Heatwaves are the most under rated natural disaster with more deaths over the last 200 years attributed to heatwaves compared with any other type of natural disaster. In 2009, the Melbourne heatwave may have caused 374 more deaths over what would normally be expected for the period, which is a 62% increase in mortality.
Australia is one of the most urbanised countries in the world, with around 86% of the population living in cities. While the percentage of population living in urban areas has only increased slightly over the past few decades, the total Australian population has doubled in the past 50 years. As cities grow more land has been transformed into built structures that absorb more heat from solar radiation. Together with heat release from other human activities, it contributes to the urban heat island effect.
“Cities are hotter compared to surrounding rural areas which means heatwaves have a greater impact on urban areas” says Dr Dong Chen, Research Leader for Thermal and Ventilation Engineering at CSIRO’s Climate Adaptation Flagship.
Research into the potential cooling effect due to vegetation by Dr Chen and his colleagues at CSIRO and Nursery & Garden Industry Australia, found that Melbourne’s CBD summer temperatures could be lowered by 0.7°C if CBD vegetation was increased significantly.
What is the Urban Heat Island Effect?
Cities are generally a few degrees warmer than rural areas due to the urban heat island (UHI) effect. It’s primarily caused by replacing natural materials with hard and impermeable materials such as concrete and asphalt which absorb and release significantly more of the suns energy than soils. Using more impermeable materials also means there is less moisture available for evaporation which has a cooling effect. Dark materials like asphalt also absorb more of the suns energy than lighter surfaces. A secondary cause of the UHI effect is due to waste heat from cars, industry and air conditioners.
Over the past fifty years Melbourne’s CBD temperatures have been creeping up by 0.2 degree per decade since 1950s’. Today CBD temperatures can be 1-2 degrees warmer than surrounding rural areas.
Using urban climate models Dr Chen and his team predicted that suburban areas were around 0.5°C cooler than the CBD, with leafy suburban areas up to around 0.7°C cooler than the CBD. Parklands such as a grassland, shrub land or sparse forest were found to be between 1.5 and 2°C cooler than the CBD.
The researchers also found that significantly increasing vegetation coverage in the CBD had a big cooling effect. Doubling CBD vegetation coverage was predicted to reduce temperatures by 0.3°C while 50% green roof coverage of the CBD area was predicted to reduce temperatures by 0.4°C temperature. Combining the two options would result in a 0.7°C reduction from normal CBD temperatures – making CBD temperatures the same as a leafy suburban area.
This research has important implications. “While greening won’t stop heatwaves – it can help to mitigate their impact. It can reduce the maximum temperature during day and night, giving heat relief in summer” says Dr Chen.
Greening up cities is not just about making them look attractive. It can reduce temperatures and minimise the impact of heatwaves, with the potential to lower heat related death rates. Increased vegetation can also lower energy use due to reduced use of air conditioners. “Urban vegetation is considered as one of the important risk mitigation measures for heatwaves and for climate adaptation as well” says Dr Chen.