This suggests that a climatic shift allowed for their onset. Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons; there are also  longer-term trends that are better understood in the modern era.
This sequence of events suggests that a climatic shift created an opportunity for their emergence. The Martian polar caps shrink and regrow in response to the seasonal temperature fluctuations; additionally, there are also more enduring trends that have been better understood through contemporary research.