Driverless Cars Might Lead to Cooler Cities

Two lessons about the future for landscape architects

             It might seem like a futuristic idea, but it’s beginning to look like driverless cars are here to stay.  Imagine having your driverless car take you downtown to shop.  There’ll be no need to look for a parking spot.  You can simply step out of the car and tell it to drive somewhere and wait for you to call it back.  It could go home and wait in your garage, go to a designated parking area outside the urban core, or better yet, it could be a shared car and you could let it transport someone else around while you’re shopping. If cars no longer need to be parked near a destination it could mean the beginning of the end for those expanses of hot asphalt in the centres of our cities.  There would be no need to park in the urban core.  If this land was converted to almost any other use it would reduce the intensity of the urban heat island.  But if the land was converted to well-watered, shady green space, urban heat islands might be converted to urban cool islands.

             An energy budget analysis helps to explain the reason for this transformation.  Let’s first compare the effect of solar radiation falling on an asphalt parking lot and an expanse of grass.  Solar radiation is either reflected or absorbed by a surface depending on its colour.  Black asphalt absorbs a greater proportion of the solar radiation that it receives than does green grass.  The energy absorbed by the surface is then converted into various streams of energy.  If the surface is wet or transpiring, like the grass, then a lot of the energy will go into evaporation.  But if the surface is dry like the asphalt then this stream is not available.  All the energy absorbed by the asphalt goes into heating the asphalt and then either heats the air next to it through convection, or is emitted as terrestrial radiation.  Both of these streams of energy help to create the urban heat island – warmer air and more terrestrial radiation.  Meanwhile, the grass, having started with less energy, and then having lost some to evaporation, has much less energy available to heat itself or the air next to it, or to emit as terrestrial radiation.  Cooler surfaces translate into less of an urban heat island. 

             Now let’s add trees to both landscapes and consider the effect.  Instead of the solar radiation falling directly on the surfaces much of it would be intercepted by the leaves and branches of the trees.  The radiation that makes its way through the canopy would then be transformed in the same way as the radiation in the previous example.  The only difference would be that there would be a lot less energy to start, and the effect on the urban climate would consequently be much less.  Even parking lots will stay reasonably cool if they are shaded by healthy trees.

             There are two things you can do to help reduce the urban heat island in your city.

             Lesson One: Plant trees that shade asphalt surfaces to reduce the effects of urban heat island intensification.

             Lesson Two: Prepare for a future of driverless cars by starting to think about transforming parking lots into well-watered, shady green spaces.

             Who knows… maybe we can unpave parking lots and put up a paradise.

Trees causing air pollution?! Say it ain’t so, Joe!

4 Species to Avoid and 3 Species to Plant It’s well known that trees remove pollutants while purifying and humidifying the air. While this is true in many cases, it turns out that there's another side to the story. And, like so many stories, this one came from Hollywood.

Starting in the early 1940s a foul smelling, throat-burning, plant-killing fog would roll into Los Angeles without warning and for no apparent reason. Residents were understandably upset. It ruined the magnificent Los Angeles weather. It took many years to solve the puzzle of why this apparent mixture of smoke and fog, which became known as smog, formed in this area. And when the mechanism was finally revealed it involved… believe it or not… trees!

Trees produce isoprene which seems to protect the leaves against damage at high temperatures. As the air temperature goes up, trees produce more isoprene and much of it escapes through the stomata on leaves and wafts into the atmosphere. During beautiful hot, sunny southern California weather large amounts of isoprene were being injected into the air by trees. Isoprene is totally natural and there is no environmental threat when trees emit it in natural settings. However, when isoprene is chemically altered through exposure to solar radiation it can then react with nitrogen oxides in vehicle exhaust to create smog. And in the 1940s with cars becoming more common, the Los Angeles basin with its low winds had all the essential ingredients for smog formation.

In suburban and rural areas the amount of nitrogen oxide is typically very small and the potential for smog production is also very small. So the species of trees growing in these locations do not need to be low isoprene producers. But in high-urban areas with large volumes of vehicular traffic and reduced wind speeds it might be wise to avoid trees that produce a lot of isoprene.

So how can we know which trees to plant and which to avoid? You can refer to a long list of trees and their ‘ozone-formation potential’ (OFP) in Landscape Architecture Graphic Standards, in an article written by Terry Gillespie and me. But here’s a shorthand version of the list:

  • Four genera with high potential to produce smog: Carya, Populus, Salix, and Quercus.
  • Three genera with low potential to produce smog: Acer, Fraxinus, and Tilia.

While it would be incorrect to say that trees actually cause smog, it certainly is correct to say that some trees contribute to the level of ozone smog in urban areas. We hope that trees will live for many decades, and while we don’t know what the level of nitrogen oxides will be in the future, it’s probably wise to use low OFP trees in high-urban areas.

Hopper, Leonard J. 2006. Landscape Architecture Graphic Standards. John Wiley & Sons, New York.