense (Figure 3). However, there are hidden by approximately 19 km (17%) by 2100 (Figure 3). In costs in the form of lost ecosystem services (e.g., fisheries, order to halt development where coastal ecosystems will water purification, carbon storage) from coastal defense occur in 2100, land must be purchased for conservation, that, if factored in, could potentially make retreat a more costing approximately AU$11.27 billion. This cost does attractive financial option. For example, defense would not include the removal of existing coastal defense lead to 5–27 km of mangrove and marsh loss, which structures as this information is not available. in 1997 was estimated to have an average global value However, if it is assumed the relevant authority decides of AU$1.05 million per km (U$9900 per ha) per year to either defend or retreat from each inundated land par- for the provision of ecosystem services (Costanza et al. cel (i.e., an adaptation option had to be assigned), cost- 1997). This would equal between AU$0.2–1.3 billion loss effective strategies are found by strategically combining in ecosystem services between now and 2100 (assuming both defense and retreat. These solutions are skewed to- linear ecosystem loss from 2014), making the total rel- ward achieving development objectives, because the cost ative cost advantage of engaging in retreat decrease or of achieving the conservation objective of protecting all disappear, thus emphasizing the benefits of an ecosystem Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 365 Planning for coastal squeeze M. Mills et al. approach to adaptation. Additionally, the risk of levees and coastal ecosystems can substantially increase the being overwhelmed or collapsing is not factored into this achievement of conservation and development objectives study. Failure of levees would mean an economic loss given rising seas, achieving approximately 70% of both equivalent to or greater than that estimated for retreat, objectives and saving billions of dollars (see Figure 3B). as development behind levees will often intensify as peo- We advance SLR adaptation studies by integrating cutting ple assume their properties are protected. edge science from multiple fields. Previous studies have While a trade-off exists between achieving conser- tended to focus on the costs of adaptation to achieve vation and development objectives, a concave trade-off a single goal (e.g., Ng & Mendelsohn 2005; Nicholls & curve (Figure 3) indicates that SLR adaptation strategies Tol 2006) or included relatively simple models defining can be allocated within the study region to increase the change, based on large assumptions and considered to delivery of one objective without a substantial impact on have high levels of uncertainty, thus providing only the other. A high proportion of both the conservation and indicative results of impacts (Fankhauser 1995; Mills development objectives could be achieved by a strategic et al. 2014). Our study provides a clear direction for allocation of a combination of adaptation strategies the implementation of adaptation options, grounded (Figure 3). Large gains of either development or conser- in the reality of the study region. Additionally, the vation objectives can be attained, for a given conservation ability to quantify the probability of inundation for or development objective, respectively, by strategically individual properties increases the ability of decision allocating coastal adaptation measures with consideration makers to effectively incorporate and communicate risk of current and future distribution of coastal ecosystems when undertaking adaptation decisions. Our results can and development. Even considering uncertainty in SLR, be incorporated in existing coastal plans, and can be the gain in the achievement of objectives through the use updated within a dynamic decision-making framework of such models can be seen when comparing these with as more information on the extent and impacts of SLR is a random allocation of adaptation options (simulating gathered. decisions of adaptation undertaken locally, without There are several important caveats to this study insight into changes in ecosystems and urban areas). For (Table S4). First, the time scales between the urban example, the objectives achieved by random allocation growth and SLR models were different (2031 vs. 2100) of coastal adaptation G (Figure 3) could be increased by resulting in a conservative estimate of the area of urban 30–45% for development or 28–40% for conservation, expansion used in the trade-off analyses and a potentially for a saving of AU$0.1–5.93 billion dollars in each low estimate of the costs of retreat, which could increase case. However, there remains a trade-off between the with increasing urban growth. In the future, researchers percentage of development that can be defended from from multiple fields should codevelop models to ensure SLR and the amount of coastal ecosystem migration that time scales match and facilitate integration. Second, pre- may be facilitated through coastal retreat. For example, dicting the future distribution of coastal vegetation in re- while 70% of both objectives can be achieved, achieving sponse to SLR is dependent on a number of assumptions more than 80% of either objective results in a rapid simplifying ecological processes that vary spatially, tem- decline of the other. Uncertainty in SLR impacts the porally, and with environmental conditions. These as- trade-off between conservation and development ob- sumptions could result in errors regarding the predic- jectives, with the impacts being increasingly uncertain tion of the coastal ecosystem extent as a whole and a when aiming to achieve over 40% of either of the two change in the predictions of ecosystem transition from objectives. Notably, this trade-off is also habitat-specific one type to another. Third, large-scale geomorphic (e.g., and is only significant for most of the vegetation types erosion and migration of channels or dunes) or climatic after around 60% of the development objectives have changes (e.g., frequency and intensity of drought and in- been achieved (Figure S4). Trade-offs between and tense storms) were not encompassed by our models and among development and conservation goals should can increase the erosion within the coastline, changing be considered simultaneously as the interactive and the predictions of ecosystem distribution. Finally, when potentially cumulative impact of multiple changes (e.g., considering adaptation, governments may need to under- ecosystem migration and coastal squeeze) can exacerbate take broader institutional reforms to expand the range negative impact on biodiversity. of available options for progressively transferring private property to public use, such as by acquiring properties and granting back licenses or leases to occupy, rezon- Discussion ing land to public use, or placing restrictions on the sale This study found that SLR adaptation strategies that con- or transmission of property (Titus 1991). Implementing sider the current and future distribution of urban areas such reform is politically difficult when societal settings 366 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze favor short-term and private economic development ob- Conti, J. (2007) Annual Energy Outlook 2007. Energy jectives (Tol et al. 2003). Information Administration (EIA) Energy Outlook, Modeling, and Data Conference, Washington, DC. Adaptation to SLR is a complex problem involving Costanza, R., dArge, R., deGroot, R., et al. (1997) The value of social and ecological dimensions and there are several the world’s ecosystem services and natural capital. Nature, important future research directions that can build on 387, 253-260. this study to better inform policies for SLR adaptation. Craft, C., Clough, J., Ehman, J., et al. (2009) Forecasting the For example, incorporating a broader range of adaptation effects of accelerated sea-level rise on tidal marsh options, (e.g., accommodation can help people live with ecosystem services. Front. Ecol. Environ., 7, 73-78. flooding while limiting impact to ecosystems), better Fankhauser, S. (1995) Protection versus retreat - the incorporation of the human response to SLR and investi- economic cost of sea-level rise. Environ. Plann. A, 27, gating how to optimally adapt through time. Understand- 299-319. ing the preference of coastal dwellers and the preference Huong, H. & Pathirana, A. (2011) Urbanization and climate structures (risk-averse or risk-seeking) will improve our change impacts on future urban flood risk in Can Tho understanding of what adaptation options are feasible. city, Vietnam. Hydrol. Earth Syst. Sci. Discussions, 8, 10781-10824. Acknowledgments Johansson, M.M., Pellikka, H., Kahma, K.K. & Ruosteenoja, MM, KM, JXL, MIS, JB, OHG, and HPP acknowledge sup- K. (2014) Global sea level rise scenarios adapted to the Finnish coast. J. Mar. Syst., 129, 35-46. port from the Australian Research Council. The authors are grateful for funding from ARC SuperScience grants LaSalle, J.L., (2012) A new world of cities, redefining the real estate investment map. World Winning Cities, Global Foresight Nos. FS100100024 and FS110200005. Series. http://www.us.jll.com/united-states/en-us/research/ 1651/a-new-world-of-cities-redefining-the-real-estate- Supporting Information investment-map. Additional Supporting Information may be found in the Leon, J.X., Heuvelink, G.B.M. & Phinn, S.R. (2014) online version of this article at the publisher’s web site: Incorporating DEM uncertainty in coastal inundation mapping. PLoS One, 9, e108727. Table S1. Review of previous studies of SLR adaptation Liu, Y. (2012) Modelling sustainable urban growth in a Table S2. Details and assumptions of random and rapidly urbanising region using a fuzzy constrained cellular systematic SLR adaptation automata approach. Int. J. Geogr. Info. Sci., 26, 151-167. Table S3. Ecosystem impacts of SLR adaptation strate- Mills, M., Nicol, S.A.M., Wells, J.A. et al. 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Reconciling Development and Conservation under Coastal Squeeze from Rising Sea Level

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Abstract

Climate change; planning; priority setting; Among the biggest global challenges for policymakers is the development of adaptation; retreat; managed realignment; land use policies robust to climate change impacts. While diverse fields can in- defend. form adaptation, integrated social-ecological assessment of the multiple adap- Correspondence tation options are rare and cannot be easily applied. Here, we build on past Morena Mills, School of Biological Sciences, The studies by undertaking an integrated fine scale and strategic allocation of sea University of Queensland, Brisbane, Queensland level rise (SLR) adaptation options that can direct policy making. We use mod- 4072, Australia. els of probabilistic SLR inundation, urban growth, and sub- and intertidal Tel: +61-422-229-074; ecosystem migration, to investigate the impacts of different SLR adaptation Fax:+61 7 3365 1655. strategies, and how these can be allocated to best achieve both development E-mail: morena.mills@uq.edu.au and conservation goals. Coastal adaptation will involve trade-offs among de- Received velopment and conservation objectives and these will vary based on the ex- 18 May 2015 tent to which sea levels rise. There will be trade-offs between conservation Accepted objectives regardless of the adaptation options chosen, however, retreat does 28 October 2015 provide opportunities for enabling the expansion of coastal ecosystems inland. Local governments can save billions of dollars and minimize political conflict Editor Richard Zabel between conservation and development goals through integrated strategic spa- tial planning. Our planning approach both informs policy and is transferable doi: 10.1111/conl.12213 to other coastal regions faced with a rising sea. Introduction as mangroves and saltmarsh, and their provision of Developing robust adaptation strategies to sea level ecosystem goods and services (e.g., Arkema et al. 2013). rise (SLR) poses a serious challenge to policy makers Although progress has been made in developing SLR globally (Nicholls & Cazenave 2010), and knowledge adaptation policies, assessments rarely include an in- from diverse fields can be harnessed to inform adaptation tegrated social-ecological assessment of the multiple options. SLR will increase the risk of permanent flooding adaptation options (Fankhauser 1995; Ng & Mendelsohn of low-lying coastal land (Nicholls 2004), resulting in 2005; Nicholls & Tol 2006) and none show how such the forced migration of tens of millions of people this assessments can be applied in practice. century (Nicholls et al. 2011). SLR will also change Resolving the trade-off between development and the distribution of vulnerable coastal ecosystems, such conservation goals is challenging in the context of SLR Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 361 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Planning for coastal squeeze M. Mills et al. as adaptation strategies can mitigate or exacerbate SLR can be strategically combined to provide a compromise impacts (e.g., loss of coastal ecosystems; Nicholls & Tol between development and conservation objectives. The 2006). Driven by the desire to protect existing infras- development and biodiversity conservation objectives tructure, coastal armoring through levees and seawalls were to allow for all coastal ecosystems and urban areas (hereafter called “defend”) has historically been the main to reach their maximum projected extent. response to an encroaching sea. This strategy typically prevents the spread of ecosystems, such as saltmarsh or mangrove inland (Nicholls & Cazenave 2010) resulting Methods in “coastal squeeze.” Coastal squeeze is defined as the loss of intertidal habitat “due to the high water mark We conduct our investigation in the coastal region of being fixed by a defense or structure . . . and the low Moreton Bay Regional Council, Queensland, Australia water mark migrating landwards in response to SLR” (Figure S1); seaward of the metropolis of Brisbane, the (Pontee 2013). Alternately, managed realignment of fastest growing “mature” city in the world (Lasalle 2012), the shore (hereafter called “retreat”) is adopted in some and landward of the coastal embayment of Moreton Bay. regions such as the United Kingdom and Germany Marine and coastal ecosystems within Moreton Bay sup- (Rupp-Armstrong & Nicholls 2007). Deciding which port significant populations of the International Union for strategy to choose is driven by a mix of economic and en- Conservation of Nature Red-listed marine species (e.g., vironmental goals, including the maintenance of coastal dugongs, turtles, and shorebirds), commercial fisheries, wetlands, sustainable flood defenses and the creation of and high biodiversity (Chilvers et al. 2005). Our study new intertidal habitats (Turbott 2006; Rupp-Armstrong & region consisted of the area in Moreton Bay Regional Nicholls 2007). Council with >5% probability of inundation (total area of Scientists from different fields have developed state-of- 193 km ). the-art, spatially explicit models that predict inundation, We combined four spatially explicit models, at a 30 urban growth and the ecosystem dynamics, all of which by 30 m resolution, to quantify the impacts of different can inform SLR adaptation. SLR inundation models coastal adaptation options to inundation by SLR and now include probabilistic estimates of the likelihood of storm surge. We focus on impacts of adaptation on inundation and can be integrated with the probability urban areas and the distribution and abundance of distribution of SLR (Leon et al. 2014; Mills et al. 2014). coastal ecosystems (Figure 1) and consider predicted Meanwhile, models of urban growth and ecosystems changes of the extent of urban areas and migration of migration have been used to inform land-use planning subtidal (seagrass) and intertidal (e.g., mangroves and decisions in the context of SLR for development and tidal marsh) ecosystems by 2100. We estimated changes conservation, respectively (Huong & Pathirana 2011; in the extent of “developed” areas to 2031, which Runting et al. 2013). To date, these modeling approaches includes urban and rural residential land, industrial and have not been integrated to resolve the growing con- commercial land using a cellular automata-based urban flict between development and conservation interests growth model (Text S1, Liu 2012). Future distributions in the coastal zone (Table S1for review of existing of coastal wetlands were modeled using the Sea-Level literature). Affecting Marshes Model (SLAMM, Text S2, Craft et al. We use systematic spatial planning to assess the 2009) and the future distribution of seagrass was based optimal configuration and trade-offs involved in SLR on a local Moreton Bay model (Saunders et al. 2013, Text adaptation, incorporating spatial models of inundation by S3). The probability of coastal inundation due to SLR and SLR, urban growth (Liu 2012), and ecosystem migration storm surge from a 100-year storm was determined using (Figure 1, SI 2, Craft et al. 2009; Saunders et al. 2013). a novel probabilistic approach combining uncertainties The analysis is based on costs of adaptation strategies from errors in mapped elevation (Leon et al. 2014) and their contributions to development and conservation and the probability distribution function for global SLR goals, translated to quantitative area-based objectives. (Johansson et al. 2014; Mills et al. 2014; Figure 2, Text Here, we assess the impacts of SLR on urban growth S4). Future SLR scenarios were based on likely SLR and coastal ecosystems and how the implementation of scenarios (66% probability of occurrence, in line with two adaptation strategies influence these impacts: (1) the Intergovernmental Panel on Climate Change (IPCC) “defend,” where areas subject to flooding are defended standard for communicating uncertainty) and encompass by building levees protecting existing housing develop- the IPCC likely range for SLR for Representative Concen- ment and (2) “retreat,” where areas subject to flooding tration Pathway scenarios 4.5, 6.0, and 8.5 (Stocker et al. are purchased to permit inland migration of coastal 2013; Figure 3). Models predicted 27 km of urban de- ecosystems. We also assess how adaptation strategies velopment (a 51% increase since 2011) and 72–102 km 362 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze Figure 1 Flow chart of the process undertaken to plan for SLR. Initially the spatial data were collected and models were processed independently. Impacts and trade-offs between different SLR adaptation strategies were assessed by combining models using geographic information system and systematic planning software. Finally, recommendations were provided of how policy should be adapted based on the results of this study. of coastal ecosystems within the inundated region, 14% objectives are achieved (Watts et al. 2009). Marxan uses and 37–53% of the area with over 5% probability of a simulated annealing algorithm to identify near-optimal inundation, respectively. spatial configurations of adaptation. The time frame used for the urban growth model Adaptation strategies were assessed individually and (2011–2031) was shorter than that used for the SLR and then in combination, providing a trade-off curve be- coastal ecosystem migration models (2011–2100) because tween the conservation and development benefits. The of uncertainties in predicting urban growth accurately SLR adaptation strategies considered were: (1) Defense: past existing infrastructure plans, which would hinder where reinforced levees were built to defend existing our ability to provide insight into different coastal adap- and projected development. The cost associated with tation policy options. We opted to assess the likelihood defense was estimated to be between AU$4,000–10,000 of potential coastal squeeze using a realistic scenario of per meter of levee (pers. comm. Dr. Ian Teakle, BMT urban growth to 2031 as opposed to a less realistic sce- WBM; SI 6.1), and the length of levee required was nario that ran to 2100 given that the more defined model calculated as that needed to surround the inundated is likely to improve our policy recommendations. Given edge of land parcels. For simplicity, we assumed the levee that the average life of a building is 75 years (Conti 2007), would be built using the most direct routes across the urban development existing in 2031 will be impacted by coastline where development (existing or future) had to and interact with other SLR impacts in 2100. We modeled be defended. We assumed that coastal ecosystems could SLR to 2100 because the impacts of SLR become more ap- no longer exist within land parcels that were defended parent over longer time frames. The mismatch in model by levees. (2) Retreat: this strategy refers to “managed time frames results in a likely underestimate of the poten- realignment” of the shoreline, where the migration of tial impact of SLR on urban development, and of urban ecosystems is facilitated though the purchase of land. For development on ecosystems. simplicity, we assumed that an adaptation strategy would We used the systematic planning software Marxan be selected for groups of land parcels (hereafter termed to allocate the two common SLR adaptation strategies planning units, n = 306) that would be collectively (Titus 1991). Marxan allows users to identify the location flooded because of their hydrological connectivity (based of multiple “zones” (i.e., adaptation strategies) and min- on topography and geographic proximity; Figure S1). imize the cost involved subject to the constraint that all Only a single adaptation strategy, defense or retreat, Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 363 Planning for coastal squeeze M. Mills et al. Figure 2 (A) Probability of inundation modeled in our study region, based on uncertainty of the digital elevation model combined with the probability distribution function representing global mean SLR (Weibull distribution scale 0.95 and shape 2.2). (B) Transect (white line in [c]), across a subsetofthe study region, indicating the different land covers encountered. The uncertainty in the digital elevation model varies across the transect and is influenced by land cover, as shown in (C). In (C), the difference between the LiDAR derived and the geostatistical simulated digital elevation model demonstratesthe uncertainty in information on surface elevation. could be allocated to a given planning unit. The cost (e.g., upper intertidal mangroves) and others almost associated with retreat was the current acquisition cost becoming locally extinct (e.g., sedgelands, Figures S2 and (improved or unimproved) of land parcels (Text S5). S3). Therefore, regardless of the adaptation response, We assumed that development was no longer allowed in as coastal ecosystems are redistributed across space as a these properties (for additional details, see Table S2). response to SLR, trade-offs will occur among conserva- We quantified the costs of using information of the tion objectives. Choices will have to be made between current and potential future spatial distribution of urban allowing for the expansion of ecosystems and actively areas and coastal ecosystems and compared it to that maintaining those ecosystems that are predicted to dis- of applying one adaptation strategy to the whole of appear (e.g., tidal marsh) by preventing the expansion of the coastal region. We also quantified the difference in others (e.g., mangroves). The latter strategy can involve benefits and costs of using a strategic versus random substantial costs and decisions are complicated by the approach to allocate different adaptation strategies across value and conservation status (e.g., saltmarsh is pro- the landscape. tected by the Environment Protection and Biodiversity Conservation Act 1999) of the ecosystems or organisms at risk of being lost. Yet by 2100, if all ecosystems could migrate inland, there would be a net increase in the Results total area of coastal ecosystems of 17–78 km (23–104%) We found that SLR is predicted to exert negative impacts compared to present day. The increase in the area of on urban development, increasing risk on inundation, some coastal ecosystems with widespread coastal retreat and mixed impacts on coastal ecosystems, increasing the can be regarded as an opportunity to increase fisheries total area of available coastal ecosystems but decreasing production, water quality, and coastal protection. the number of ecosystems types. Approximately 18 km The costs and benefits of defend versus retreat contrast (12%) and 10 km (6%) of existing and future urban ar- markedly when strategies are considered individually. A eas in the planning region is more than 50% and 95% strategy of defense across all potentially inundated areas likely to be inundated, respectively. of the study region would require 321 km of levees and While SLR negatively impacts most coastal ecosystems would cost between AU$1.28–3.21 billion (depending on (on an ecosystem by ecosystem basis), there are marked the cost of levees). With defense, the extent of all coastal contrasts in the predicted response of coastal ecosys- ecosystems decreases significantly, ranging from a loss of 2 2 tems to SLR, with some increasing in area dramatically 0.01 km of sedgelands to loss of 71 km of mangrove 364 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze Figure 3 (A) Probability distribution function representing global mean SLR (Weibull distribution scale 0.95 and shape 2.2; Johansson et al. 2014; Mills et al. 2014). The blue shading on the curve represents the trade-off based on “likely” SLR scenarios (66% probability of occurrence, from 0.44 to 1.23 m). (B) Trade-off curve between conservation objectives (average % area of each ecosystem) and development (% area of urban development). Trade-offs are calculated for different SLR scenarios based on the global mean SLR probability distribution function represented in (A). The blue shading on the graph represents the trade-off based on “likely” SLR scenarios (66% probability of occurrence, from 0.44 to 1.23 m). Points represent the configuration of SLR adaptation strategies along the Pareto frontier (e.g., point A to F within the 0.98 m scenario). Point A and F represent scenarios where all land parcels have been selected for defense and retreat, respectively. The black crosses represent random allocation of coastal adaptation strategies (point G is illustrated by the map of ∗ ∗∗ the bottom left of the figure). “ ”and “ ” illustrate the gain in development and conservation objectives, respectively, by moving from a random allocation to a strategic approach to coastal adaptation. The range of costs for each scenario is calculated as the cost of land acquisition plus the cost of building a levee (AU$4,000–10,000 per meter). (Tables S1 and S3, Figure S3). For the adaptation strategy expanding ecosystems by retreat is generally higher than of retreat, coastal ecosystems were predicted to expand that of defense (Figure 3). However, there are hidden by approximately 19 km (17%) by 2100 (Figure 3). In costs in the form of lost ecosystem services (e.g., fisheries, order to halt development where coastal ecosystems will water purification, carbon storage) from coastal defense occur in 2100, land must be purchased for conservation, that, if factored in, could potentially make retreat a more costing approximately AU$11.27 billion. This cost does attractive financial option. For example, defense would not include the removal of existing coastal defense lead to 5–27 km of mangrove and marsh loss, which structures as this information is not available. in 1997 was estimated to have an average global value However, if it is assumed the relevant authority decides of AU$1.05 million per km (U$9900 per ha) per year to either defend or retreat from each inundated land par- for the provision of ecosystem services (Costanza et al. cel (i.e., an adaptation option had to be assigned), cost- 1997). This would equal between AU$0.2–1.3 billion loss effective strategies are found by strategically combining in ecosystem services between now and 2100 (assuming both defense and retreat. These solutions are skewed to- linear ecosystem loss from 2014), making the total rel- ward achieving development objectives, because the cost ative cost advantage of engaging in retreat decrease or of achieving the conservation objective of protecting all disappear, thus emphasizing the benefits of an ecosystem Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. 365 Planning for coastal squeeze M. Mills et al. approach to adaptation. Additionally, the risk of levees and coastal ecosystems can substantially increase the being overwhelmed or collapsing is not factored into this achievement of conservation and development objectives study. Failure of levees would mean an economic loss given rising seas, achieving approximately 70% of both equivalent to or greater than that estimated for retreat, objectives and saving billions of dollars (see Figure 3B). as development behind levees will often intensify as peo- We advance SLR adaptation studies by integrating cutting ple assume their properties are protected. edge science from multiple fields. Previous studies have While a trade-off exists between achieving conser- tended to focus on the costs of adaptation to achieve vation and development objectives, a concave trade-off a single goal (e.g., Ng & Mendelsohn 2005; Nicholls & curve (Figure 3) indicates that SLR adaptation strategies Tol 2006) or included relatively simple models defining can be allocated within the study region to increase the change, based on large assumptions and considered to delivery of one objective without a substantial impact on have high levels of uncertainty, thus providing only the other. A high proportion of both the conservation and indicative results of impacts (Fankhauser 1995; Mills development objectives could be achieved by a strategic et al. 2014). Our study provides a clear direction for allocation of a combination of adaptation strategies the implementation of adaptation options, grounded (Figure 3). Large gains of either development or conser- in the reality of the study region. Additionally, the vation objectives can be attained, for a given conservation ability to quantify the probability of inundation for or development objective, respectively, by strategically individual properties increases the ability of decision allocating coastal adaptation measures with consideration makers to effectively incorporate and communicate risk of current and future distribution of coastal ecosystems when undertaking adaptation decisions. Our results can and development. Even considering uncertainty in SLR, be incorporated in existing coastal plans, and can be the gain in the achievement of objectives through the use updated within a dynamic decision-making framework of such models can be seen when comparing these with as more information on the extent and impacts of SLR is a random allocation of adaptation options (simulating gathered. decisions of adaptation undertaken locally, without There are several important caveats to this study insight into changes in ecosystems and urban areas). For (Table S4). First, the time scales between the urban example, the objectives achieved by random allocation growth and SLR models were different (2031 vs. 2100) of coastal adaptation G (Figure 3) could be increased by resulting in a conservative estimate of the area of urban 30–45% for development or 28–40% for conservation, expansion used in the trade-off analyses and a potentially for a saving of AU$0.1–5.93 billion dollars in each low estimate of the costs of retreat, which could increase case. However, there remains a trade-off between the with increasing urban growth. In the future, researchers percentage of development that can be defended from from multiple fields should codevelop models to ensure SLR and the amount of coastal ecosystem migration that time scales match and facilitate integration. Second, pre- may be facilitated through coastal retreat. For example, dicting the future distribution of coastal vegetation in re- while 70% of both objectives can be achieved, achieving sponse to SLR is dependent on a number of assumptions more than 80% of either objective results in a rapid simplifying ecological processes that vary spatially, tem- decline of the other. Uncertainty in SLR impacts the porally, and with environmental conditions. These as- trade-off between conservation and development ob- sumptions could result in errors regarding the predic- jectives, with the impacts being increasingly uncertain tion of the coastal ecosystem extent as a whole and a when aiming to achieve over 40% of either of the two change in the predictions of ecosystem transition from objectives. Notably, this trade-off is also habitat-specific one type to another. Third, large-scale geomorphic (e.g., and is only significant for most of the vegetation types erosion and migration of channels or dunes) or climatic after around 60% of the development objectives have changes (e.g., frequency and intensity of drought and in- been achieved (Figure S4). Trade-offs between and tense storms) were not encompassed by our models and among development and conservation goals should can increase the erosion within the coastline, changing be considered simultaneously as the interactive and the predictions of ecosystem distribution. Finally, when potentially cumulative impact of multiple changes (e.g., considering adaptation, governments may need to under- ecosystem migration and coastal squeeze) can exacerbate take broader institutional reforms to expand the range negative impact on biodiversity. of available options for progressively transferring private property to public use, such as by acquiring properties and granting back licenses or leases to occupy, rezon- Discussion ing land to public use, or placing restrictions on the sale This study found that SLR adaptation strategies that con- or transmission of property (Titus 1991). Implementing sider the current and future distribution of urban areas such reform is politically difficult when societal settings 366 Conservation Letters, September/October 2016, 9(5), 361–368 Copyright and Photocopying: 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. M. Mills et al. Planning for coastal squeeze favor short-term and private economic development ob- Conti, J. (2007) Annual Energy Outlook 2007. Energy jectives (Tol et al. 2003). Information Administration (EIA) Energy Outlook, Modeling, and Data Conference, Washington, DC. Adaptation to SLR is a complex problem involving Costanza, R., dArge, R., deGroot, R., et al. (1997) The value of social and ecological dimensions and there are several the world’s ecosystem services and natural capital. Nature, important future research directions that can build on 387, 253-260. this study to better inform policies for SLR adaptation. Craft, C., Clough, J., Ehman, J., et al. (2009) Forecasting the For example, incorporating a broader range of adaptation effects of accelerated sea-level rise on tidal marsh options, (e.g., accommodation can help people live with ecosystem services. Front. Ecol. Environ., 7, 73-78. flooding while limiting impact to ecosystems), better Fankhauser, S. 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World Winning Cities, Global Foresight Nos. FS100100024 and FS110200005. Series. http://www.us.jll.com/united-states/en-us/research/ 1651/a-new-world-of-cities-redefining-the-real-estate- Supporting Information investment-map. Additional Supporting Information may be found in the Leon, J.X., Heuvelink, G.B.M. & Phinn, S.R. (2014) online version of this article at the publisher’s web site: Incorporating DEM uncertainty in coastal inundation mapping. PLoS One, 9, e108727. Table S1. Review of previous studies of SLR adaptation Liu, Y. (2012) Modelling sustainable urban growth in a Table S2. Details and assumptions of random and rapidly urbanising region using a fuzzy constrained cellular systematic SLR adaptation automata approach. Int. J. Geogr. Info. Sci., 26, 151-167. Table S3. Ecosystem impacts of SLR adaptation strate- Mills, M., Nicol, S.A.M., Wells, J.A. et al. 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