For a long time, people didn’t agree about whether global warming was really happening and, if it was, whether people were to blame. Now that has changed. We have lots of proof that the Earth’s climate is changing. And it seems almost certain that humans have caused it.

Can we actually see signs of global warming?

Have you heard grown-ups say that they remember when winters seemed snowier? Or when summers didn’t seem to be so hot? People all around the world are noticing changes that seem to be related to rising global temperatures. Glaciers are shrinking in many places. Birdwatchers report that migrating birds are returning earlier in the spring and leaving later in the fall. Gardeners say that flowers are blooming earlier in the spring than they used to. These observations all seem to be part of a pattern of shorter winters, longer summers, and higher temperatures overall. But there’s scientific proof, too.

How do scientists measure global warming?

Scientists have been tracking the amount of carbon dioxide in the atmosphere over time. Since 1958, they have measured the Earth’s carbon dioxide levels from an observatory on Mauna Loa, a volcano in Hawaii.

They have measurements for thousands of years before that, too—frozen in ice! The thick ice sheets of Antarctica, Greenland, and other always-frozen places are like time capsules. Scientists drill out long cores of ice and study the bubbles in each layer to find out about the temperature and the amount of carbon dioxide in the atmosphere at the time the ice froze.

The measurements show that the amount of carbon dioxide has increased sharply since the early 1800s—the time when people started burning fossil fuels. And the Earth’s average temperature follows the same pattern. That’s just what we would expect, because we know that more carbon dioxide in the atmosphere means a stronger greenhouse effect.

Does that mean it’s really getting hotter?

It looks that way. Over the past 50 years, the temperature has increased at the fastest rate in history. 2006 was the hottest year ever recorded in the continental United States.