<human>: Why is there an ice age?
<bot>: The Earth’s climate has been changing over time. Ice ages are periods of long-term cooling that occur when the Northern Hemisphere receives less sunlight, which reduces the amount of heat energy reaching the Earth’s surface and oceans. This causes the air to cool and the sea level to fall, resulting in more land area exposed to the cold ocean water. These changes can cause glaciers to grow and expand, which further decreases the average global temperature.