This activity supports the 2014 Holiday Lectures on Science: Biodiversity in the Age of Humans. Human activity is causing the concentration of atmospheric carbon dioxide to increase at an alarming rate. In addition to its role as a greenhouse gas, carbon dioxide reacts with water to form a weak acid called carbonic acid which lowers the pH of seawater. In this activity, students simulate this process by blowing through a straw into a beaker of artificial seawater and measuring the change in pH. Using the provided graph, they determine the atmospheric carbon dioxide concentration that would produce the same pH change in the oceans. Finally, students determine how many years it would take to reach this concentration at the current rate of emissions.