High on Mauna Loa, a volcano in Hawaii, sits a small building where scientists measure the air. On May 10, 2013, their instruments showed something that had never been recorded in human history. The amount of carbon dioxide in the atmosphere reached 400 parts per million. Carbon dioxide is a gas that plants use to grow. It also traps heat from the Sun, keeping Earth warm enough for life. This is called the greenhouse effect. Without some carbon dioxide, Earth would be frozen. But burning coal, oil, and natural gas has added huge amounts of extra carbon dioxide to the air. Since the Industrial Revolution began around 1760, humans have increased atmospheric carbon dioxide by about 50 percent. Scientists began measuring carbon dioxide at Mauna Loa in 1958, when the level was 316 parts per million. It has risen every year since. The 400 mark was significant because climate scientists had warned for years that crossing it could lead to more extreme weather, rising sea levels, and shrinking ice caps. Scientists compare carbon dioxide to a blanket. A thin blanket keeps you comfortable. Piling on more blankets makes you too hot. Extra carbon dioxide does that to the climate.