The City or the Planet? Asking About Hotter Summers

A Plan for Brightwater

1Brightwater is a fast-growing city on the plains of the central United States. Its planning office is writing a 20-year plan for how the city will use water and energy, and it asked a youth council of middle school students for ideas. Council members Ngozi Eze and Felix Bergström were given one topic. Older residents say Brightwater's summers are much hotter than they used to be. Is that true, and if so, what is causing it?

2Cities are often warmer than the countryside around them. Dark roofs and pavement absorb sunlight and give off heat, even at night. Tall buildings block breezes, and there are fewer trees to shade the ground. This local warming is called an urban heat island.

3At the same time, scientists have measured a rise of about 1.2 °C in the average temperature of Earth's surface since the early 1900s. Several factors can change Earth's temperature, including changes in the Sun's energy, large volcanic eruptions and the amount of carbon dioxide (CO₂) in the air. CO₂ is a gas that traps heat near Earth's surface. Measurements show that CO₂ in the air rose from about 303 parts per million (ppm) in 1920 to about 414 ppm in 2020. Much of it comes from burning coal, oil and natural gas, which is how most of Brightwater's electricity is made.

4Ngozi found old records from a weather station downtown and from a farm station in Pine Valley, 60 km away. Each temperature in Table 1 is a 10-year average centered on the year shown. Felix collected the warming from 1920 to 2020 at several more places for Table 2.