Extreme Climate Events Impact Marine Habitat Forming Communities Along 45% of Australia’s Coast

From Frontiers in Marine Science

Original Research ARTICLE

Front. Mar. Sci., 24 July 2019 | https://doi.org/10.3389/fmars.2019.00411

Severe Continental-Scale Impacts of Climate Change Are Happening Now: Extreme Climate Events Impact Marine Habitat Forming Communities Along 45% of Australia’s Coast

Russell C. Babcock^1,2*, Rodrigo H. Bustamante¹, Elizabeth A. Fulton³, Derek J. Fulton³, Michael D. E. Haywood¹, Alistair James Hobday³, Robert Kenyon¹, Richard James Matear³, Eva E. Plagányi¹, Anthony J. Richardson^1,4 and Mathew A. Vanderklift^3,5

• ¹CSIRO Oceans and Atmosphere, Brisbane, QLD, Australia
• ²School of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA, Australia
• ³CSIRO Oceans and Atmosphere, Hobart, TAS, Australia
• ⁴Centre for Applications in Natural Resource Mathematics, School of Mathematics and Physics, The University of Queensland, St. Lucia, QLD, Australia
• ⁵CSIRO Oceans and Atmosphere, Indian Ocean Marine Research Centre, Crawley, WA, Australia

Recent increases in the frequency of extreme climate events (ECEs) such as heatwaves and floods have been attributed to climate change, and could have pronounced ecosystem and evolutionary impacts because they provide little opportunity for organisms to acclimate or adapt. Here we synthesize information on a series of ECEs in Australia from 2011 to 2017 that led to well-documented, abrupt, and extensive mortality of key marine habitat-forming organisms – corals, kelps, seagrasses, and mangroves – along >45% of the continental coastline of Australia. Coral bleaching occurred across much of northern Australia due to marine heatwaves (MHWs) affecting different regions in 2011, 2013, 2016, and 2017, while seagrass was impacted by anomalously high rainfall events in 2011 on both east and west tropical coasts. A MHW off western Australia (WA) during the 2011 La Niña extended into temperate and subtropical regions, causing widespread mortality of kelp forests and seagrass communities at their northern distribution limits. Mangrove forests experienced high mortality during the 2016 El Niño across coastal areas of northern and north-WA due to severe water stress driven by drought and anomalously low mean sea levels. This series of ECEs reflects a variety of different events – MHWs, intense rainfall from tropical storms, and drought. Their repeated occurrence and wide extent are consistent with projections of increased frequency and intensity of ECEs and have broad implications elsewhere because similar trends are predicted globally. The unprecedented and widespread nature of these ECE impacts has likely produced substantial ecosystem-wide repercussions. Predictions from ecosystem models suggest that the widespread mortality of habitat-forming taxa will have long-term and in some cases irreversible consequences, especially if they continue to become more frequent or severe. The abrupt ecological changes that are caused by ECEs could have greater long-term impacts than slower warming that leads to gradual reorganization and possible evolution and adaptation. ECEs are an emerging threat to marine ecosystems, and will require better seasonal prediction and mitigation strategies.

Introduction

Extreme climate events (ECEs), statistically rare or unusual climatic periods that alter ecosystem structure and/or function well outside normal variability (Smith, 2011), are receiving increasing attention as drivers of change in ecological and evolutionary communities (IPCC, 2012; van de Pol et al., 2017). ECEs are also associated with climate change, becoming more frequent and more intense (e.g., Herring et al., 2018). In coastal marine systems, heatwaves and floods could have greater ecosystem and evolutionary impacts than the more gradual effects of climate change (Campbell-Staton et al., 2017; Grant et al., 2017), and essentially represent a “pulse vs. press” dichotomy in terms of climate impact regime (Harris et al., 2018). For example, heatwaves compound the effects of underlying warming trends and provide little opportunity for organisms to acclimate or adapt, whereas slower warming is more likely to allow time for these processes to occur (Walther, 2010). Although there is a common perception that high-latitude areas will be most affected by climate change because the magnitude of warming is greater there (Burrows et al., 2011), low-latitude areas with dampened seasonal cycles, have the greatest emergence of extreme heat (Diffenbaugh and Scherer, 2011) and host many species that inhabit environments already close to the limits of their thermal tolerance (Sunday et al., 2011, 2012; Frieler et al., 2013; Pinsky et al., 2019). Global warming is increasingly affecting marine ecosystems including habitat forming sessile organisms (Poloczanska et al., 2013), which are often key ecosystem engineers and particularly vulnerable to heatwaves as individuals cannot physically move to cooler locations (Mislan and Wethey, 2015).

Here we synthesize the unprecedented large-scale impacts of a series of ECEs on coastal marine habitats around the Australian continent (Figure 1) between 2011 and 2017, spanning both strong El Niño and La Niña phases of the El Nino Southern Oscillation (ENSO). We also model how impacts on habitat forming organisms propagate through food webs and ecosystems under a range of impact scenarios. The impact of ECEs throughout most of northern Australia has broad implications as the climate change phenomena driving the ECEs are being experienced globally (Oliver et al., 2018a). These conditions are also a precursor of a future in which ECEs are increasingly common, since ECEs are episodic, thus providing little or no time for acclimation and evolution, thus potentially exacerbating damage through the shocks they create within ecosystems (IPCC, 2012).

FIGURE 1

Figure 1. (A) Cumulative habitat impact map (created by overlaying the individual event maps detailed in Figure 2). (B) Cumulative percentage of the Australian marine domain that experienced the maximum monthly SST for the period between 1985 and 2017, based on NOAA High Resolution Daily Sea Surface Temperature. The analysis was performed for each 0.25 × 0.25 degree cell in the domain (10–45° S, 110–160° E) for the period 1985–2017. The year in which the warmest month was recorded for each cell. The plot is the cumulative sum of these warmest years across all cells. For example, 50% of the ocean around Australia has experienced its warmest month between 2008 and 2017.

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