Turning up the heat: rapid intensification and long-lasting impacts of marine heatwaves

Over a decade ago, the highly diverse and productive marine ecosystems that fringe the Western Australian coastline were subjected to the highest magnitude warming event (‘marine heatwave’) ever recorded in the region. Sea temperatures were up to 5°C warmer than average, and extreme temperatures were observed along more than 2,000 km of coastline for around 10 weeks over summer and autumn. At the time, marine biologists including ourselves documented a wide range of ecological impacts and unusual observations that were attributed to the extreme temperatures recorded during the marine heatwave. These included sightings of warm-water megafauna such as tiger and whale sharks much further south than normal, mass die-offs of fish, abalone, kelp, and seagrass, unprecedented coral bleaching, and increased abundances of warm-tolerant fish, sea urchins, and seaweeds (Fig. 1). Overall, we observed a rapid shift in ecosystem structure, with major losses of cold-water seaweeds and seagrasses occurring across hundreds of kilometres of coastline. This, in turn, affected coastal communities and economies, particularly through impacts on fisheries, aquaculture, and tourism.

Figure 1. Two photos of the same reef in Kalbarri before (left) and after (right) the 2011 marine heatwave. Before the MHW reefs in the region were covered by a dense kelp canopy and pink coralline understorey. After the MHW the kelps had disappeared and all reef surfaces were covered by small sediment-packed filamentous and foliose seaweeds. More than 10 years later the kelps have not come back. © Thomas Wernberg (University of Western Australia).

Sustained ecological monitoring in the region following the marine heatwave has shed new light on the capacity of ecosystems to withstand and recover from extreme warming events. Alarmingly, the kelp forests that were most adversely affected have shown little sign of recovery after >10 years, and these habitats are now dominated by weedy turf-forming algae and some warm-tolerant Sargassum and coral species. Since the high-profile event in Western Australia, the number of marine heatwaves impacting species and ecosystems around the world has soared, as has the research effort to better understand the physical drivers and biological impacts of marine heatwaves. For example, significant marine heatwaves have been recorded in the northwest Atlantic in 2012, along the entire northeast Pacific coastline in 2014–2016 (‘the Blob’), across tropical Australia in 2016, and in the Mediterranean Sea in multiple recent summers. In June 2023, much of the North Atlantic reached marine heatwave status, with coastal sea temperatures around the UK and Ireland peaking at 5°C warmer than the long-term average (Fig. 2). Documenting, understanding, and predicting ecological impacts of marine heatwaves is challenging, given the complex nature of ecosystems and the dynamic ocean environment. What is clear, however, is that extreme temperatures can have devasting effects on marine life and drive widespread, dramatic shifts in biodiversity and ecosystem structure.

Figure 2. Sea surface temperature anomaly (°C) for the month of June 2023, relative to the 1991-2020 reference period. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF.

Marine heatwaves, as with other ‘weather’ events, are naturally occurring phenomena driven by a range of complex physical processes interacting across the global climate system. However, anthropogenic global heating caused by greenhouse gas emissions has led to rapid warming of the upper layers of the global ocean, which have absorbed around 90 per cent of the excess heat trapped in the atmosphere. As such, when extreme climatic events now occur, they are superimposed onto long-term warming trends and supercharged by record-high ocean heat content. Marine heatwaves are predicted to continue to intensify in the coming decades as a direct consequence of anthropogenic climate change. Improving our understanding of how these extreme events impact marine ecosystems is vital to inform approaches to adaptation, management, and conservation.