Key messages

• Heatwave frequency is projected to increase across NSW by the middle of the century under all emissions scenarios.
• Under a medium-emissions (SSP2-4.5) and high-emissions (SSP3-7.0) scenario heatwave frequency will increase to the end of the century.
• Under a low-emissions (SSP1-2.6) scenario, although heatwave frequency still increases, the rate at which it is increasing is projected to slow down towards the end of the century, highlighting the importance of reducing emissions.
• The highest heatwave frequency is expected in the north-east of the state.
• The higher the heatwave frequency, the greater the implications for human health, agriculture, and infrastructure.
• Heatwave frequency projections can be used to assess how often populations and ecosystems are exposed to extreme heat events. This supports adaptation planning and the development of risk management strategies.

Heatwave background

What is heatwave frequency and how is it measured?

Heatwave frequency, or HWF, is the number of days each year that contribute to a heatwave event. For example, if there are two heatwave events in a year that each last three days, the HWF would be six days for that year.

A heatwave is determined using the excess heat factor (EHF)¹ which considers:

• Comparison of the average temperatures for a 3-day period against the hottest days on record at that location with respect to the annual temperature threshold at the location (above the 95th percentile).
• The observed temperatures at that location over the past 30 days.

Key findings 

Changes to heatwave frequency

Heatwave frequency is projected to increase across NSW by the middle of the century under all climate models and emissions scenarios when compared to the baseline periodⁱ (1990–2009). In the second half of the century, heatwave frequency continues to increase under a medium-emissions (SSP2-4.5) and high-emissions (SSP3-7.0) scenario, while under a low-emissions scenario (SSP1-2.6), that rate of increase slows resulting in only a minor increase by the end of the century when compared to mid-century (Figure 1).

Figure 2. Projected change in average annual number of heatwave frequency (days) under a low-, medium- and high-emissions scenario for the time periods 2020–2039 (near-future), 2040–2059 (mid-century), 2060–2079 (late century) and 2080–2099 (end of century). Change projections are relative to historical baseline 1990–2009.

By the middle of the centuryⁱⁱ

• The frequency of heatwave days is projected to be more than double the baseline frequency under all emissions scenarios.
• The number of annual heatwave days across the state could increase by over 22 days from the baseline under a high-emission scenario.

By the end of the centuryⁱⁱⁱ: 

• The state could see an average mean of 23.9 heatwave days per year under a low-emissions scenario, an average of 42.4 heatwave days per year under a medium-emissions scenario and an average of 60.3 heatwave days per year under a high-emissions scenario.
• Under a high-emissions scenario the number of heatwave days across the state could increase by 49.7 days above the baseline period.

Heatwave frequency geographic differences across NSW

Average heatwave frequency will increase across the state. The greatest heatwave frequencies are expected in the north-east of the state, to the west of the Great Dividing Range. 
 

Implications of increased heatwave frequency

A greater frequency of heatwaves increases the implications to human health, agriculture, and infrastructure. For example, instances of heat stress could increase for the public, and the health sector may be required to deal with more frequent heat-related illness.³ Crops and livestock will also be exposed to more heatwave days, which could impact production in the industry.⁴ A modelling study for Australian sheep estimates that 2.1 million potential lambs are lost annually due to heat stress alone, increasing to 3.3 million with 3°C warming over the historical baseline.⁵  

Additionally, more frequent and intense heatwaves can expose infrastructure to temperatures beyond the operating conditions assumed in historical design and planning, placing stress on infrastructure that may have been designed for historical temperatures, and may result in increased wear and material degradation over time.⁶ Financial losses from the 2009 heatwave in southeast Australia have been estimated at $800 million, mainly due to power outages and disruptions to the transport system.⁷

Extended hot summer weather can cause significant demand on the electricity system, water services and the cost of living. This strain has increased over recent decades with the growing use of air conditioners in homes and businesses.⁸

Heatwaves have the biggest impact on the electricity grid during January and February, when increased cooling demand places additional pressure on the interconnected electricity system. If reserve capacity becomes limited and is compounded by generator or transmission outages, involuntary load shedding – the temporary disconnection of electricity supply to some customers to maintain system stability – may be required. This can result in controlled or rolling blackouts to maintain grid security.⁹

Uses of heatwave frequency climate data

Heatwave frequency projections can be used to assess how often populations and ecosystems are exposed to extreme heat events each year.

For example, information on heatwave frequency can be used by the health sector to plan for and prevent heat stress, while the agricultural sector may use the information to plan for impacts on crops and livestock. Information on increases to heatwave frequency can also inform urban planning, to ensure infrastructure is resilient to changing conditions.