The cities where we live are heating up, but the places where we live need to be cool. What does the urban heat island look like here in Dallas? Over 100 people recently gathered at the Grey To Green conference, hosted by the Texas Trees Foundation, to hear from Dr. Brian Stone, Jr., the author of The City and the Coming Climate (see my book review here). Dr. Robert Haley, David Hitchcock and Matt Grubisich also addressed the consequences of a warming climate and the critical role of trees and green infrastructure in urban heat management.
David Hitchcock is Senior Research Scientist on Sustainable Communities at the Houston Advanced Research Center. He participated in a national climate change study on urban heat islands that led to the publication in 2009 of a report on the Dallas Urban Heat Island as part of the EPA’s Sustainable Skylines Initiative. (See http://www.harcresearch.org/work/Dallas_Urban_Heat_Island.) Mr. Hitchcock showed us a kinetic thermal map, a snapshot of Dallas County on Sept. 28, 2006, when the official temperature was 88°, shows surface temperatures ranging from 42° to 181°:
What accounts for such a great difference? Heat islands occur where there are heat-absorbing materials (hot roofs and paving) and minimal vegetation. The hottest areas (clusters of red) include warehouse concentrations, retail centers, parking lots and airports; the cooler areas include older neighborhoods, river basins, and major parks and conservation areas – places where there are trees.
A comparison of surface temperatures and land features identifies sources of heat and opportunities for heat mitigation. This aerial photo of an area south of downtown and north of the Trinity River shows a hot spot where there is an industrial area with a large tract of cleared land and no vegetation; the cooler spot is the forested area along the Trinity River with a large tree canopy.
Simply put, trees cool cities. Therefore, they are a top priority for managing urban heat. Trees cool cities by providing shade that reduces the absorption of heat by roofs, walls and pavement. Shaded roofs are cooler by 20-45°, and cars parked in the shade can be cooler inside by 45°. A large area of trees, such as a park, can be 5-10° cooler than surrounding areas. Trees also cool through transpiration, or moisture release from the leaves, similar to the way our bodies cool themselves with perspiration. Trees can transpire thousands of gallons of water per year to help cool our cities.
Conservation of existing trees is the primary heat management strategy he advocates because it has the greatest benefits; yet it remains a difficult challenge for cities as new development can offset tree planting goals. The benefits of larger, older trees cannot be replaced by most tree planting programs and projects, although such projects do serve the very important goal of raising awareness of the need for tree conservation.
Cool roofs are the most straightforward method of reducing urban heat island impacts, and a combination of incentives and policies can be deployed to effect large-scale replacement over time of heat-absorbing roofs with those that reflect heat.
Finally, cool paving is another heat management strategy that has water management benefits in addition to its cooling effects.
View my full report from the conference here on my blog.
Klyde Warren Park image via tripadvisor.com