Heat generated by people, vehicles, and the Sun is easily trapped by the materials used to build houses, industrial buildings, sidewalks, and parking lots. This heat often makes cities significantly warmer than surrounding rural areas, a phenomenon known as the urban heat island effect. Now, a new study of the highly developed Yangtze River Delta in southern China examines a less studied but related impact of city building: the desiccation of the local atmosphere.
Spanning more than 210,700 square kilometers, the Yangtze River Delta region contains the largest concentration of metropolitan areas in the world, including megacities such as Shanghai. It is urbanizing at a breakneck pace: Between 2000 and 2010, for example, the proportion of land covered by cities and suburbs increased by 75%, replacing flooded paddy fields, cropland, and forests.
Here Hao et al. hypothesize that in addition to increasing local temperatures in urban areas, this rapid development has altered the flow of water between the ground and the atmosphere, making built-up regions drier. To test that hypothesis, the researchers obtained data from the Global Land Surface Satellite (GLASS), products derived from multiple satellite imageries. They also obtained more than 50 years of climate data from 33 weather stations spanning the delta and data from Chinese government records on urbanization and the amount of land being used to grow paddy rice.
The team performed a correlation analysis—a statistical test used to study the strength of a relationship between two variables—to analyze the relationship between urbanization and relative humidity (a measure of moisture in the air) between 2001 and 2014. They found that urbanizing regions of the Yangtze River Delta grew dramatically drier as wetlands, rice paddies, and forest were replaced by cities. These findings indicate that as they design growing cities, urban planners need to account not only for the urban heat island effect but also for a related “urban dry island” effect, the team argues. (Water Resources Research, https://doi.org/10.1029/2018WR023002, 2018)
—Emily Underwood, Freelance Writer