The impact of urban growth and climate change on heat stress in a sub-tropical Australian city

Abstract

Extreme heat kills hundreds of people each year, and during extreme heat waves the death toll can rise to the tens of thousands. Urban residents are particularly vulnerable to heat stress due to the urban heat island (UHI), which is when cities are warmer than the surrounding countryside. As cities grow, the intensity of the UHI increases, exposing urban residents to a greater risk of heat stress. In urban areas, heat stress will increase due to both urban growth and climate change. Despite this, most previous work has focussed on either climate change or urban growth, not both. This underestimates the future temperature increases in cities and limits the ability to plan mitigation of and adaptation to rising temperatures.In this thesis, the combined impact of urban growth and climate change on the UHI and heat stress of Brisbane, a sub-tropical Australian city, is simulated using CCAM, a high-resolution climate model. This thesis aims to understand how urban growth and climate change together will increase temperature in a sub-tropical city, and if climate change will alter the UHI. Heat stress is also calculated to evaluate the human health impact of temperature increases. While only one city is examined, the results are applicable to other sub-tropical cities around the world.Climate change may alter the UHI if urban and rural areas warm by different amounts in response to climate change. If this is the case, urban areas need to be included in climate models otherwise urban temperatures will be inaccurately represented by climate simulations. In Chapter 2, I systematically review the previous research on the UHI and identify processes that may lead to urban and rural areas responding differently to climate change. I also identify key limitations in previous research, which the rest of this thesis builds on. These limitations are: the exclusion of anthropogenic heat (AH) from urban climate simulations, focussing on expansive forms of urban growth and not densification of urban areas, the lack of consideration of the combined impact of urban growth and climate change, and a lack of consideration of heat stress.Anthropogenic heat is a key component of the UHI. Despite the importance of AH to urban temperatures, the review in Chapter 2 identified that it has often been excluded from urban climate simulations because estimates of AH are often not available. In Chapter 3, I estimate AH for four Australian capital cities, Brisbane, Sydney, Melbourne and Adelaide. These estimates were intended for use in the climate simulations in this thesis, and for future research into Australian urban climate.The systematic review identified that few studies have considered the impacts of urban densification on future urban temperatures. In Australia, future urban growth is planned to be by densification. In Chapter 4, I develop two scenarios of urban growth through densification and simulate their impact on Brisbane’s climate using CCAM. The results show that even small increases in density, such as in the Medium Density scenario, are enough to increase night-time temperatures in the city-centre. During extreme events, the temperature increase was higher than during average conditions, increasing heat stress risk at a time when urban residents would already be facing higher heat stress risks.The systematic review identified that most previous research into urban climate has focussed on urban growth or climate change, not both, and has not considered heat stress. In Chapter 5, I use CCAM to simulate the impact of climate change (RCP 8.5) and urban growth on Brisbane’s UHI in 2041 – 2050. When urban growth and climate change were considered together, the increases in urban and rural temperatures was higher than when climate change was considered alone. If future urban growth is ignored, temperature will be under-estimated, not only in urban areas but also in nearby rural areas. The UHI decreased in the climate change scenario due to a drying trend and rural temperatures increasing more than urban temperatures. In both scenarios, the heat stress indices showed that activity will need to be limited during the day to limit the risk of heat stress.This thesis explored the combined impact of climate change and urban growth on temperatures and heat stress in a sub-tropical city. It demonstrates the interaction between climate change and the UHI and shows that temperatures increase more with climate change and urban growth than with climate change alone. It also shows that the heat stress to urban residents will increase in the future, despite reductions in the intensity of the UHI. Of the urban parameters examined, vegetation cover had the strongest impact on urban temperatures. Maintaining and restoring vegetation therefore is a key strategy in mitigating the UHI. Incorporating measures to reduce the UHI into urban planning should be a priority to ensure that urban planning and design does not increase the heat stress risks urban residents will already face due to rising temperatures from climate change.