At a Glance
Heat waves (HWs) are among the most damaging climate extremes to human society. For urban residents, the urban heat island (UHI) effect further exacerbates the heat stress resulting from HWs, and these risks are even greater if HWs interact synergistically with UHIs. Combining climate model simulations and a new analytical framework, the Oppenheimer group has investigated the synergistic effects, sometimes positive and sometimes negative, between UHIs and HWs at a large scale under climate change. The study also uncovered the physical mechanisms underpinning these synergistic effects.
Among the many damaging environmental extremes, including hurricanes, floods, and tornadoes, heat waves (HWs) are the deadliest in the US. Assuming no acclimatization and adaptation, extreme heat stress in a changing climate has the potential to cause a substantial increase in human mortality, morbidity, energy demand, and perhaps civil conflicts.
In recognition of these concerns, better understanding is needed of the risks of future HWs including physical mechanisms, temporal structure, and potential adaptation and mitigation strategies to better manage changing risks over time.
Climate models consistently project that HW frequency, severity, and duration will increase markedly over this century. For urban residents, the heat stress resulting from HWs are amplified by the urban heat island (UHI) effect, and these risks are even greater if HWs interact synergistically with UHIs (Figure 3.1.1). Future impacts from such events can be mitigated through adaptation and risk management efforts informed by an improved understanding of the response of UHIs to HWs.
STEP postdoctoral research associate Lei Zhao, with his advisor, Michael Oppenheimer, used an Earth system model to investigate the interactions between UHIs and HWs in 50 US cities under current climate and future warming scenarios. The results show significant sensitivity to local background climate and warming scenarios. Sensitivity also differs between daytime and nighttime (Figure 3.1.2). In today’s climate, there is a positive synergy in all regions day and night, significantly stronger in some cities at night. These synergistic effects, however, change in complex ways in future warmer climates. For cities in the eastern half of the US, in the daytime, positive synergies will become negative synergies, while at night, many positive synergies will become more strongly positive. For cities in the western half of the US, which consist mostly of dry climates, synergies are generally weak in today’s climate, but will become stronger in a future warmer one.
An analytical method was used to disentangle the mechanisms behind the interactions between UHIs and HWs that explain the spatiotemporal patterns of the interactions. Results show that evaporation plays a key role. Over a water-sufficient surface (such as moist soils or wet surfaces), an increase of air temperature favors increasing latent heat flux, a change in phase such as evaporation, rather than increasing sensible heat flux, a change in temperature. Over a water-limited surface (such as dry soils or concrete surfaces), sensible heat flux dominates.
In the present-day climate, despite ample precipitation in temperate regions, cities in these regions are usually water limited due to the large fraction of impervious surfaces; whereas their surrounding rural surfaces are water sufficient. Therefore, during HWs (i.e., temperature increases) evaporation increases less over urban surfaces than over rural surfaces, resulting in an enhanced UHI effect, which can be measured as the urban-rural gap in air or surface temperature. In dry regions where both urban and rural surfaces are water limited, the UHI effect due to differences in evaporation is weaker.
Under future warming scenario, for cities in the temperate region, the enhancement of evaporative contribution to the urban-rural difference during HWs is diminished near the end of this century under RCP 8.5 (a greenhouse gas concentration trajectory adopted by the Intergovernmental Panel on Climate Change). The reason lies in the model-projected increase in precipitation in this region in future warmer climates. The increase in precipitation, to some extent, turns cities into water-sufficient surfaces, so that evaporation over cities can increase as much as over their surrounding rural surfaces when HWs come in. The enhanced anthropogenic heat release during HWs, present in temperate and dry climate regions, is primarily the result of higher air-conditioning energy use to cope with the heat extremes.
At night, the enhanced release of stored heat (in built structures) and anthropogenic heat during HWs are the primary contributors to synergistic effects.
This work highlights the heat risks that urban residents face now and in the projected future. HWs have detrimental impacts on human society and natural ecosystems. The synergistic effects that were found in the current study augment these impacts. The daytime synergistic effect alone leads to a 3.2% increase in mortality risk in the current climate for temperate cities. The nighttime synergistic effect in the temperate region leads to an increase of 2.2% in mortality risk in the current climate and is projected to increase to 4.3% by the end of this century under RCP 8.5.