Australia just recorded its hottest Spring on record, with temperatures 2.5 degrees above the preindustrial average. It was also humid. This should be a wake-up call, not just because it sets a new high, but because it signals a dangerous trend that will worsen as heating accelerates. One of the most concerning aspects of rising temperatures is their disproportionate impact on cities, particularly areas like Western Sydney where I have been working recently and which are hit much harder than coastal areas. The Urban Heat Island effect, especially when combined with the risk of wet bulb temperature moments, poses a serious threat to public health and liveability.
What is the Urban Heat Island effect?
The Urban Heat Island effect occurs when cities and suburban areas become significantly warmer than their rural surroundings. This additional heat is driven by the built environment:
• Paved surfaces like roads and roofs absorb and radiate heat.
• Lack of vegetation means less shade and minimal cooling through transpiration.
• Waste heat from air conditioners, vehicles, and industry adds to the temperature spike.
In places like Western Sydney, climate change, rapid urbanisation and sparse tree cover are already amplifying the Urban Heat Island effect. The result? Heatwaves are becoming more intense and prolonged, pushing local temperatures to extremes.
The Danger of Wet Bulb Temperatures
Linked to the Urban Heat Island effect is the insidious risk of wet bulb temperature moments which occur when dangerous heat combines with humidity. For every 1°C of global warming, the atmosphere holds approximately 7% more water vapour, intensifying humidity levels. When temperatures get close to or above 37°C and humidity levels are high, it becomes physiologically impossible for humans to regulate their body temperatures. Our natural cooling mechanism of sweating stops working, even in the shade or with ventilation. Prolonged exposure to such conditions can lead to heatstroke and death, regardless of fitness or hydration levels.
During heatwaves, inland built-up areas like Penrith and Blacktown face much higher risks than coastal suburbs like Bondi or Manly, where sea breezes provide relief. Already, Western Sydney has seen temperatures soar to 50°C during heat events, and as temperatures continue to rise, the frequency and intensity of these events will increase. This summer will be a test.
Australia Needs to Get Ready
Heatwaves are already Australia’s deadliest natural hazard. And the reality of climate change means that hotter, longer, and more humid heatwaves are unavoidable, even if we drastically reduce emissions today. Preparing for this reality is critical, especially in places where populations are growing and where the Urban Heat Island effect and wet bulb moment risks intersect. Here’s what Australia must do:
1. Invest in Urban Greening
Increasing tree canopy cover is one of the most effective ways to mitigate the Urban Heat Island effect. It’s cooler sitting under a tree than in the middle of Westfield a car park, right! Trees provide shade and cool the air through transpiration. With Western Sydney lagging behind other regions in tree cover, urban greening programs can reverse this trend. Replacing concrete with trees and biodiverse grasses and shrubs should be prioritised - especially along streets, in parks, and around schools and public spaces.
2. Rethink Urban Design
Urban planning must prioritise heat resilience. This includes designing buildings with reflective materials, incorporating green roofs and walls, and ensuring adequate airflow in built environments. Community spaces should be designed as cooling hubs, with access to water, shade, and air conditioning during heatwaves.
3. Prepare for Heat Emergencies
Local governments and health services need comprehensive heatwave plans. This includes public education campaigns, ensuring access to cooling for vulnerable populations, and strengthening emergency response systems.
4. Transition to Renewable Energy
The more we rely on fossil fuels, the hotter our planet becomes. In contrast, transitioning to renewables and electrification is a win-win: it reduces greenhouse gas emissions and lowers the Urban Heat Island effect by reducing waste heat. Almost half a million cars can be pumping out heat on the roads in Sydney at any one time. If these were EVs, their heat impact would be reduced significantly.
5. Advocate for Climate Action
Local action alone cannot solve this crisis. Heat vulnerability highlights the urgent need for national and global efforts to reduce emissions and limit global warming.
A Call to Action
As Australia reels from its hottest Spring, places like Western Sydney provide a stark example of the challenges we face in adapting to a warming world. The Urban Heat Island effect and wet bulb temperatures moments are not distant threats - they are already here. I was in Blacktown last month and it was already 39°C and humid. Without bold action, our bodies and health systems are going to be under more and more strain.
We owe it to ourselves and the most vulnerable to act now. Urban greening, climate-resilient planning, and an urgent transition to renewables are essential.
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