From 2009 to 2°C: Climate change, mortality and morbidity in Australia

Climate change does not simply increase average risk. It reshapes the distribution of health outcomes. This challenges how we think about mortality, morbidity and health system resilience.

In late January 2009, the state of Victoria experienced one of the most severe heatwaves in its recorded history. Over a number of days, temperatures in Melbourne exceeded 43°C.

The impact on health outcomes was researched in some detail. There were 374 ^[1] excess deaths, a 46% ^[1] surge in ambulance emergency caseloads over the three hottest days and significant increases in emergency department (ED) presentations and hospital admissions.

It was not a marginal public health event. It was a system stress test.

17 years on, the climate is warmer, the population older and heat extremes more frequent. If a 2009-style event were to occur in a world that is around 2°C warmer, the consequences would likely be materially worse. Not simply because average temperatures rise, but because the tails of the distribution expand.

The tail matters more than the mean.

This article distinguishes clearly between mortality, morbidity and system strain; highlights the populations most at risk; explores compounding hazards such as bushfire smoke; and reflects on model risk, attribution challenges and adaptation.

Mortality, morbidity and system strain: three different lenses

Heatwaves are often discussed in terms of excess deaths. Mortality is visible, countable and emotionally salient. But mortality is only one dimension of a much larger challenge.

Mortality captures excess deaths above expected seasonal baselines. In 2009, Victoria’s 374 excess deaths were calculated by comparing observed deaths during the heatwave with historical averages for the same period. This equates to a 62% increase in mortality over the days considered, most of which related to people over 75, with cardiovascular causes prominent.

Morbidity refers to non-fatal health impacts. During the same event, ambulance call-outs rose 25%, emergency department (ED) presentations increased 12% and hospital admissions rose for dehydration, renal failure, cardiovascular events and respiratory conditions ^[1] .

The Alfred Hospital reported an 81% ^[2] increase in medical admissions during the event. 

System strain is the third layer of impacts during these events. Heatwaves create peaks in healthcare demand: ambulance services surge, EDs crowd, beds fill and elective procedures are deferred. Night-time temperatures remain elevated, reducing recovery for both patients and staff.

Evidence from the Victorian heatwave and similar events suggests that for every excess death there are typically two to three additional hospital admissions, several emergency department presentations and around ten ambulance call-outs.

From an actuarial standpoint, morbidity and system strain are often more economically material than mortality. Health insurers see utilisation before deaths. Public systems incur surge costs, which can have cascading longer-term impacts. Understanding climate impacts therefore requires modelling all three layers.

Frequency and severity: shifting risk distributions

Climate change affects both the severity and the frequency of extreme events.

Mean temperature increases of 1–2°C may appear modest. However, mortality risk does not increase linearly with temperature.

Climate change shifts and stretches the distribution. Although warmer winters may reduce some cold-related mortality, increases in extreme heat, hot nights and prolonged heatwaves are projected to substantially increase heat-related mortality. These impacts are expected to be amplified by Australia's ageing population and increasing urban exposure.

Hot nights are particularly dangerous. Humans can tolerate short bursts of heat if the body cools overnight. But when night-time temperatures remain high, the body cannot recover and physiological stress accumulates over several days. ^[3]

The Australian National Climate Risk Assessment projects that severe heatwave days are expected to approximately double under 2°C of warming, with hot nights and prolonged heatwaves becoming increasingly common.

For actuaries, this dynamic is familiar. Loss distributions change not only because events become more severe, but because the probability of extreme outcomes increases.

Which populations in Australia are most vulnerable?

Heat impacts are not evenly distributed.

The strongest risk factors include:

• Age: The over 75 cohort carries disproportionate risk. Australia’s population aged 75 and over is projected to roughly double by mid-century, meaning exposure to heat-sensitive populations is rising.
• Chronic disease: Cardiovascular disease, chronic kidney disease, respiratory illness and diabetes increase vulnerability.
• Socioeconomic disadvantage: Poor housing insulation, low tree canopy, energy poverty and the access to air conditioning amplify the exposure.
• Urban heat islands: Inland growth corridors such as Western Sydney experience significantly higher temperatures and hotter nights than coastal areas.
• Residential aged care: Facilities without adequate cooling or surge staffing are particularly vulnerable.

Climate risk and demographic risk are multiplicative, not additive. An older, more urbanised Australia in a warmer climate creates compounding exposure.

Compounding and cascading risks

The broader impacts of heatwaves are important. The 2019–20 Black Summer bushfires demonstrated how heat and smoke can combine to amplify health impacts. Across eastern Australia, prolonged bushfire smoke exposure was estimated to contribute to approximately 417 excess deaths, more than 3,000 respiratory and cardiovascular hospital admissions, and over 1,300 asthma-related emergency department presentations. ^[4]

When smoke coincides with extreme heat, the health burden compounds.

Other cascading risks include:

• Heat + power outage: Loss of air conditioning during extreme heat can turn vulnerable households into acute medical cases.
• Heat + water stress: Dehydration risk increases in drought-affected regions.
• Heat + workforce fatigue: Health workers themselves face heat stress, increasing operational risk.

These are not simply health events; they are examples of systemic risk. Multiple hazards interact with demographic, infrastructure and health system vulnerabilities, producing outcomes that are greater than the sum of their individual parts. For actuaries, this is a familiar challenge: understanding dependency, correlation and tail risk, rather than treating risks as independent events.

How should actuaries model the health risk given deep uncertainty?

The impacts are highly uncertain. What emissions pathway are we on? What is the resulting impact on average and extreme climate outcomes? What are the key interdependencies? How will we adapt in the meantime?

Even estimating historical temperature-related mortality is challenging. Different studies use different reference temperatures, time lag structures, expected mortality and statistical models. These modelling choices can materially impact the conclusions. 

For actuaries, this type of model risk is familiar. Longevity models, catastrophe models and insurance pricing assumptions all involve structural uncertainty.

Climate-health modelling presents a similar challenge: multiple layers of uncertainty interacting over long time horizons.

For example, the impact of a future heatwave depends not only on temperature, but on whether hot nights persist, how many older people are exposed, whether homes and aged-care facilities can stay cool, whether the power system remains reliable, and how quickly health services can respond. These uncertainties interact, which is why climate-health risk is best understood through scenarios.

Adaptation as a variable, not a constant

Future mortality outcomes depend critically on adaptation.

Heat Health Plans, early warning systems, urban greening, increased air conditioning penetration and targeted outreach can materially reduce impacts. Victoria’s post-2009 heatwave reforms are widely regarded as having improved preparedness, with evidence suggesting lower excess mortality in subsequent severe events.

Humans are adaptable. Communities living in hot climates have long developed cultural and behavioural strategies for coping with heat.

However, modern heat risk arises from a different combination of factors like large urban populations, ageing demographics and sustained heat events that limit night-time recovery.

Adaptation also introduces new dependencies. Air conditioning depends on energy affordability and grid reliability. Urban greening takes decades. Behavioural responses vary, and aged-care infrastructure remains uneven.

Effective planning must treat adaptation as dynamic. The relevant actuarial question is not simply what temperatures will be, but how will exposure, vulnerability and adaptation evolve and impact one another?

Conclusion

The lesson of 2009 is not that heatwaves are rare tragedies. It is that extreme heat can rapidly stress mortality, morbidity and health systems.

With warming of 2°C, such events are likely to become more frequent and more intense, with highly uncertain short- and longer-term impacts.

For actuaries, this is not an abstract environmental issue. Climate-related health risk involves shifting probability distributions, compounding hazards and deep uncertainty; challenges that are central to actuarial practice.

The direction of travel is clear.

The question is whether our models, understanding and public systems are equipped for a world in which extreme events become more consequential.

Climate-health risk is not simply another emerging risk. It is an opportunity for the actuarial profession to apply its core strengths: understanding uncertainty, modelling complex systems, and informing long-term decision-making on one of Australia's most important resilience challenges.

References

^[1] Department of Human Services, Victoria. (2009). January 2009 heatwave in Victoria: An assessment of health impacts. https://www.aph.gov.au/DocumentStore.ashx?id=14ec91a8-6a8f-492e-89ba-c1c2352d8420

^[2] Lindstrom, S. J., Nagalingam, V., & Newnham, H. H. (2013). Impact of the 2009 Melbourne heatwave on a major public hospital. Internal Medicine Journal, 43(11), 1246–1250. https://doi.org/10.1111/imj.12275

^[3] Vardoulakis, S., Dear, K., Hajat, S., Heaviside, C., Eggen, B., & McMichael, A. J. (2015). Comparative assessment of the effects of climate change on heat- and cold-related mortality in the United Kingdom and Australia. Environmental Health Perspectives, 122(12), 1285–1292. https://doi.org/10.1289/ehp.1307524

^[4] Borchers Arriagada, N., Palmer, A. J., Bowman, D. M. J. S., Morgan, G. G., Jalaludin, B. B., & Johnston, F. H. (2020). Unprecedented smoke-related health burden associated with the 2019–20 bushfires in eastern Australia. Medical Journal of Australia, 213(6), 282–283. https://doi.org/10.5694/mja2.50545