This lesson is optional. In this lesson, students calculate percentages of increase or decrease based on information about how some different places on Earth have changed over time. Students choose which pairs of values to compare to make sense of the situation (MP1). Students attend to precision of language as they calculate and report values to describe these changes. (MP6).

The Warm-up and main activity focus on changes with the Aral Sea, which has been drying up since the 1960s. The last activity is optional because it provides an opportunity for additional practice while focusing on the Great Barrier Reef.