2021 State of the Environment Report Marine Chapter – Case Study - Marine heatwaves

The Marine chapter of the 2021 State of the Environment (SoE) report incorporates multiple expert templates developed from streams of marine data. This metadata record describes the Case Study "Marine heatwaves". 
<strong><em>A PDF of the full Case Study, including figures and tables (where provided) is downloadable in the "On-line Resources" section of this record as "CASE STUDY 2021 – Marine heatwaves"</em></strong>



DESCRIPTION OF THE CASE STUDY
The physical characteristics of the ocean, such as temperature, salinity, oxygen content, and pH are clearly changing in Australia’s oceans as a result of climate change. Summaries from the IPCC and other publications clearly show long term trends that are clearly attributable to increased greenhouse gases. The southeast and southwest of Australia are global warming hotspots (Hobday &amp; Pecl, 2014), with rates of warming above the global average. More recently, extreme events in the ocean environment have received considerable attention as they often cause widespread ecosystem impact and represent examples into future environmental conditions. Marine heatwaves, intense upwelling, deoxygenation, and coastal flooding are examples of extreme events and have already affected habitats around more than 40% of the Australian coastline (Babcock et al. 2019). 
Periods of extreme ocean warm-water events known as marine heatwaves (MHWs) can have significant impacts on marine ecosystems and industries. The formation of MHWs is a result of heat flux into a region from the atmosphere, or via advection of warm water, often from lower latitudes (Holbrook et al. 2019). MHWs are widely defined based on differences with expected temperatures for the location and time of year, and so may occur during any season, not just summer (Hobday et al 2016). A quantitative MHW definition now often used is when seawater temperatures exceed a seasonally varying threshold (the 90th percentile) for at least 5 consecutive days (Hobday et al 2016). Based on this definition, MHWs increased in frequency (34%) and duration (17%) from 1925 to 2016, resulting in a 54% increase in annual MHW days globally (Oliver et al 2018). These trends can largely be explained by increases in mean ocean temperatures (Oliver et al. 2019), with further increases in MHW days projected to occur under continued global warming with many parts of the ocean reaching a near-permanent MHW state by the late 21st century (Oliver et al. 2019).  

ISSUES OF IMPORTANCE
To aid in the description of MHWs, a categorisation system analogous to earthquake or cyclones has been developed (Hobday et al. 2018). Based on this system, Australia has experienced strong marine heatwaves in recent years, including Western Australia in 2011 (Wernberg et al 2016), the Coral Sea and northern Australia in 2016 (Oliver et al 2017), and the Tasman Sea in 2015/16 (Oliver et al 2017). Coral bleaching on the Great Barrier Reef was associated with marine heatwaves in successive years (Hughes et al 2018) resulting in impaired recruitment and recovery of reefs (Hughes et al 2019). 
The contribution of climate change to marine heatwaves can be calculated using the fraction of attributable risk method, which has been used for extreme events since around 2005 (Oliver et al. 2017; Perkins-Kirkpatrick et al. 2019). For example, the Tasman Sea 2015/16 MHW was over 300 times more likely as a result of climate change.  

DATA STREAM(S) USED IN CASE STUDY
Literature and www.marineheatwaves.org/tracker

Tags: case study, marine heatwaves, state of the environment (SoE)
Organization: Australian Ocean Data Network