A heatwave occurs when both maximum and minimum temperatures are unusually high over a 3-day period at a given location, based on historical records (BOM 2021a). The combination of very hot days followed by hot nights means that people, and native biodiversity, have less chance to recover before the high temperatures of the following day. Heatwaves on land cause significant impacts and distress. Elevated temperatures also affect freshwater systems, and marine heatwaves (Figure 6) are recognised as having major impacts, particularly on reef communities.

Figure 6 Mean sea surface temperatures and temperatures over land in the Australian region, 1910–2020

Terrestrial heatwaves

Heatwaves cause more human deaths in Australia than any other extreme weather event, and they are increasing in frequency and duration (Steffen & Hughes 2013, Coleman 2016). Some people are more vulnerable than others – age, health status and socio-economic disadvantage all contribute to heatwave vulnerability (Beggs et al. 2019). Australia’s potential vulnerability to heat exposure is high and increasing, with total deaths, lost working hours and mental health outcomes all increasing with measures of heat (Beggs et al. 2019). Presentations at hospital emergency departments peak on heatwave days, and there is a significant increase in presentations for up to 2 weeks after a heatwave event (Watson et al. 2019).

The effects of heatwaves on human populations, and by inference on animals, are exacerbated by poor air quality (e.g. Patel et al. 2019) (see the Air Quality chapter) and by urban overheating (e.g. Santamouris et al. 2021) (see the Urban chapter). The Reducing Illness and Lives Lost from Heatwaves project (PERN 2021) reflects on the complexity of heatwaves as a driver of mortality and morbidity, and the multiple contributing factors, including the geographical correlates of health, environmental epidemiology, and the built environment as a local context.

Native animals are susceptible to the effects of heatwaves. Heat stress has been responsible for large numbers of deaths in flying foxes across several species and several states in recent years (Mo & Roache 2021), as well as deaths in Carnaby’s black cockatoo (Calyptorhynchus latirostris) (Saunders et al. 2011). Small arid-zone birds are at particular risk because many already occupy habitat where they are living at close to their physiological limit; heatwaves can cause loss of condition, abandonment of nests and increased mortality (Sharpe et al. 2019). Heatwaves also impact freshwater habitats, affecting water temperature, dissolved oxygen content and conditions for bacterial growth (see case study: 2019 Menindee fish kills).

Domestic animals are also affected by heat stress; dairy cattle are less productive when the temperature–humidity index increases above threshold levels. Days of modest heat stress level are increasing in number per year, and the number of consecutive heat stress days is increasing: the frequency doubled between 1960–70 and 2000–08 (Nidumolu et al. 2010). Shading and spraying water over a concrete floor to increase local evaporative cooling mitigate impacts to some extent (Little & Campbell 2010), but, under higher greenhouse gas emission scenarios, the consequences across the entire Murray–Darling herd are for significant decreases in milk production over coming decades. United States studies show that late-gestational heat stress can impair future milk production for at least 2 generations (Laporta et al. 2020).

Heat stress also affects plant productivity. Wine Australia (2020) has released a climate atlas to support decadal decisions about grape varietal management for future market resilience.

Case Study 2019 Menindee fish kills

Klaus Joehnk and Tapas Biswas, CSIRO

Fish deaths are a relatively common phenomenon in Australian waterways, usually caused by low oxygen levels in low-flow waters during summer or drought conditions, blackwater events after large-scale floodplain inundation during warm-season post-drought flooding, or sediment- and ash-laden run-off after bushfires. However, the 2018–19 ‘Menindee fish kill’ event was an exception on an unprecedented scale (Sheldon et al. 2021). Although the exact number of fish killed is not known, it is estimated that more than 1 million fish died during this event. Between December 2018 and January 2019, 3 separate fish kill events occurred, with the most devastating one occurring on 5–6 January 2019 in the pool upstream of Weir 32 near the town of Menindee on the Darling River (Figure 7). The fish kill event covered a 40-kilometre stretch of the Darling River. It affected native fish, including Murray cod and golden perch, as well as invasive carp, which can survive lower oxygen levels. This event was of significant concern, with a high negative impact on local communities.

Figure 7 Fish kill in the Darling River near the Menindee Lakes, 2018–19

Photo: Graeme McCrabb

The event arose from a series of compounding events, including the low-flow conditions of the Darling River, poor water quality and a sudden change in temperature. However, there was also a history leading to these events. High-flow years in 2012 and 2016 led to an increase in fish abundance in the Menindee Lakes. Increasing drought conditions and low flow in the following years led to the draining of Lake Menindee. Fish moved into the weir pool above Weir 32, where they were trapped to some extent between Weir 32 and the upstream Menindee Main Weir, despite the presence of fishways. Furthermore, water quality deteriorated, leading to large algal blooms in the lakes and river in the months before the fish kills.

Low-flow conditions in the weir pool allowed the water column in the river reach to stratify, developing a warmer upper layer heated by the sun above a cooler lower layer, an effect commonly seen in lakes or reservoirs. Such stratification restricts oxygen exchange between the 2 layers – that is, low oxygen levels in the lower layer cannot be replenished from above. In rivers, such stratification is usually not persistent because flow will destroy it, leading to a mixed water column. However, under low-flow conditions, stratification can persist. Even then, if the air temperature falls significantly below the water temperature, typically at night, convective mixing due to cooling of the surface water will occur and, together with wind action, will mix the water column. Stratification can persist over several days when there is a long stretch of stable, warm weather with high daytime and night-time temperatures, combined with low flow. A combination of heatwave conditions and the low-flow conditions prevailed at the Menindee Lakes when the fish kills happened.

Another factor was necessary in leading to conditions that resulted in the fish kills: dissolved oxygen had to drop below a threshold to be lethal for fish – that is, the water column had to become hypoxic. Organic materials in the water column and on the riverbed are constantly decomposed by microbial action, which consumes oxygen. The sediment oxygen demand and sunken dead fish can draw down oxygen levels within a few days. Furthermore, although algae thriving in the upper water column produce oxygen during daytime via photosynthesis, they respire oxygen at night. In an ideal situation, this leads to a strong day–night cycle of oxygen saturation in the water column. In a stratified system, it leads to the lower water layer rapidly becoming hypoxic or even anoxic (no available oxygen) unless the water column is mixed. At the same time, the top water layer is well oxygenated during daytime but cannot replenish the oxygen in the bottom layer due to stratification. At night-time, the top layer loses oxygen, but levels are still sufficient for fish to survive unless the system suddenly mixes the low-oxygen top water with the anoxic bottom water, resulting in overall hypoxic conditions under which fish suffocate. Under such stratified circumstances, several potential causes for sudden mixing exist, including strong flow (which was not the case in the Darling River at the time of the fish kill), sudden high wind conditions, or a significant drop in temperature leading to convective mixing.

In the massive Menindee fish kills, drought conditions led to low or no flow, and a heatwave caused the build-up of a persistent stratification in the Menindee weir pool lasting for several days and resulting in anoxic conditions in the lower layer of the water column. On the surface, a large blue–green algae (cyanobacteria) bloom reduced night-time oxygen levels in the top layer, further aggravating the situation (see Figure 8). Finally, a sudden drop of air temperature during the night of 5–6 January (and likewise during the previous and following fish kill events) from 26–30 °C during the day to 16 °C at night, which was significantly lower than the ambient water temperature of around 28 °C, caused stratification of the water column to break down, leading to a fully mixed hypoxic water layer that triggered a fish kill.

Figure 8 Satellite image (Sentinel-2B) of the Menindee Lakes and Darling River at the time of the fish kills, 4 January 2019

Note: Menindee Lake is drained. Lake Pamamaroo and Lake Tandure shine bright green, signalling high blue–green algal content.

Source: Geoscience Australia & CSIRO Data61 (2021)

The Menindee fish kills were an extreme consequence of drought, heatwave, sudden change in weather conditions (with a significant drop in overnight temperatures) and very high blue–green algal growth. The question remains whether such events will happen in future – perhaps more often – and what can be done to reduce such adverse outcomes or even prevent them. With changing climate, drought spells and therefore low- or no-flow conditions might become more common. Intervention measures such as artificial refuges for fish and aerators, as implemented after the events, have only a local, short-term effect. Increased monitoring with in situ sensor networks and remote sensing can give early warning and provide knowledge for potential management options. Higher flows in the river channel can prevent such extreme events by reducing the risk of persistent stratification, resulting in higher oxygen re-aeration rates and reduced algal blooms. However, under drought conditions, there is only limited water available upstream to manage such situations effectively.

For further information, see AAS (2019) and Vertessy et al. (2019).

Marine heatwaves

Like temperatures on land, the physical environment of the oceans is changing because of climate change (see the Coasts and Marine chapters). Periods of extreme ocean warm water are known as marine heatwaves. They affect species’ distribution, reproduction success and persistence in some habitats. Marine heatwaves were recorded on the Great Barrier Reef in 2015–16, 2016–17 and 2020, and were accompanied by significant coral bleaching events (BOM 2020). Recent work has improved the accuracy and lead time of forecasts of extreme marine temperatures in 2 acknowledged global warming hotspots: Western Australia and the Tasman Sea (Hobday & Pecl 2014, CSIRO 2021).