Question for Dr. Boulter

Dr. Boulter (or anyone else who may have an answer to share),

Last week, I came up with a question concerning the reading in the Climate Change: Picturing the Science book. In chapter 1 on page 26, the book mentions the shrinking of the stratosphere due to it's decrease in temperature. My first question is what is causing the stratosphere to get cooler? My only guess is that it's on the outside of the "blanket" of emissions that is trapping the heat. Second, is there any risk that comes with having a smaller, denser stratosphere? The book jokingly says, "Chicken Little might not have approved," but that's not reassuring to me.

Thank you.