PARKING LOTS

Green Parking Lots: Mitigating Climate Change and the Urban Heat Island

Forest Service research in the late 1990s documented elevated temperatures in neighborhoods up to a quarter mile downwind of unshaded parking lots. This asphalt absorbs more heat and re-radiates that heat load throughout the day and into the evening when the sun goes down. As a result, residential areas near large parking lots, warm up quicker, leading to potentially higher energy use during the summer months to stay cooler (air conditioning usage).

Those same parking lots are also sources of large quantities of polluted stormwater that degrade our waterways, scouring and eroding streambanks and adding heavy metals, gasoline, oils, and hydrocarbons that impact aquatic life and downstream water quality.

As climate change continues to warm our communities, elevating temperatures, especially seasonal low and evening temperatures, parking lots are exacerbating the problem by trapping heat that affects human health and energy use. One relatively easy and inexpensive way to mitigate these environmental issues is to design green parking lots with plenty of large tree canopy cover that provides shade along with stormwater interception and infiltration (if the plantings are designed properly). USDA Forest Service research in Davis, California showed that air temperatures were 4-8 degrees cooler in shaded parking lots. The shade from trees in those parking lots reduced surface asphalt temperatures by 36 degrees F. and vehicle interior temperatures by 47 degrees F. The cooler parking lot temperatures reduced ozone concentrations and hydrocarbon emissions from parked cars (fuel evaporation).

Unshaded parking lots become mini-urban heat islands (a dome of elevated temperature) that can adversely affect human health. Excessive heat can induce heat stokes, impact respiratory illnesses, and increase stress and anti-social behaviors.

The Role of Trees:

Healthy large canopy trees provide shade that keeps air temperatures and asphalt surfaces cooler during the day. With less heating of asphalt surfaces during the day, there is less heat radiating from those surfaces after sunset, allowing air temperatures to cool faster into the evening and overnight.

Trees also transpire water through their leaves, increasing the surface area contributing to evaporation. When a molecule of water evaporates, it takes with it some heat that energy that would otherwise be used to warm the nearby environment. Trees provide an evaporative cooling effect that can decrease local air temperatures by several degrees Fahrenheit. This effect typically reaches its peak when evaporation levels are highest during the middle of the day.

Trees in parking lots also have a positive impact on air and water quality by intercepting particulate matter, absorbing ozone, nitrogen dioxide, and sulfur dioxide gases and intercepting rainfall in their canopies. If parking lot planting areas are designed properly, trees and other landscape plants can allow stormwater to infiltrate into the soils, providing much needed moisture for the plants while filtering out pollutants.

Other benefits:

Improve Customer Experience

The presence of trees on your industrial or commercial property completely changes the look and the feel of the space. With strategically-placed trees, you can turn a warehouse or an office building into an attractive space where people feel welcome. They also provide shade on sunny days. These things are so important for your employees – who spend every workday on-site – as well as your visitors.

A more attractive outdoor space gives customers the best first impression, shows that you care about your brand, and makes them more inclined to return. All of that can be achieved with parking lot trees and car park landscaping!

Natural Sound Buffer

Don’t forget that trees also act as a great natural buffer for noise. Perfect and strategically placed parking lot trees planted in areas that ensure they act as a natural noise buffer, blocking the sound of busy roads and making your commercial property a nicer place to work and do business.

Retrofits and De-paving

Many parking lots have been poorly designed for successful tree and landscape plantings or incorporation of trees was never given any thought during the parking lot design process.  Retrofitting existing parking lots to enhance tree canopy can and should be done. It just takes planning, partnerships, communications, and resources. It is not impossible, and we see it completed all the time. It is often best to start with municipal-owned parking lots, which can be used as an example of what can be done. Discussions with private commercial properties about greening up parking lots can then follow, if private properties complete retrofits, they can often lead to a stormwater credit, good publicity, and increased business.  Research by Dr. Kathleen Wolf at the University of Washington illustrated that greener commercial districts across the country led to an increase in shoppers, more frequent visits, longer stays, and increases in spending from shoppers and visitors.

When retrofitting parking lots, it might be as simple as digging out landscape islands that are not functioning well, creating a shallow below-grade bowl, replacing soils and plants and cutting notches in the curbing so water can enter the curbed island.  It is often best to utilize some large stone where the curb cuts are made to reduce the velocity and impact of the water entering the landscaped area. 

If there were no landscape islands, look for room in the parking lots where cars cannot fit (corners and dead zones or linear strips between the parking stalls), or consider removing (or de-paving) an entire parking space and creating a landscaping island that is usually 10 feet wide by 20 feet long.  Losing a few parking spaces in large parking lots is typically not detrimental because the lots are designed for overly large capacities. 

https://extension.psu.edu/green-parking-lots-mitigating-climate-change-and-the-urban-heat-island

https://earthdevelopmentinc.com/best-parking-lot-trees-for-your-wisconsin-business/