A City's Fight Against Itself
Follow and help New York City in its efforts to address its role as an urban heart island
What is the urban heat island effect?
If you've ever traveled from a densely populated metropolitan area to one more rural one nearby during the summer or vice-vera, you may have noticed that one tends to be warmer. The urban heat island (UHI) effect is a common environmental issue faced by metropolitan and urban areas alike, leaving them to experience high temperatures compared to their suburban and rural counterparts (up to a 22°F difference). This is mainly due to incoming solar radiation being absorbed and trapped by dark, impermeable materials used for construction, while vegetation continues to be removed to make way for these new facilities. Unfortunately, this leads to a myriad of issues, including elevated greenhouse gas emissions, as well as heat-related illnesses and even death. Although there have already been many plans implemented, we have to recognize that this is not at all a passive issue and that more needs to be done to ensure that our environment and our communities will heal and continue to prosper for many more years to come.
The map on the right displays urban heat islands for U.S. cities. Zoom in to view a city's UHI.
Causes/Influences
- geographic location
- local weather trends
- reduced vegetation
- changes in radiative/thermal attributes of urban materials
- urban geometry (how buildings affect local micro-climates through wind flow, absorption of solar energy, and solar reflectance)
Effects
- local climate change
- longer growing seasons
- higher energy demands, especially in the summer
- increased air pollution and greenhouse gas emissions
- poor human health (dehydration, heat exhaustion/stroke, death etc.)
- lower water quality by way of thermal pollution
The image on the right depicts U.S. fatalities based on hazards from 2006 to 2015, with heat waves as the most likely cause and tornadoes a close second.
NYC's Heat Island
As seen in the previous image, the most hazard related deaths in NYC between 2006 and 2015 have been associated with heat waves. Currently, the annual average of emergency visits due to heat is 450 and 13 amounting to heath-stroke fatalities. These risks are only expected to increase by the 2050's. The city's panel on climate change anticipates there to be twice as many days with temperatures above 90°F and face an overall 5.7°F increase in its daily temperature.
NYC Heat Vulnerability Index by District. Zoom in to view the names of specific neighborhoods.
Heat Vulnerability Index (HVI) was developed to show neighborhoods susceptible to injury/harm from the heat and warm weather conditions, verified using health data. This helps to compare and identify which communities are more at risk, usually consisting of those in high poverty areas. This includes, but is not limited to the elderly, people already in poor health, and the lack of available air conditioning.
In order to combat the UHI effect, several strategies have been put in place, such as programs implementing green infrastructure, planting trees and vegetation, cool roofs, green roofs, and cool pavements.
Green Infrastructure
Thermal infrared satellite and vegetation data collected on August 14, 2002 of NYC. Areas with more vegetation are associated with cooler temperatures, whereas the UHI effect is strongest in areas with sparse vegetation. Source: NASA Landsat Image Gallery, Maps by Robert Simmon
Green infrastructure (GI) is a network of open, green spaces primarily intended for water management, including capturing stormwater. However, it can also curtail UHI and its effect. This is due to the installation of GI in the public right-of-way (ROW), with rain gardens replacing sidewalk sections. These rain gardens typically consist of a mixture of flowers, grasses, shrubs, and trees. GI also involves green roofs, which will be discussed under the "Cool Roofs vs. Green Roofs" section.
Pocket parks and plazas can be found scattered throughout the city, acting as small-scale open-spaced parks and outdoor areas. In addition to improving the microclimate, they provide social benefits and areas for community activity. However, their construction is impeded by what might be adjacent (buildings or other types of land use), affecting levels of exhaust and shade, for example. These will be important to keep in mind for assignment in the upcoming "Mapping Assignment - Advanced Site Selection Exercise" section, as well as construction in general.
Tree and Forest Vegetation
Consist of multiple species of trees and plants that help to cool climates by providing shading and evapotranspiration. In terms of shading, leaves and branches can absorb up to 30% of solar radiation in the summer and 20% in the winter, with the rest going through to whatever material is below the canopy. As a result. shaded areas can cooler by 20°F to 40°F than their unshaded counterparts.
Evapotranspiration is the total transference of water from the earth's surface to the atmosphere through evaporation and transpiration. Evaporation involves the movement of water to the atmosphere from thing like soil and water bodies, while transpiration is when water moves inside a plant and ultimately exits as water vapor. If considered alongside shading, this can result in a reduction of temperatures by 2°F to 9°F.
Changes in tree canopy cover in NYC between 2010-2017. Zoom in to see what replaced areas that were lost or areas replaced in exchange for gains in coverage.
Cool Roofs vs. Green Roofs
Cool Roofs
Consist of extremely emissive and reflective coating and roofing material, staying around 50°F to 60°F cooler than other more conventional roofing material, like asphalt, which can reach temperatures of 150°F to 185°F during the summer. By reducing the heat transferred into buildings, the building ultimately uses less energy and produces less waste-heat from air conditioning systems. When found in clusters, they can also lower ambient air temperatures.
This map highlights the different cool roofs currently existing in NYC, with an option to zoom in to see how they each look.
In a study simulation, it was determined that if 50% of NYC's roofs became cool roofs, then the average temperature would be reduced by 0.3°F city-wide.
Study conducted under NYC °CoolRoofs Program: city workers and Columbia University worked together to identify and prioritize prospective building owners of 61.98 acres of roofing for cool roof outreach and implementation in heat vulnerable areas by
- Using HVI data to determine geographic areas of focus
- Deciding to form clusters of potential roofs
- Locating and form list of priority participants
- Forming outreach strategies and securing participants
- Creating tools to help the participants better understand how the program would help them and their buildings
This image shows the priority list of locations in heat-vulnerable areas to consider for potential cool roofs within the Bronx, Central Brooklyn, and Northern Manhattan.
Green Roofs
Functions similar to trees and vegetation everywhere else and consists of vegetation growing atop a roof. These plants and their growing medium prevent solar energy from reaching the roof layer below (shading). The same plants also partake in evapotranspiration, resulting cooler air temperatures.
This map highlights the different green roofs currently planned, in construction, and constructed with or without maintenance in NYC, as well as an option to zoom in to see how they each look.
A study determined that if all roofs in NYC were converted to green roofs, then the average temperature for the city would reduce by 0.4°F city-wide.
Cool Pavements
Junior High School 185 in Flushing, Queens, where dark asphalt areas have been converted to lighter concrete and planted spaces. Source: NYC DEP
Originally consisting of highly reflective pavement with light coloring, its definition has expanded to include permeable pavements. Such pavements allow air, water, and water vapor to seep through into the soil, vegetation, or other support materials below. Reflective surfaces are beneficial in that they limit heat gain from solar energy, while permeable surface are beneficial in that they allow for evaporative cooling when wet. Additional benefits include cooler vehicle temperatures resulting in less evaporative emissions from said vehicles, less heated stormwater runoff and filtering of pollutants, preserving aquatic ecosystems and wildlife easily susceptible to excessive heat.
City simulations have determined that if cool pavements are used alongside cool roofs, trees, and vegetation, then the average temperature will reduce by 4°F to 9°F. Cool pavements are also expected to last longer due to lower temperatures and produce less stress unto street trees.
A city's surface area can consist of up to 40% conventionally dark asphalt pavements. In order to mitigate this, between 2007 to 2016, the NYC Department of Transportation (DOT) managed to convert over 68.87 acres of dark asphalt roads to those of a light color or had them replaced with planted areas.
