Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: 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. Answer: pleistocene ice age
A:
Glaciers have been melting since what period?