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The Best Line Of Defense Against Blistering Heat

Keeping Our Cool With Green Roofs And Park Breezes

In November, Denver voters passed the Green Roof Initiative by a vote of 54.3 percent to 45.7 percent.

The initiative – a citizen led effort – requires that new buildings over 25, 000 sq. ft. dedicate a percentage of their roof to green, vegetative space. The goal is to assist in reducing urban heat, stormwater runoff and improving biodiversity for insects and birds.

Analysis shows that extreme heat events are increasing across the United States due to climate change. In Colorado, heat-wave days are expected to occur five times more than they currently do, jumping from 10 days per year to 50 by 2050.

In the report “1001 Blistering Future Summers” published on Aug. 1, 2014 by the journal Climate Central, by the year 2100 summers in Denver could be 10.9 degrees Fahrenheit hotter than they are today.

In a report released last June by the Rocky Mountain Climate Organization, the City and County of Denver’s Department of Environmental Health noted that Denver will experience much more extreme heat. Heat-related illnesses cause more deaths than any other natural disaster.

Clearly the outlook for Denver is hot and dry. As a city we will need to have innovative and creative approaches to reduce our impact on climate change and the ability to bring the temperature down. The Green Roof Initiative is one of the many possible solutions.

Heat Can’t Escape

Denver has the third worst Urban Heat Island Effect (UHIE) in the country. The city on average is five degrees hotter than surrounding areas.

The UHIE refers to higher temperatures and air pollution in urban areas caused by structures. When land surface and natural vegetation are reduced, cities lose the ability for heat to escape. Concrete and buildings trap heat and add to the warming effect.

As the heat increases citizens crank up the air conditioning, which then releases more heat into the air. That further contributes to air pollution and reduces air quality. There is a direct relationship between the UHIE and heat-related illness and deaths. Each passing year the brown cloud seems to get a bit worse and the vicious cycle continues.

Urban areas are also drier than rural areas due to lack of green space and impervious areas. More of the sun’s energy is absorbed at the surface, which then raises air temperature versus evapotranspiration (the word “evapotranspiration” refers to a cooling process of water uptake and loss by plants).

As plants transpire they produce a cooling effect. When cities are covered in concrete and structures the ability for heat to escape and to absorb rainfall is lost. Due to impervious surfaces our drainage systems cannot handle severe storms – which are also often the result of global warming.

Additional causes of the heat island in addition to air conditioning are cars and the obstruction of airflow by the built up surfaces.

Plants help cool the air

The American Planning Association (APA) executive summary regarding the UHIE notes four key factors in reducing the heath island effect.

1) Parks are the first and best line of defense. Well-vegetated parks in a variety of forms and sizes mitigate the impact. Increasing vegetation in cites by creating and expanding parks and open space will counter the effect by cooling the air.

2) Parks enhance local wind patterns in cities through the park breeze. Parks may act as microscale “non-urban areas” by creating a circulation known as the “park breeze.” New York City has recently made an investment in two large waterfront parks that may enhance the urban breeze.

3) Parks mitigate local precipitation anomalies amplified by the UHIE. Warmer air and higher concentrations of particulates over cities can cause more frequent precipitation events. Increased rain over cities can help clean the air of pollutants and provides cooling but also will lead to flooding due to lack of pervious surfaces.

4) Parks sequester carbon and other pollutants trapped by urban heat. Trees and vegetation act as a pollutant sink. Project EverGreen has shown that within one year an acre of trees can absorb enough carbon dioxide to equal the amount produced by driving a car 11,000 miles. Trees also remove other pollutants.

The APA’s conclusion states that well-vegetated parks help reduce the amount of pollutants, increase positive benefits to human health, and mitigate climate change.

Denver needs to implement policies within our neighborhoods that increase park space, expand the green roof initiative and create a walkable city. As citizens, we need to demand it.

Tracey MacDermott is chair of the board of Greater Park Hill Community, Inc. Active in the Registered Neighborhood Organization for many years, MacDermott was the 2012 recipient of the Dr. J. Carlton Babbs Award for Community Service. She was trained as a Climate Reality Leader in 2017.


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