The Quest for Cooling in Southern Nevada
Southern Nevada, particularly the city of Las Vegas, has been grappling with the harsh effects of extreme heat. The region has experienced a significant increase in heat-related deaths and emergency room visits in recent years. In response, the city has been planting trees at an unprecedented rate, aiming to cool down the urban heat islands and provide relief to its residents. However, a new study suggests that trees alone may not be enough to make a meaningful difference in the daytime.
The Cooling Effect of Trees at Night
While trees can cool surrounding air temperatures up to nearly 35 degrees at night and provide shade, their cooling effect during the day is negligible. The study found that air temperature cooling is hard to achieve, even with the most common adaptation measures. “When you measure air temperature, the cooling effect of trees is usually very, very low,” said Juan Henao Castaneda, a postdoctoral researcher at the Desert Research Institute (DRI) and the study’s lead author. “Reducing air temperature below one degree or two degrees is hard for any of the most common adaptation measures.”
A Comprehensive Approach to Heat Mitigation
The bottom line is that trees come with their own baggage as an extreme heat solution and shouldn’t be the only one urban planners implement. “They need a lot of care,” said John Mejia, a study author and DRI climatologist. “They are expensive to take care of. In many other cities, they have had challenges in keeping trees alive, even in more humid and more friendly environments for trees.”
The Importance of Nighttime Cooling
Long-term extreme heat response in Southern Nevada, while only a handful of years in the making, is mostly limited to bolstering tree canopy in the region’s most densely paved urban heat islands. However, trees still provide a crucial cooling effect during nighttime, which is often overlooked. Peter Ibsen, a U.S. Geological Survey research ecologist, published a nationwide study last year that found Las Vegas could experience a cooling benefit from trees more than any other hot city in the country.
A Multi-Faceted Approach to Heat Mitigation
In an interview, Ibsen said the DRI study is a worthy addition to the conversation of urban forestry and extreme heat, though the study methods and findings were different. Careful consideration should be put behind where trees can reap the most benefit, and in which direction they should be planted for maximum shade. “Trees are one part of the heat mitigation toolkit,” Ibsen said. “Trees also do many other things. A shade structure will provide shade, but will it provide biodiversity habitat? Will it be aesthetically pleasing? We focus on heat, but it’s not easy to measure, and there’s many different ways we can approach this question.”
In the short term, Clark County’s cooling centers — usually a community center or library — open their doors when the National Weather Service issues an extreme heat warning. The new study affirms that trees have never been a silver bullet to cool cities, said Ariel Choinard, coordinator of the Nevada Heat Lab, a Desert Research Institute research initiative formed in 2023 that directs heat mitigation efforts throughout the state.
She pointed to the nighttime cooling effect on air temperatures outlined in the study — an important piece of the equation as nights heat up in Las Vegas at an unprecedented rate. The cumulative effects of heat on the body when nights don’t provide cooling relief are often ignored, Choinard said. “There’s an emotional reaction to hearing that trees weren’t the one weird trick to solve Las Vegas being one of the fastest warming cities in the nation,” Choinard said. “Trees are a solution. They are not the solution, but they still will provide people with incredible relief.”
Many trees may not survive climate change. A 2019 Southern Nevada Water Authority study found about 38 percent of the 100 most common plants used in landscape conversions won’t be able to withstand hot temperatures by 2055. Some neighborhood groups, including the Water Fairness Coalition, have been critical of the water authority’s conservation programs, such as the removal of “nonfunctional turf” before 2027 as mandated by the Nevada Legislature.
Among their chief concerns are fewer green spaces and trees to cool off urban heat islands. Previously, the water authority said in an interview that trees are dying across the valley because of mismanagement on private property. The agency offers rebates for new trees planted on lawns converted to desert landscaping and tips for plant selection. Cayenne Engel, an urban forestry expert, recently joined the Nevada Heat Lab, and said having any hyperlocal study is a benefit to better understanding the unique effect trees have on Las Vegas. Trees are often a popular adaptation method to heat because people have an emotional and cultural connection to them, Engel said. When the study came out, Engel said community members reached out with many questions. “The response is potentially surprising,” Engel said. “The results are that trees don’t significantly change the air temperature any more than built-environment shade; that makes sense when you dig a little bit into tree physiology. Trees here are working at night instead of during the day. The science of it does make sense, and the story is complicated.”
In no way does the study suggest that trees are not a valuable tool to protect individuals from extreme heat, Engel said. Henao Castaneda and Mejia will continue to research different extreme heat adaptation measures, hopefully equipping city planners and decision-makers with data specific to Las Vegas. As required by Assembly Bill 96, every municipality with more than 100,000 residents must update their master plans to include extreme heat mitigation. “The beauty of what we are doing is we are creating basic information to help guide policy in the future,” Mejia said. By continuing to research and develop effective adaptation measures, cities like Las Vegas can build a more resilient future and provide relief to their residents during extreme heat events.
