A CHANGE IN AVERAGE TEMPERATURE: WHY DO ONLY A FEW DEGREES MATTER? 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 1.0°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 3.0°C colder. At that time there was an ice sheet 1.6 kilometres thick as far south in North America as Chicago.