Specific Connection in Energy-Water Nexus

Because a city, on a warm summer day, can be 6º to 8°F warmer than its surrounding areas, scientists sometimes refer to cities as urban heat islands. The physics behind urban heat islands is simply that dark or black materials used on roofs and road surfaces absorb more sunlight than do light-toned surfaces. The temperatures of roofs and road surfaces can be as much as 70°F higher than the air temperature. These hot surfaces then heat the surrounding air.

The higher temperatures found in a city increase the demand for electricity (for air conditioning), the demand for water (for lawns and pools), and smog.

Resource Quantities

Urban heat islands increase the consumption of both water and energy, as noted below.

• Urban heat islands are responsible for 5 to 10 percent of air conditioning use nationwide.
• Water consumption for lawns in heat islands is 5 percent higher than in surrounding areas.
• Evaporation from swimming pools in heat islands is about 10 percent higher than in surrounding areas.
• Mitigating the effects of urban heat islands could reduce urban cooling energy and water use by about 20 percent.

As power plants burn more fossil fuels to meet the high demand for air conditioning in urban heat islands, they increase the production of both pollutants and greenhouse gases. In addition, the formation of smog is sensitive to temperature: the higher the temperature, the greater the formation and, hence, concentration, of smog. When temperatures are below 70°F in Los Angeles, the concentration of smog (measured as ozone) is below the national standard. At temperatures of about 95°F, all days are smoggy. Cooling the city by about 5°F would greatly lower energy consumption.

Sector Challenges and WETT Solutions

Much of the fundamental scientific research and development of materials for cooling our nation’s cities can be credited to Lawrence Berkeley National Laboratory (LBNL). LBNL has performed extensive research to develop reflective surfaces that reduce the temperatures of roofs and pavements, thereby reducing energy and water consumption in urban areas. Using the city of Los Angeles to model the effects of lighter materials and increased plantings, our scientists have found that temperatures could be reduced by 6ºF at 3 pm on a typical summer day in Los Angeles. Because the rate of smog formation depends on temperature, this same model indicated that a 6-degree temperature reduction would reduce smog by about 10 percent, the equivalent of removing three to five million cars from the roads. We currently are enhancing the model by adding the capability to estimate impacts of lighter colors and lower temperatures on water consumption.