Global Warming

How can things change so quickly?

Glaciers and ice sheets are melting. This can be seen clearly in photographs. The glaciers advanced during the Pleistocene ice age. They have been melting back since then, except for during the Little Ice Age. However, the rate of melting has increased tremendously in recent decades.

Temperature Variations

Since the end of the Pleistocene, Earth’s temperature has risen. The image below shows how it changed over just the last 1500 years (Figure below). The Medieval Warm Period was a time of somewhat higher average temperatures. The Little Ice Age was a time of somewhat lower average temperatures. But for each time, the anomaly (the difference from average temperature) was less than 1° C (1.8° F). Still, the overall trend is slightly upward.

Recent Temperature Trends

Since the mid 1800s, Earth has warmed up quickly (Figure below). By the end of 2017, 17 of the 18 hottest years on record have all occurred since 1998! This temperature rise is what is usually meant by global warming.

Summary

• Average global temperature has been rising since the end of the ice ages.
• The largest temperature rise has been in the past two decades.
• The rise in Earth's temperature since the late 19th century is referred to as global warming.

Review

1. What have been the temperature trends in the past 1500 years?
2. How is the rise in temperatures in the past two decades different from the rise of the past 1500 years?
3. What is global warming?