Earth's climate has undergone gradual changes over long periods of time. Historically, Earth has gone through several ice ages and periods of warming. Data from ice cores and other proxy measurements provide us with temperature records going back hundreds of thousands of years. You will learn more about proxies in Lesson 2.
In addition to proxy measurements, we have real temperature data going back 120 years. The Historical Temperatures learning tool shows changes in surface temperature anomalies from 1880 to 2000.
Question: If the Earth's climate has always been changing, why is the change we are observing now such a concern?
Previous changes have been dramatic, but very gradual (spanning thousands of years). The biggest concern with climate change today is the rate at which it is occurring and the potential for it to continue accelerating. We will explore this further in Lessons 2 and 5.