L.A. Island

High Temperatures

Cities all over the world have been warming up in the summer over the years. Los Angeles, is a striking example of how a city was transformed into an urban heat island.

With increasing irrigation and orchards, Los Angeles cooled until major urban development began. (The pronounced temperature depression in the late 1880-90's is due to the a volcanic eruption.) Temperatures are ten-year averages, including the previous four years and the following five.

In the 1930s, Los Angeles was an area covered with irrigated orchards. The high temperature in the summer of 1934 was 97°F. Then, as pavement, commercial buildings, and homes replaced trees, Los Angeles warmed steadily, reaching 105°F and higher in the 1990s.

Energy Use

Air conditioning demand increased as temperatures increased. On warm afternoons, the demand for electric power rises nearly 2% for every degree Fahrenheit the daily maximum temperature rises. Also, peak power rises 3% for every 0.5°F rise in daily maximum temperature. In total, we estimate that about 1-1.5 gigawatts of power are used to compensate the impact of the heat island. This increased power costs the Los Angeles ratepayers about $100,000 per hour, about $100 million per year.

The increased summertime temperatures increased cooling requirements. The data is the peak load for Southern California Edison in 1988.

Air Quality

As power plants burned more fossil fuels, they increased both pollution levels as well as energy costs.

The impact of these pollution levels is seen in smog. The formation of smog is highly sensitive to temperatures; the higher the temperature, the higher the formation and, hence, the concentration of smog. In Los Angeles at temperatures below 70°F, 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 have a dramatic impact on smog concentration.

As temperature rises, so does the likelihood that smog will exceed the national standard. In Los Angeles, for every degree Fahrenheit the temperature rises above 70°F, the incidence of smog increases by 3%.

Modeling: Simulating Solutions

What would happen in Los Angeles if some roofs and pavements were resurfaced with light materials and the right kind of trees were planted in several communities? The Heat Island Group has been painting the town and shading its homes--all by computer simulation--to answer that question.

Staff scientist Haider Taha has run meteorological and air quality models for the Los Angeles basin. Taha divided the L.A. basin into hundreds of portions and estimated how much vegetation and reflective surfaces could be added to each location. Then he added trees and lightened surfaces in only about 15% of the possible areas. Summer temperatures at 3 p.m. dropped 6°F.

About 40% of the area in the LA basin is covered by buildings and roads which could realistically be made 30% more reflective during their next resurfacing. If this were done, summer temperatures in LA at 3 p.m. on August 27 could become 5 to 9°F (or 3 to 5°C) lower. Then LA would consume 1/2 to 1 GW less in peak power, energy worth at least $100,000 per hour. Most areas would also have improved air quality, and the population-weighted average predicts an ozone reduction of 10 to 20% overall.

Because the rate of smog formation depends on temperature, this same model was used to estimate the effect on the region's smog, taking into consideration wind patterns, moisture, and other factors specific to the area. The results showed an overall reduction in smog by about 10%, the equivalent of removing three to five million cars from the roads. Similar modeling studies are being performed for Houston, Dallas, Chicago, Atlanta, Washington D.C., Baltimore, Philadelphia, New York City, Miami, Phoenix, and Tucson.