Governments around the world have implemented measures to slow down the spread of COVID-19, resulting in a substantial decrease in the usage of motorized transportation. The ensuing decrease in the emission of traffic-related heat and pollutants is expected to impact the environment through various pathways, especially near urban areas, where there is a higher concentration of traffic. In this study, we perform high-resolution urban climate simulations to assess the direct impact of the decrease in traffic-related heat emissions due to COVID-19 on urban temperature characteristics. One simulation spans the January–May 2020 period; two additional simulations spanning the April 2019–May 2020 period, with normal and reduced traffic, are used to assess the impacts throughout the year. These simulations are performed for the city of Montreal, the second largest urban centre in Canada. The mechanisms and main findings of this study are likely to be applicable to most large urban centres around the globe. The results show that an 80% reduction in traffic results in a decrease of up to 1 °C in the near-surface temperature for regions with heavy traffic. The magnitude of the temperature decrease varies substantially with the diurnal traffic cycle and also from day to day, being greatest when the near-surface wind speeds are low and there is a temperature inversion in the surface layer. This reduction in near-surface temperature is reflected by an up to 20% reduction in hot hours (when temperature exceeds 30 °C) during the warm season, thus reducing heat stress for vulnerable populations. No substantial changes occur outside of traffic corridors, indicating that potential reductions in traffic would need to be supplemented by additional measures to reduce urban temperatures and associated heat stress, especially in a warming climate, to ensure human health and well-being.
Climate change is projected to increase the frequency and intensity of extreme weather events, including heat waves (IPCC, 2013). These projected changes in heat waves coupled with the urban heat island (UHI) effect, as manifested by elevated near-surface air temperatures in urban areas compared to their non-urban surroundings, exposes urban dwellers to additional heat stress. The higher urban temperatures are largely related to thermal and radiative properties of built surfaces, substantially different from its surrounding natural environment, and to a lesser vegetation coverage with limited evaporative cooling. UHI is further enhanced by heat emitted from transportation, heating and air conditioning systems (Oke, 1982). Adaptation and mitigation strategies considered to reduce UHI, which could reduce the impacts during heat waves, include increasing the reflectivity of urban regions and increasing vegetation coverage (Alexandri & Jones, 2008;Costanzo et al., 2016;Touchaei & Akbari, 2013). Development of effective adaptation and mitigation strategies will require information of mean and extreme temperature changes at super-resolution (<250 m). Climate change information at such super-resolution are not available for cities, primarily due to the inadequate representation of urban regions in climate models due to their coarse resolution. Climate processes are complex-more so in urban regions (Bai et al., 2018). Super-resolution climate modeling that includes good representation of urban regions is required to capture the urban-climate feedbacks. With significant developments in high performance computing, it has now become possible to execute regional climate simulations at 4 to 1 km resolutions, but for shorter periods. High computational cost continues to be
<p>Extensive degradation of near-surface permafrost is projected during the 21st century, which will have detrimental effects on northern communities, ecosystems and engineering systems. This degradation will expectedly have consequences for many processes, which most previous modelling studies suggested would occur gradually. Here, we project that soil moisture will decrease abruptly (within a few months) in response to permafrost degradation over large areas of the present-day permafrost region, based on analysis of transient climate change simulations performed using a state-of-the-art regional climate model. This regime shift is reflected in abrupt increases in summer near-surface temperature and convective precipitation, and decreases in relative humidity and surface runoff. Of particular relevance to northern systems are changes to the bearing capacity of the soil due to increased drainage, increases in the potential for intense rainfall events and increases in lightning frequency, which combined with increases in forest fuel combustibility are projected to abruptly and substantially increase the severity of wildfires, which constitute one of the greatest risks to northern ecosystems, communities and infrastructure. The fact that these changes are projected to occur abruptly further increases the challenges associated with climate change adaptation and potential retrofitting measures.</p>
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