Coastal resilience in the face of increasing wildfires: a land-sea perspective

Coasts are biodiversity hotspots at the nexus of compounding stressors from anthropogenic activity and climate change. Changing wildfire frequency and severity linked to climate change and land management can degrade water quality and alter coastal waters, leading to economic and social consequences for human well-being. However, previous coastal vulnerability studies have not considered wildfire. We present a global-scale index incorporating fire weather, population, commercial fisheries activity, and marine biodiversity hotspots to assess coastal wildfire vulnerability. Approximately 33% of moderate to highly vulnerable ecoregions sustain indigenous seafood consumption > 1,000 tons per year, of which Southeast Asia, Indonesia, and the Philippines are particularly at risk, surpassing 2,500 tons annually. This highlights regions requiring closer inspection for marine protections that are not currently capturing vulnerability, and that may not be apparent using index results alone. Implications of these findings are discussed with consideration of filling information gaps for improved coastal resilience. Coastal ecosystems are major hubs of biodiversity containing approximately 1 million fish, bird, and animal coastal species identified, with an estimated additional 9 million organisms remaining to be discovered [1]. Nearly 4 billion humans live near coasts or depend on them for their well-being and livelihoods, benefitting from ecosystem services such as maritime trade, fisheries, and recreation [2]. Coastal waters receive terrestrial exports of carbon, nutrients, and other runoff, which contribute to their high rates of respiration and ecosystem productivity [3]. Coasts are integral to the global carbon cycle, playing a critical home to processes of carbon transformation, outgassing, and sequestration [3–4]. For example, kelp is a foundational species present along 25% of the planet’s coastlines that sequesters between 61 and 268 teragrams of carbon per year, have high rates of primary productivity and biodiversity, and provides nursery grounds and habitat for several marine species including critical fish stocks [5–6]. Due to their importance to people and the planet, the United Nations (UN) recognizes coasts in the 2030 Agenda for Sustainable Development Goals 14.2 and 14.5 and the UN Climate Action Pathways, which call for management and action to strengthen coastal resilience, restore, protect and conserve coastal and marine ecosystems [7–8]. Anthropogenic stressors, most notably from population growth and corresponding development, subject coasts to habitat loss, increased erosion, nutrients, and pollutants, and hydrology changes that adversely impact ecosystem function [1–2, 9 and references therein]. Climate change further compounds these issues introducing additional stressors including sea level rise, ocean acidification, warming, and circulation change, and climate hazards such as hurricanes [9–10 and references therein]. An important knowledge gap exists in understanding how increasing climate hazards will impact coastal ecosystems and the communities dependent upon them [10–11]. For example, excess nutrient supply from rivers following a wildfire could result in increased sedimentation and algal blooms, smother marine species recruitment, reduce light availability for primary producers, compromise recreational water use, and in severe cases induce marine life mortality [12–15]. In this scenario, prevention, mitigation, and management efforts are challenged by the sporadic nature of wildfires, overlapping local, regional, and national policies and authorities, and public presence in impacted areas (i.e., beachgoers, coastal settlements). Wildfire regimes - characterized by patterns of extent, intensity, severity, frequency, and seasonality - are intensifying globally due to climate change and human land use changes [16–19]. Though wildfires are essential to the ecological function of terrestrial ecosystems, severe wildfires, particularly under the legacy of historical and ongoing fire suppression, can endanger human lives, cause massive property damage, alter biodiversity, and release pollutants [20–22]. Between 2001 and 2019, wildfires caused over 110 million hectares of global forest loss [23]. Fire-prone areas are expected to expand by 29% globally, with boreal and temperate zones increasing by 111% and 25%, respectively [18]. Worldwide, 15% of terrestrial and freshwater species face exacerbated extinction risks due to changing fire regimes, with savannas, grasslands, shrublands, and forests most at risk [24]. Australia's 2019–2020 burned area was 800% above the 1988–2001 average, while United States (US) wildfire burned areas have nearly quadrupled in the past 40 years [25–26]. In 2020, California wildfires cost $149 billion across economic, health, and environmental sectors, contributing to a 7% decline in the state’s forests since 1985 [22 and references therein, 27]. Record-breaking 2023 Canadian wildfires emitted 1.3 petagrams of CO₂ in addition to other air pollutants [21]. Australia’s 2019–2020 fires incurred $75 billion in losses and affected over 30% of the habitat for 70 vertebrate species, including 21 endangered species [22 and references therein]. These examples highlight the escalating impacts of wildfires on ecosystems and dependent organisms as climate change progresses. Amplified wildfire occurrence and intensity impact the timing, quantity, type, and transport of key biochemical constituents such as carbon, sediments, nutrients, and pollutants at the land-sea interface, altering coastal biogeochemical cycles and habitat quality for marine species [28–34]. As such wildfires may affect water quality, primary production, biodiversity, and marine carbon sequestration, threatening the life-sustaining functions of coastal ecosystems and jeopardizing their services to people. Specifically, vegetation loss, reduced infiltration, and increased surface runoff from wildfires in coastal watersheds can increase sediment, nutrient, and pollutant delivery to nearshore waters, elevate water temperatures, disrupt water supply services, and damage critical water infrastructure [28–32]. These physical and chemical changes in riverine discharges to the coast can imperil the health of nearshore ecosystems [14–15, 29 and references therein, 35]. Furthermore, climate change-driven global precipitation extremes may occur up to 40% more often by 2100 increasing periods of drought and heavy rainfall, which play a major role in fire weather and post-fire water impacts such as debris flows [36–38].