Urbanization dries the atmosphere: the urban dry island effect

An international collaborative study suggests that urbanization not only causes more storm runoff and urban ‘heat islands’, but also dries out the air in cities, resulting in urban ‘dry islands’.  This environmental change is related to land cover change that alters land surface water and energy conditions. Maintaining the hydrological functions of wetlands and other ecosystems (cooling the air and improving water storage) is critical for mitigating negative impacts of urbanization under a changing climate. 

Shanghai urbanization 1984 to 2017Urbanization often makes cities significantly warmer than surrounding rural areas, a phenomenon known as the urban heat island effect. Forest Service scientists at the Eastern Forest Environmental Threat Assessment Center and partners at the US-China Carbon Consortium examined another related impact of urbanization: drying of the local atmosphere in one of the world’s most developed urban regions. In the Yangtze River Delta in southern China, wetlands including large areas of rice paddies are being rapidly converted to urban uses. Researchers obtained satellite images for detecting land use and land cover change and also acquired more than 50 years of climate data from 33 weather stations spanning the study region. Results showed that the air in the urbanizing regions grew dramatically drier as wetlands, rice paddies, and forest were replaced by cities. The observed change in humidity was explained by the decrease in water loss from vegetation and wetlands into the atmosphere. These findings indicate that as cities grow, urban planners need to consider not only the urban heat island effect and storm runoff issues, but also a related ‘urban dry island’ effect.


Pictured: Urban sprawl in Shanghai between 1984 (left) and 2017 (right). A new study describes how urbanization influences the flow of water from the ground to the atmosphere. Images prepared by Joshua Stevens and Jesse Allen, courtesy of NASA Earth Observatory.

Related publications:

External Partners/Collaborators: Nanjing University of Information Science and Technology.

Contact: Ge Sun, Research Hydrologist, Eastern Forest Environmental Threat Assessment Center, ge.sun@usda.gov.

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