Suspiciously similar to this book...
I was describing Part 1 to my 12-year-old son, who said it sounded like a book in his school library that the librarian had recommended this past year. I figured out which book it was, and here's a description from the Barnes & Noble website:
Life As We Knew It
Miranda is a normal 16-year-old girl whose main concerns in life are schoolwork, swim meets and whether or not she will be asked to the prom. But Miranda's world is literally ripped apart when an asteroid hits the moon, shifts it from its orbit and throws the earth into chaos. Millions die due to tsunamis and earthquakes. Millions more perish because of an early, devastatingly cold winter, brought about by ash thrown into the atmosphere by hundreds of volcanic eruptions. The story, told through a series of entries in Miranda's journal, chronicles the heroine's and her family's efforts to survive in a world where staying warm and having enough to eat and drink becomes the day-to-day priority. . . etc., etc.
Maybe this topic is prevalent in certain scientific circles, and this is just a coincidence? I realize that the plots aren't identical, but still...