On land and in the ocean, the average world temperature has been increasing since the nineteenth century. In many parts of the world, average temperatures are reaching historic highs. Perhaps these are natural 5 changes over time, and long-term climate cycles are not completely understood. However, most scientists conclude that this average temperature increase stems from the production of carbon dioxide by human activity. Carbon dioxide is a greenhouse gas. It traps energy 10 from the sun, thus preventing that energy from escaping into space. This is fortunate. Without carbon dioxide, we would bake by day and freeze at night. But since the nineteenth century, human activity has increased the amount of carbon dioxide in the air by 40 percent. This 15 rise is reflected in the increase in average temperatures.