A groundbreaking study from China has uncovered alarming evidence that the rapid growth of cities worldwide is significantly contributing to more intense drought conditions, particularly in urban areas. This phenomenon is exacerbating urban heat and water stress, highlighting the urgent need to rethink how we design and build our cities to ensure they are more livable and resilient in the face of climate change.
The study, which analyzed an extensive 40 years of weather station data from both urban and rural regions across the globe, reveals a stark connection between urbanization and worsening drought conditions. The findings indicate that larger cities with less green cover are experiencing even more severe droughts, with the Greater Sydney region being one of the most affected.
Sydney, along with five other cities from around the world, was selected for detailed model simulations to explore how urbanization is worsening local drought conditions. The results are striking. On January 4, 2020, the western Sydney suburb of Penrith recorded a blistering 48.9°C, making it the hottest place on Earth that day. This extreme temperature is a clear example of how urban development is intensifying heat and drought in cities.
The study also sheds light on a few exceptions to this trend, such as cities along the US west coast, the Mediterranean, and South-East Queensland. These regions benefit from their proximity to the coast, where ocean breezes help cool the land and bring moisture, mitigating the effects of urbanization.
Impact of Urbanization on Heat and Moisture Levels
As more than half of the world’s population (56%) now resides in cities, understanding the impact of urbanization on local climates is more critical than ever. Cities are known to influence local microclimates in numerous ways. The urban heat island effect, where urban areas are significantly warmer than their rural counterparts, is one of the most well-documented consequences of urbanization. This effect is primarily due to the loss of natural vegetation, replaced by man-made materials like buildings, roads, and parking lots, which absorb and retain heat.
Urban development also disrupts the natural movement and storage of water in urban areas, a phenomenon known as urban stream syndrome. Impervious surfaces, such as roads and rooftops, prevent rainwater from soaking into the ground, leading to drier and hotter soils in cities. This reduced moisture retention contributes to the urban dry island effect, a lesser-known but closely related phenomenon to the urban heat island effect.
The study highlights the physics behind these changes: as air temperature rises in urban areas, the air can hold more water vapor, increasing the likelihood of heavy rain and storms. However, after these intense downpours, the lack of soil moisture absorption in cities often leaves plants and trees parched, exacerbating the drought conditions.
One of the study’s most significant findings is the critical role that urban green spaces play in mitigating these adverse effects. Plants, particularly trees, help cool the air through a process known as evapotranspiration, where water absorbed by roots is released as water vapor through leaves. This natural cooling effect not only lowers air and soil temperatures but also adds much-needed moisture to the urban atmosphere.
However, as cities grow and green spaces are replaced by buildings and infrastructure, the cooling effects of plants diminish. The study points out that the loss of vegetation associated with urbanization decreases urban evapotranspiration, intensifying local atmospheric dryness and contributing to more severe droughts.
Towards a More Resilient Future for Cities
The research underscores the importance of integrating green spaces into urban planning. Parks, gardens, and reserves are not just essential for the well-being of urban communities but also play a crucial role in reducing the severity of droughts. By maintaining and expanding urban green spaces, cities can mitigate the impact of the urban heat and dry island effects, making them more resilient to the challenges posed by climate change.
This study, grounded in real data from a vast number of weather stations worldwide, provides compelling evidence that urban areas are not just passive victims of climate change but active contributors to the intensification of droughts. As cities continue to grow, the need to incorporate sustainable urban design practices that prioritize green spaces and natural cooling mechanisms becomes increasingly urgent.
In a world where many cities are already struggling to provide sufficient water for their residents, and mega-cities like Mexico City are facing the threat of “day zero” when they could run out of water, the findings of this study offer a crucial roadmap for creating more livable, sustainable, and resilient urban environments.
Reference: https://www.preventionweb.net/news/urban-growth-leading-more-intense-droughts-most-worlds-cities-and-sydney-case-study-areas-risk