The recent brutal heat waves on the Indian subcontinent, in western North America, and in western Europe are instructive reminders of an often forgotten challenge for an urbanizing human population in a warming world: alleviating urban heat stress. Cities are durable and costly to change, so what we do now to reduce risk in a future with more numerous and more dangerous heat waves that will directly affect future generations. Why is this topic important to focus upon now?
Humans are rapidly moving to cities. As human populations urbanize and the planet warms, the effects of urban heat islands (UHIs) will become more important to our societies. Warmer cities will cause increased human discomfort, require more energy to cool building envelopes, increase air pollution which degrades human cardiopulmonary health, and use more water to keep vegetation alive.
UHIs are a phenomenon of much warmer temperatures in cities than in the surrounding landscape; the increased temperatures are caused by unshaded buildings and pavement reflecting, absorbing and storing the sun’s energy – cities are hotter during the day from reflected heat and warmer at night due to the stored heat in pavement and buildings.
Emergency managers, regional organizations, national governments and adaptation professionals across the world are starting to prepare local climate adaptation plans that include strategies to mitigate UHIs in a future world with stronger heat waves. It will be important in a warmer world to plan for the fact that crowded cities need to maintain (and increase) vegetation to create shade and cool the surrounding air by evapotranspiration. Cities need to ensure cooling breezes flow through, scouring out heat and air pollution. Cities need to have places for at-risk people to go when their dwellings become too hot to bear during heat waves. And cities need to ensure that new buildings are well-insulated against added heat. Planning for uncertainty and many moving parts in cities is a challenging task. Is there anything you and I can do to help alleviate the UHI?
It turns out that human preferences for a certain type of landscape help with reducing the effects of the UHI. Environmental psychologists hypothesize that our landscape preferences come from the vegetation types found in the African savanna where early hominids evolved: scattered tall, spreading trees, some understory plants, and grassland. Now look around at the vegetation in nice neighborhoods: tall trees (but not too many), some understory plants (often flowering), and turfgrass. The very landscape types that humans prefer can help cool cities. The value of a large shade tree goes beyond shading your dwelling and keeping it cool – large, healthy trees raise the property values of the area, greenery is restorative, and many trees in a neighborhood induce us to get out and go for a walk. Designing cities with these needs in mind has multiple, reinforcing benefits for city residents.
In our neighborhoods, we can plant a large tree directly to the west of our dwelling for most effective shading (and avoiding shade on solar collectors). The tree will also add value to the property and the properties surrounding it. Many cities have local groups that have programs that offer shade trees – volunteers and citizens who plant trees report higher satisfaction with their neighborhoods. A well-treed neighborhood also has more residents engaged in physical activity. Getting involved locally has a wide range of benefits.
In our cities, we can advocate for good design to allow large trees to flourish. Trees need an adequate amount of soil to grow, and ensuring local codes and laws are written to provide enough soil for trees will not only benefit the tree, but will reduce infrastructure conflicts, saving taxpayer money. In many places, however, there is a disconnect between the planning and implementation, so we must ensure our urban planners are aware of the importance of trees, designing for tree health, and awareness of what other agencies are doing about the UHI. Good city design at larger scales benefits all city residents – we can advocate for plans that ensure local resilience against future heat waves and alleviating UHIs, as this results in a more livable city for everyone. Getting involved in local decision-making for trees results in positive changes in the way cities are designed.
Lastly, we can insist on a change in building standards that allow for cool roofs. Cool roofs absorb less heat, reducing the UHI and the energy load on buildings. This is an easy to accomplish standard, and easy to grasp for elected officials.
Urban Heat Islands will become more problematic in the future as the planet warms. There are things we can do directly to cool down cities. We can also hold our leaders’ feet to the hot pavement to effect change. The important thing to remember is that there are a number of things we can do. Let us start to do them now, while we remember the brutal heat of 2015.
Image Credit: Atlanta Daytime thermal by NASA. Public domain via NASA/Goddard Space Flight Center Scientific Visualization Studio.
I agree with the concept of promoting shade trees to help reduce cooling loads in cities during the summer, as trees also do not negatively impact the heating load in winter. I also agree that adding insulation works to reduce both heating and cooling energy demand. However, the writers’ assertion about the benefits of cool roofs should be limited to cooling dominated southern climates where they can reduce natural resource consumption and therefore decrease CO2 emissions. The heating penalty associated with white roofs in central and northern climates leads to increased natural resource consumption and an increase in CO2 emissions. A Stanford University study simulated what would happen if all the roofs in the U. S. were converted to white and found that the sunlight reflected back up into the atmosphere by white roofs causes heat to mix with black and brown soot particles to further warm the atmosphere, thus increasing global warming. The bottom line is that white reflective roofing can be a net benefit in southern cities, but can be the wrong choice in northern cities due to the heating penalty. My company manufactures both black and white roofing.