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The veiled ecological danger of rising sea levelsAs sea levels rise, human displacement and subsequent land-use change may be as ecologically significant as the direct impacts of climate change. New work suggesting that mean sea level will rise further and faster than previously thought calls attention to the importance of these indirect processes for ecology and conservation.

Steven L. Chown and Grant A. Duffy

Although contemporary reviews continue to place global sea-level rise by 2100 at less than a metre1,

recent work has shown that larger changes may be likely. Three studies stand out from an ecological process perspective. First, Deconto and Pollard2 demonstrated that when the role of atmospheric warming on ice shelves and ice cliffs is considered Antarctica could contribute more than a metre of sea-level rise by 2100 and 13 metres by 2500. In their work, they suggested that the Larsen C ice shelf would be lost by 2055 — reports suggests that a crack in the ice shelf may foreshadow instability3. Second, Galeotti et al.4 demonstrated that the stability threshold for marine-based sectors of the Antarctic ice sheet is 400 ppm

atmospheric CO2. Atmospheric CO2 concentrations have been above 400 ppm since December 20155. Third, though much debated now and during its open review, Hansen et al.6 suggested that multi-metre sea-level rise will be practically unavoidable within 50–150 years if greenhouse gas emissions continue to grow, and storms are likely to become more powerful, with important implications for storm surges.

Together these works indicate that a course has been set for dangerous climate change and sea-level rise. Despite the much-needed developments at the Paris COP21 meeting, especially the agreement to aim to limit climate change to 1.5 °C above pre-industrial levels, the signs remain

worrying. Not only is the Paris Agreement fragile7, but other developments suggest it may be a challenge to meet. The 2016 World Energy Outlook8 concluded that, although recent actions may slow the projected rise in CO2 emissions, they are insufficient to limit warming to < 2 °C. The US$8.8 trillion cumulative investment in renewable energy projected to 2040 remains just 20% of total cumulative investment in energy supply. Current, more typical forecasts for global sea-level rise of < 1 m in 50 years1 thus seem optimistically low.

These new forecasts for sea-level rise can readily be incorporated into understanding of their direct ecological consequences. Coastal areas and small islands obviously face the direct threat of total submergence or more frequent seawater inundation from storm surges, but the secondary effects9 of sea-level rise, as humans attempt to protect, relocate and rebuild existing infrastructure, settlements and agriculture have not been as extensively examined and warrant more explicit attention. As sea levels rise, new land for human settlements, industry and agriculture will have to be sought. Thus, previously low-use areas, many of which have been set aside for conservation purposes, including in biodiversity hotspots co-located with populous areas10, will come under pressure. These secondary effects constitute a veiled ecological process that is unlikely to scale directly with areal loss through inundation (Fig. 1) and may have increasingly profound consequences for terrestrial systems in the Anthropocene.

Development and displacementThe twenty-first century is witnessing the fastest rate of infrastructure expansion in human history. Logging, mining, agriculture, biofuel demand and local development, including the construction of new cities, account for this expansion.

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Fig. 1 | Estimated population displaced and area inundated, either permanently or with increasing frequency, under a range of sea-level-rise scenarios and with a focus on 2 m and 5 m scenarios. Symbols and lines are coloured by the classification of the land that is either inundated or from which people are displaced. Each filled humanoid symbol represents displacement of 5 million individuals (approximately the current population of Sydney, Australia). Each area tile represents 50,000 km2 (approximately twice the area of Vermont, USA). Estimates are based on a flood-fill model assuming additive effects of mean sea-level rise and tidal variance (see Supplementary Methods). Estimates rounded to the nearest 5 million individuals or 50,000 km2. Protected area loss was calculated independently and may, therefore, overlap with other land-use classifications.

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Most of these activities are taking place in the tropics, many in biodiversity hotspots. Although suggestions for and attempts at impact mitigation are being made, the biodiversity consequences of these developments are profound11. Furthermore, the effects of climate change will exacerbate all of these development impacts. In particular, rising sea levels will require agricultural production and urban infrastructure to be relocated over the long term to avoid loss by inundation. Nonetheless, few investigations have sought to examine the combination of current development impacts with those that will follow from the need to replace inundated infrastructure and agricultural land.

An urban population larger than that of the London metropolitan area (~14 million persons; Fig. 1) would be displaced by sea levels two metres above current means. Sea levels five metres above current means, which are within foreseeable estimates2, especially considering the additive effects of rising mean sea levels and existing tidal variance, would displace more than 318 million people worldwide, equivalent to the estimated population of the United States in 2014. These values assume a static human population held at current levels and are, therefore, likely underestimates of future impact given projected human population growth and urban migration12. Furthermore, our estimates are based on a relatively coarse spatial-resolution (30 arcsecond)

flood-fill model that assumes a homogenous rise in mean sea level across the globe despite historical sea-level change showing geographic variation (see Supplementary Information for full details of model methods). Geographic variation of future sea-level rise will make some areas more prone to indirect ecological impacts than others. Such variation is expected to mirror historical variation, but confidence in predictions is inconsistent across the globe and often low for regions that lack historical tide-gauge data13. Regional-scale studies of developed countries may overcome these limitations using high-resolution, high-confidence data and dynamic models of population growth and migration14. Dynamic high-resolution analyses at a global scale, however, remain challenging for a variety of reasons, but are urgently required.

Most people displaced by sea-level rise under the assumptions of our model live in cropland areas (Fig. 1) and over 80,000 km2 of arable land is predicted to be impacted by a two-metre sea-level rise. Thus, not only would those displaced require relocation and rehoming, but some agricultural areas would no longer be as productive as they once were. Additionally, at least 15% of urban agglomerations with populations exceeding 1 million individuals lie within 20 km of the coast and incorporate land that is below 10 m elevation (Fig. 2a). Short-term adaptation responses, such as

the construction of flood barriers, are widely analysed as part of forecast scenarios, but for many areas such infrastructure development may prove economically unfeasible, especially in developing countries. The growing numbers of informal settlement residents12 will be especially vulnerable, leaving them with few choices that do not include further migration.

For agriculture, any reduction in current area for production implies its expansion elsewhere, thus precluding scenarios where, without inundation, the potential exists for meeting global food requirements without areal expansion. Indeed, considerations of sea-level rise impacts are absent from the most recent assessment of how the global human population can be fed without further deforestation15. Urban and agricultural expansion is likely to encroach into unutilized or currently protected areas from their margins. Even without consideration of sea-level rise, expectations are that urban land area within 50 km of a protected area will increase from > 450,000 to 1,440,000 km2 by 203010. Among various geometries of area loss, marginal loss leads to the greatest biodiversity impacts16. In addition, many protected areas will face direct threats from sea-level rise, even excluding consideration of land-use change to accommodate relocated infrastructure and agriculture (Figs. 1, 2). Such pending impacts suggest that gains which may have been made via Aichi Target 11 (on protected

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Fig. 2 | threat assessment for urban agglomerations and protected areas under a 2 m sea-level rise scenario. a,b, Urban areas with greater than 1 million inhabitants (a); and protected areas that will lose more than 50 km2 of land with a 2 m rise in mean sea level (b). Agglomerations whose urban centres are within 20 km of the coast and incorporate low-lying land (< 5 m elevation; hatched shaded area) are particularly at risk from inundation. Urban centres within 50 km of the coast, particularly the most expansive cities that incorporate land less than 10 m above sea level (shaded area), may also be subject to areal loss from rising sea levels or more frequent flooding from storm surges. See Supplementary Methods for more information.

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areas) of the Strategic Plan for Biodiversity 2011–2020 may be compromised.

achieving human aspirationsExactly how human adaptation to sea-level rise will play out as an ecological process affecting species, ecosystems and their conservation is far from clear. What is apparent, though, is the need for much greater attention to assessment of these indirect effects of climate change, given that habitat alteration has substantial effects on biodiversity17. To be effective, such assessments must be regional; include consideration of what the biodiversity impacts might be from higher-yield, more-intensive farming, which can be beneficial; and focus on urbanization questions, including means to improve informal settlements and accommodate migration.

Both for their general significance and ease of measurement, the direct effects of temperature and precipitation change have made up much of what is known about the ecological processes underlying climate change impacts. By contrast, the direct and indirect effects of habitat change

associated with rising sea levels and human adaptation to them will need a similarly concerted, process-based research effort. Modelling and empirical investigations both require development. Most significantly, however, the broader message from these considerations is that mitigation remains the less complex means to achieve human aspirations while securing the future of biodiversity. ❐

Steven L. Chown* and Grant A. DuffySchool of Biological Sciences, Monash University, Victoria 3800, Australia. *e-mail: steven.chown@monash.edu

Published online: 22 August 2017DOI: 10.1038/s41559-017-0267-7

References 1. Slangen, A. B. A. et al. Surv. Geophys. 38, 385–406 (2017). 2. DeConto, R. M. & Pollard, D. Nature 531, 591–597 (2016). 3. Tollefson, J. Nature 542, 402–403 (2017). 4. Galeotti, S. et al. Science 352, 76–80 (2016). 5. The Keeling Curve. Scripps Institute of Oceanography

https://scripps.ucsd.edu/programs/keelingcurve/ (accessed on 10 July 2017).

6. Hansen, J. et al. Atmos. Chem. Phys. 16, 3761–3812 (2016). 7. Sanderson, B. M. & Knutti, R. Nat. Clim. Change 7, 92–94 (2017).

8. World Energy Outlook 2016 (International Energy Agency, 2016).

9. Wetzel, F. T., Kissling, W. D., Beissmann, H. & Penn, D. J. Glob. Change Biol. 18, 2707–2719 (2012).

10. Güneralp, B. & Seto, K. C. Environ. Res. Lett. 8, 014025 (2013). 11. Laurance, W. F. et al. Curr. Biol. 25, R259–R262 (2015). 12. World Urbanization Prospects: The 2014 Revision (United Nations,

New York, 2015). 13. Kopp, R. E., Hay, C. C., Little, C. M. & Mitrovica, J. X. Curr. Clim.

Change Rep. 1, 192–204 (2015). 14. Hauer, M. E. Nat. Clim. Change 7, 321–325 (2017). 15. Erb, K.-H. et al. Nat. Commun. 7, 11382 (2016). 16. Keil, P., Storch, D. & Jetz, W. Nat. Commun. 6, 8837 (2015). 17. Maxwell, S. L., Fuller, R. A., Brooks, T. M. & Watson, J. E. M.

Nature 536, 143–145 (2016).

AcknowledgementsWe thank G. Ayers, M. McGeoch and C. White for comments on a previous version of the manuscript. This work was supported by The Wellcome Trust (grant 201791/Z/16/Z).

Author contributionsS.L.C. developed the initial idea, G.A.D. carried out the modelling, both authors contributed to further development, and drafting of the text.

Competing interestsThe authors declare no competing financial interests.

Additional informationSupplementary information is available for this paper at doi:10.1038/s41559-017-0267-7.

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