A shift of only a few degrees in the average global temperature will likely result in more frequent and extreme heat waves. Compared to the mid-20th-century average, we have increased about 1.7°F (~1.0°C). To put this in perspective, at the end of the last ice age 10,000 years ago, Earth’s average temperature was only about 5.5°F (3.0°C) colder. At that time there was an ice sheet 1.6 kilometers (1 mile) thick as far south in North America as Chicago.