One of the near-term expressions of climate change is increased frequency and intensity of extreme heat events. Extreme heat impacts can be particularly large in cities due to preexisting urban heat island effect and high concentration of people. These impacts include heat-related mortality and morbidity, degraded air quality, reduced mental health and worker productivity, and strain on energy infrastructure, which raise an urgent need for a better understanding of extreme heat risk in cities and strategies to reduce it.

The evolution of extreme heat risk in urban areas depends on the interactions of large-scale climate dynamics and urban micro-climates as well as the distribution of population within these micro-climate regions. We use California as a testbed where we employ a suite of satellite-supported, high-resolution (1.5 km) regional climate simulations coupled with an urban canopy model (UCM) and a spatially explicit population projection to investigate the interacting effects of climate change and population dynamics on exposure to extreme heat events in cities by the mid-century.

We find that climate change and population growth reinforce with one another to drive substantial increases in future exposure to heat extremes, which are shown to become more frequent, longer, and more intense. We estimate the future exposures to extreme heat days, heat waves, extreme heat waves, and cooling degree days and their attribution to climate change, population dynamics, and their interactions. For instance, we show that by the mid-century, exposure to events analogous to historic high-mortality extreme heat waves increases by 3.5-6 folds.

We next assess the potential for cool roofs, a heat mitigation strategy, to push back the rising extreme heat risk, associated with climate change and population dynamics. We find that widespread implementation of cool roofs can offset a substantial fraction (51-100%) of the increased heat exposure and associated building energy demand, represented by cooling degree days, owing to climate change in urban regions.