In an inconvenient truth, the most likely reason the author uses the heading a silent alarm is to show that there is a problem that many do not know about
Explanation:
The term silent alarm shows that the topic is something that is not at plain sight for everyone, in this documentary Al Gore travel around giving conferences about the threat that we are facing with the global warming and how we are blindfolded because we are taking nature for granted and we are really used to live in the technology and practical world.
