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 implies that a climatic change enabled their emergence. Both polar caps contract and expand in response to the temperature fluctuations of the Martian seasons; there are also longer-term patterns that are better comprehended in the contemporary era.