Feeling the Heat

A phenomenon called the urban heat island effect is to blame for higher summer temperatures in cities than in surrounding areas—and the solution lies in increasing green space among the concrete.

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As Charm City residents know all too well, the heat of a Baltimore summer can be intense. But what they may not know is that on any given summer day, the heat they're experiencing is significantly higher than that of nearby suburban and rural areas. This phenomenon is known as the urban heat island effect, and it occurs when metropolitan areas, like Baltimore City, get a lot hotter on summer days than surrounding areas.

The main cause of the urban heat island effect is the transformation of former forest habitat into a hardscape urban environment made of materials that collect and retain heat from the sun, such as concrete and asphalt. By absorbing heat-producing infrared wavelengths that are found in sunlight, these materials—found in higher concentration in cities—increase the temperature in urban areas.

Urban heat islands can be problematic for many reasons, including:

  • Soaring temperatures lead to an increased use of air conditioners, and more energy consumption means higher energy bills for businesses and residents. Many city residents cannot afford air conditioning and have to endure heat waves or retreat to cooling centers.
  • Much of the electricity needed to keep businesses and residents cool is generated by the burning of fossil fuels and releases harmful greenhouse gases into the atmosphere. This exacerbates the urban heat island effect and creates pollutants that negatively affect air quality.
  • Decreased air quality—combined with higher temperatures—poses serious health risks, especially for the elderly and people with asthma. High heat can also exacerbate underlying health conditions, including lung and heart diseases; additionally, heat exhaustion and heat-related stroke can be fatal.

To further complicate these issues, not everyone bears the burden of heat evenly in an urban area. According to a 2019 investigation by NPR and the University of Maryland's Howard Center for Investigative Journalism, the temperature in the hottest neighborhoods in Baltimore—where there are sprawling blocks of heat-absorbing asphalt and concrete and little green space—can be as much as 10 degrees higher than the coolest. What's more, low-income, minority neighborhoods disproportionately bear the brunt of the more extreme heat.

This disparity is found not only in Baltimore, but in cities across the United States. According to the NPR and University of Maryland investigation, dozens of U.S. cities exhibit this same pattern: those who live in low-income areas are also more likely to experience higher temperatures than wealthier neighborhoods.

It's clear that urban heat islands are harmful—particularly for those in poorer neighborhoods—and with heat waves predicted to be more frequent and extreme in coming years as a direct result of climate change, it's not difficult to feel alarmed. There is good news, though. Increasing green space among the city hardscape can lessen the harmful effects of urban heat islands, for the following reasons:

  • Trees act like massive shade umbrellas. Trees and their branches reduce the amount of solar radiation that reaches the ground below the canopy; in summer, about 10 to 20% of the sun's energy reaches the ground below the tree, with the remainder being absorbed by leaves and used for photosynthesis. Some sunlight is also reflected back to the atmosphere.
  • When trees absorb rainwater through their roots and emit it through their leaves—a process known as transpiration—it has a cooling effect.
  • Trees also help to cool things down through evaporation. Heat is removed from the air when water evaporates from the moist soil below the tree and from rainfall that covers its leaves.
  • Urban trees can also help to combat the problem of climate change through the direct removal of carbon dioxide, the main greenhouse gas, out of the atmosphere. Trees act as carbon sinks by storing carbon in their leaves, trunks, bark and roots.

With an increase in Baltimore's tree canopy in recent years and an eventual goal of increasing the canopy to cover 40% of the city by 2037, Baltimore is well on its way to more green space throughout the city.

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