Making nature-based solutions climate-ready for the 50 °C world
Introduction
Nature-based solutions (NBSs), the use of urban vegetation and blue-green infrastructures, are increasingly implemented to foster urban sustainability and liveability (Scott et al., 2016; Keeler et al., 2019). Urban nature can mitigate many socio-ecological problems by providing diverse ecosystem services (Ozer et al., 2008; Pataki et al., 2011; Matthews et al., 2015; Aronson et al., 2017).
Among the goals of NBSs, those related to climate change are amongst the most ambitious and challenging (European Commission, 2015; Hobbie and Grimm, 2020). NBSs can mitigate effects of extreme weather events and contribute to climate adaptation and resilience (McPhearson et al., 2015; Kabisch et al., 2017). Coastal NBSs (e.g., wetlands and mangroves) can contribute to urban protection from storm surge (Zölch et al., 2017; Masi et al., 2018; Threlfall et al., 2021). Urban forests can cool landscapes, reducing energy consumption and CO2 emissions (Vailshery et al., 2013; Salmond et al., 2016; Richards and Edwards, 2017). Local, regional and national governments worldwide are adopting NBSs to help alleviate climate disasters; extreme heat, drought or flooding (Solecki et al., 2005; Cohen-Shacham et al., 2016; Kabisch et al., 2016; Scott et al., 2016; Emilsson and Ode Sang, 2017). NBSs are recognized as adaptation, mitigation and resilience strategies toward climate change in an increasing number of global policy assessments and frameworks (Seddon et al., 2020).
However, NBSs are themselves vulnerable to the climate challenges they are meant to address (McPhearson et al., 2015; Calliari et al., 2019). Unlike natural ecosystems, often adapted to natural disturbance regimes (i.e., drought, wildfires), urban NBSs may be ill-equipped to cope with additional stressors (Burley et al., 2019). NBSs face a number of urban stressors, such as disturbance and pollution, before having to deal with climate extremes (Meineke et al., 2016). Heatwaves, exacerbated by urban heat islands, may push species past their critical thresholds and destroy expensive NBSs. Increased frequency and intensity of extreme events under climate change may ultimately limit the ability of NBSs to survive and recover because climate extremes could recur well before NBSs can recover (Seddon et al., 2020). Thus, while NBSs can provide significant benefits, climate extremes could negate these benefits due to reduced ecological functioning and loss of adaptive capacity (Gill et al., 2007).
Under climate change NBSs globally may be overwhelmed by the same climatic factors they hope to mitigate. Many cities already experience more than twice as much warming as non-urban regions (e.g., urban heat islands), with temperature extremes leading to unbearable conditions for urban nature and humans, particularly during heatwaves (Ossola et al., 2021b). Cities are warming at a faster rate than previously projected with some of the world’s cities soon to exceed temperatures of 45−50 °C (Lewis et al., 2017).
Australian cities deal with the dual challenge of high urbanization and extreme climate conditions. In the summer of 2020, western Sydney recorded the highest day temperature in the world that day peaking at 48.9 °C (BOM, 2020). Similarly, many other urban centres are hitting temperature extremes dangerously close to 50 °C conditions (Fig. 1). Summer heatwaves in major Australian cities are expected to routinely reach highs of 50 °C by 2040 (Lewis et al., 2017). Additionally, the major droughts of the late 20th and early 21st centuries in southern Australia, with little precedent over the past 400 years (Freund et al., 2017), have reduced water availability to support NBSs during hot periods in most urban regions. Despite longer and more severe droughts, climate change will most likely determine more intense precipitation extremes in parts of Australia (Alexander and Arblaster, 2017), with severe thunderstorms predicted to increase by 14 % for Brisbane, 22 % for Melbourne, and 30 % for Sydney by 2100 (Allen et al., 2014).
The magnitude of these negative impacts on current and future NBSs is largely unknown. Thus, the question arises on how to support NBS where nature itself is already or likely to be disrupted by climate change. Critically, we must reflect to what extent NBS can be viable solutions to climate change in increasingly extreme climates; what are the interventions cities can implement to help NBS deliver functions, processes, services under extreme climate conditions; and if we should moderate our expectations of the role NBSs can play in a 50 °C world. To address these critical considerations, in this commentary, we discuss first, the need to better understand tolerance and adaptability of NBS elements (i.e., natural, technological, governance and social elements) to new climate regimes; and second, we consider how NBS can be better designed to survive climate extremes. This includes considering climate-safety margins in NBS design – the difference between the maximum climatic tolerances of urban ecological, social or technological elements and the climate experienced at a location – and how to better prototype and implement NBS. We present examples from Australian cities approaching a 50 °C world to spur a much-needed discussion on how cities around the world must imagine, design and govern NBSs and highlight the steps necessary to navigate NBS into new climate futures.
Section snippets
Assessing the tolerance and adaptability of NBS elements to new climate regimes
NBSs rely on natural elements - species, communities and ecosystem processes - that have intrinsic functioning, tolerance and adaptability to specific climate conditions (Keesstra et al., 2018). These natural elements work together with structural, infrastructural, technological elements, as well as human, social and governance elements, all which will be impacted by extreme climate conditions (Depietri and McPhearson, 2017). Thus, it is important to “maintain natural adaptability when
Designing NBS for weather extremes and climate-safety margins
Designing NBSs with ample climate-safety margins is a challenging yet necessary task. A preliminary assessment and specification of the climate-safety margins of the different NBS components is required to understand the adaptability and resilience of the NBS system to future climates (Fig. 3). When planning and designing NBSs it is critical to consider species’ survivability to both current and future climates, at least within the expected NBS lifetime (Table 1). The climate-safety margins of
Conclusions
Given the complex climate challenges facing many urban areas, city leaders are in urgent need of proactive and long-term solutions, especially actions that are cost-effective, equitable, deliver multiple benefits and provide systemic redundancy. NBSs still remain one of the most effective and multi-functional solutions that cities have to adapt to climate change while also providing important nature and green spaces that deliver a multitude of ecosystem services. However, as outlined in this
Declaration of Competing Interest
The authors report no declarations of interest.
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