In her book The Weather of the Future, climatologist Heidi Cullen describes the impacts of global climate change on New York City. She focuses in part on the city’s aging and mostly immutable sewage infrastructure. Since sewage water and storm water use the same pipes, the system is more vulnerable to above-normal flooding, i.e. the kinds anticipated from greater rainfall and more forceful tidal surges. She writes:
When it’s not raining, sewage treatment plants can handle all the sewage and clean it up. But when it rains, the vast amount of rainwater that goes into the sewers exceeds their capacity, so some of it has to be released into the rivers untreated. If rainfall becomes more intense-as observed data and climate models suggest will happen-the sewer system could be overwhelmed. That would result in more flooding of streets and basements, and more untreated waste would enter rivers.
Last year’s catastrophic floods in Pakistan and this spring’s historic flooding along the Mississippi River lends visible credulity to the future impacts of flooding. But if the sewage infrastructure of NYC can’t reasonably be modified, how does the city keep from going underwater?
The answer may be green roofs. A study released by Colombia University in January reveals that converting NYC’s 1 billion square feet of roofs into green roofs would divert more than 10 billion gallons of water per year from the sewage system. The vegetation retains 30% of the water that falls on it, then releases it back into the atmosphere as vapor. The heat required to vaporize the liquid is drawn from either the building or the sun, which in either case cools the building and lowers its energy bill. The study estimates the cost of maintaining a green roof means that they could divert a gallon of water for only 2 cents per year.