Heatwave duration

Heatwave duration, or HWD, is projected to increase across NSW by the middle of the century under all climate models and climate scenarios.
Under a medium-emissions scenario (SSP2-4.5) and high-emissions (SSP3-7.0) scenario, HWD will continue to increase by the end of the century, while it is projected to level out under a low-emissions (SSP1-2.6) scenario.
The greatest increases to heatwave duration are expected in the north-east of the state.
Longer heatwave durations impact human health by limiting the ability for people to recover from heat stress.
Heatwave duration projections can be used to assess the longest given period annually that populations, ecosystems and infrastructure are exposed to extreme heat events without heat relief. This supports adaptation planning and the development of risk management strategies.

Heatwave background

What is heatwave duration and how is it measured?

Heatwave duration, or HWD, is the length of the longest heatwave in a year, measured by the number of days it lasts. For example, a heatwave duration of 5 means the longest heatwave in a year was, or is, projected to be 5 days long.

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 duration

Heatwave duration 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 to 2009). In the second half of the century, heatwave duration is projected to continue to increase under a medium-emissions (SSP2-4.5) and high-emissions (SSP3-7.0) scenario. Under a low-emissions (SSP1-2.6) scenario, heatwave duration will plateau in the decades following the middle of century.

By the middle of the centuryⁱⁱ

The longest heatwave annually is projected to last 9.7 days for the low -emissions scenario, 11.1 days under both a medium-emissions and 11.9 under a high-emissions scenario (SSP3-7.0).

Figure 2. Projected annual average heatwave duration (left) increase (right) – NSW (2040–2059). The bold number is the ensemble average for the period. Underneath the average is the ensemble range. Heatwave duration increases are additional to the historical model baseline of 7.2 days.

By the end of the centuryⁱⁱⁱ:

The longest heatwave annually is projected to last 10 days under a low-emissions scenario, 14.6 days under a medium-emissions scenario and over 20 days under a high-emissions scenario.
This approximately represents a doubling in length under a medium-emissions scenario and almost a tripling in length under a high-emissions scenario compared to the baseline.
The longest heatwave annually is projected to last an additional 13.2 days from the baseline under a high-emissions scenario.

Figure 3. Projected annual average heatwave duration (left) and increase (right) for 2080–2099 (NSW). The bold number is the ensemble average for the period. Underneath the average is the ensemble range. Heatwave duration increases are additional to the historical model baseline of 7.2 days.

Heatwave duration – geographic differences across NSW

Average heatwave duration will increase across the state. The greatest increases to heatwave duration are expected to be observed to the west of the Great Dividing Range in the north-east of the state, such as in Tamworth.

Figure 4. Projected change in average annual number of heatwave 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. Source: NARCliM2.0

Implications of longer heatwaves under climate change

Longer duration heatwaves can have more severe implications for human health than shorter more intense heatwaves. Most people can survive one extremely hot day, however when heatwaves last for multiple days, paired with high nighttime temperatures, this prevents the body from recovering, which can lead to serious health impacts.²

During heatwaves electricity demand often increases by the third or fourth day of a heatwave event as air conditioners work harder to offset the heat that has accumulated in buildings. This increased cooling demand places additional pressure on the interconnected electricity system.³ 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.⁴

Uses of heatwave duration climate data

Information on the projected changes to heatwave duration in NSW can be used to assess the persistence of extreme heat events in a year. The information may be used by the health sector to assess the increased risk to public health, or by the agricultural sector to anticipate impacts such as crop damage or yield reduction. Data on the changes to heatwave duration can also help inform energy demand and transport system needs, to improve infrastructure resilience.

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Technical information

Technical description

Heatwave indices using NARCliM2.0 are calculated over a 5-month austral summer period (November to March) per year, due to the high impacts of heatwaves over Australia during this period. It also represents how high temperatures are compared to expected summertime conditions. Heatwave indices are calculated using the Excess Heat Factor (EHF) method.

Excess Heat Factor

Excess Heat Factor (EHF) is based on a three-day-averaged daily mean temperature (DMT) and is intended to capture heatwave intensity as it applies to human health outcomes. The index is described and placed in a climatological context to derive heatwave severity.

EHF incorporates two ingredients. The first ingredient is a measure of how hot a three-day period (TDP) is with respect to an annual temperature threshold at each location. If the daily mean temperature averaged over the three-day period is higher than the climatological 95th percentile for DMT, then the TDP and each day within in it are deemed to be in heatwave conditions.

The second ingredient is a measure of how hot the TDP is with respect to the recent past (specifically the previous 30 days). This considers the idea that people acclimatise (at least to some extent) to their local climate, with respect to its temperature variation across latitude and throughout the year but may not be prepared for a sudden rise in temperature above that of the recent past.

Because of this definition, it is not enough to be ‘very hot’ to get a heatwave. It must be very hot compared to usual conditions at the given location, which means you can get heatwaves in cool alpine or coastal areas as well as in arid, inland areas. The EHF is calculated with reference to two different measures of temperature, so it has the unit of measure °C². Once a heatwave is identified, the EHF provides information on the characteristics of such heatwaves.⁵

Heatwave duration for EHF heatwaves (EHF-HWD)

Description: The index Heatwave Duration is the length of the longest heatwave, as defined by Excess Heat Factor (EHF), in each year identified by Heatwave Number (HWN)6.

Utility: HWD helps assess the persistence of extreme heat events in a year. It is useful for public health (increase risks), agriculture (crop damage and yield reduction), infrastructure resilience (energy demand, transport systems) and climate change monitoring (changes in heat intensity and length).

A limitation of HWD is that it does not account for humidity and may under-represent heatwave impacts during humid-heat periods or for areas of high humidity.

Units: Number of days

Frequency in NARCliM2.0: Yearly

NARCliM2.0 time periods

Time periods in this document

i. Baseline period: The modelled average for each climate variable from 1990–2009, used for comparison with future projections.

ii. Middle of the century: The projected annual average for 2040 to 2059. This is compared against a historical model baseline period. The projections for each time period represent averaged data across all 10 NARCliM climate models.

iii. End of the century: The projected annual average for 2080 to 2099. This is compared against a historical model baseline period. The projections for each time period represent averaged data across all 10 NARCliM climate models.

NARCliM2.0

New South Wales and Australian Regional Climate Modelling (NARCliM)2.0 provides nation-leading climate model data that spans the range of plausible future changes in climate. It offers:

climate projections to the year 2100, and simulations of the past
4-km scale projections for south-east Australia, 20-km scale projections for the broader Australasian region
projections under low (SSP1-2.6), medium (SSP2-4.5), and high (SSP3-7.0) emissions scenarios to understand how climate risk differs depending on emissions pathways (Shared Socioeconomic Pathways, SSPs).