The cause-and-effect signal word "therefore" signals to the reader that the idea of how much time should be spent at recess is debated due to different research findings.
In this excerpt, the word "therefore" is indicating a cause/effect relationship. Furthermore, what it is trying to convey is that different research findings have caused a heated debate about how much time children should spend at recess. In other words, the heated debate about that issue is the effect of different research findings. Moreover, the signal word "therefore" is a synonym of "consequently" and "as a result".