Google Trends for Sustainability

Google Trends can offer fascinating insights into the world's dynamics. Although not designed in-house, the graph below shows the relative number of times the term "Sustainability" was searched in Google since 2004. Can you spot the cyclic pattern? Why is "Sustainability" mostly searched in April and November?

Note that unfortunately, Google script to embed the visualization does not let me vary the size of the window, and therefore the page may look a little clunky from smaller devices.

Update: Since this visualization was added to the website, the cyclic pattern has kept happening! Simply fascinating!