CITIES WITH GREEN ROOFS

A segment on last night's PBS Newshour piqued my interest.  Part of the news program's "Coping with Climate Change" series, the segment spotlighted the city of Chicago, which has experienced an overall 2.5 degrees Farenheit (dF) increase in annual average temperature since 1945.  Summers within the past decade have been especially hot ~ part of a national and global trend.

Chicago is in the vanguard of cities which are researching and implementing innovative ways of cooling the local microclimate, the urban heat island which all cities create because concrete and pavement absorb heat from the sun during the day, and release it at night ~ making cities much warmer than surrounding rural areas.  Logically, heat-absorbent architecture must be modified to reverse this effect, and that is exactly what Chicago is experimenting with.

One example may be seen above (click to enlarge) ~ half of the rooftop of Chicago's City Hall has been turned into a thriving green zone of plant life which absorbs solar energy and cools the building interior.  That's 23,000 square feet populated by over 100 plant species.  "Some areas of the roof have rolling terrain with an added 18-inch layer of soil to support trees and shrubs.  A rainwater collection system irrigates the roof and several bee hives pollinate the many flower varieties."  The temperature difference between the green and paved portions of the roof can reach 80 dF in summer.  Energy savings with just half a green roof are around $3600 yearround.  The experiment is so successful that the city encourages rooftop greenery on all buildings, a move enthusiastically supported by residents of the 359 buildings with green roofs in the city.  That's a combined area of 5.5 million square feet, more than any other city in North America.

Chicago's efforts to cool the city don't stop there. The city's streets, which occupy 23 percent of available land area, are in for a makeover.  City planners are "designing new streetscapes that integrate technology and design elements, from widened sidewalks for increased pedestrian traffic to tree and plant landscaping that provide shade.  The (new) pavements are made of a light reflective material mix that .... keeps the street from absorbing so much heat.  Chicago's 1,900 miles of alleyways traditionally absorb heat and cast away potentially cooling rainwater.  But new 'green alleys' use permeable pavement that absorbs rainwater.  As that underground water evaporates, that also keeps the alley and air around it cool."  (See image below.)

The report doesn't even mention the substantial benefit of increased oxygen production, and increased carbon dioxide absorption by all that plant life.  You can see the complete video segment, along with a written transcript, here.  There is also an embedded photo gallery of a variety of green rooftops, including 24.5 acre Millenium Park, the largest green roof in the world, covering two parking garages, a railway, and an opera hall.