Study says climate crisis will cause falling humidity in global cities

A new study has suggested that urban regions around the world are likely to see a near-universal decrease in humidity as the climate changes. The research published in Nature Climate Change says that building green infrastructure and increasing urban vegetation might be a safe bet for cities looking to mitigate rising temperatures.

The results of the study demonstrate the inter-model robustness of specific levels of urban warming over certain regions under climate change. Under a high-emissions scenario, cities in the United States, Middle East, northern Central Asia, northeastern China and inland South America and Africa are estimated to experience substantial warming of more than 4 K— larger than regional warming — by the end of the century, with high inter-model confidence, it says. 

Researchers say the findings highlight the critical need for multi-model global projections of local urban climates for climate-sensitive development and support green infrastructure intervention as an effective means of reducing urban heat stress on large scales.

Half of the world’s population lives in urban areas, but cities only account for about 3% of global land surface. 

According to Lei Zhao -- a scientist from the University of Illinois and the lead author of the paper -- this means that previous climate models have not produced data specific to cities.

“Almost all the models do not have urban representation,” Zhao said, adding: “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.”