Urban regions around the world are likely to see a near-universal decrease in humidity as the climate changes, a study has found.
The research suggests that building green infrastructure and increasing urban vegetation might be a safe bet for cities looking to mitigate against rising temperatures.
Half of the world’s population lives in urban areas, but cities only account for about 3% of global land surface. Lei Zhao, a scientist from the University of Illinois and the lead author of the paper published in Nature Climate Change, says this has meant that previous climate models have not produced data specific to cities.
“Almost all the models do not have urban representation,” Zhao said. “Although cities occupy such a small area, that’s where a lot of the human impact [of global warming] takes place. So we closed this gap by providing multi-model climate projections which are specific to urban areas.”
Scientists and urban planners have known for a long time that temperatures in cities are higher than in rural areas. Infrastructure such as dark asphalt and concrete surfaces absorb more solar radiation, while reduced tree coverage contributes to what is called the “urban heat island effect”. This means that temperatures in cities can be up to 5C (9F) warmer than in the surrounding rural areas.
However, Zhao explains that urban and rural climates differ in more ways. “The urban heat island is one of the reasons why urban warming signal is different from other landscapes,” Zhao said. “But it’s not just temperature, it is also humidity. A lot of urban climate variables are different from other landscapes.”
The model predicts that green infrastructure would be a good investment for nearly all cities. Trees and vegetation help to reduce temperature by releasing water into the atmosphere, which cools down the air. This was seen as having a limited effect in places which are already humid, but the new model predicts that air in most non-coastal cities will become drier in the next century.
This would make surface evaporation more efficient, meaning increased levels of urban vegetation would be more effective at fighting global heating.
Zhao hopes the data will allow urban planners and policymakers to make more informed decisions about mitigating rising temperatures in their cities.
“Some strategy might work for a city, but not necessarily for your city,” he said. “When you look at large-scale projections, you can see if the warming signal is different from other places, and how humidity levels vary, so it can help you form the strategy differently.”
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