Why does the professor begin the lecture by describing the destructive force of an asteroid impact?

[Professor (female)] Class, let’s jump right into the lecture for today. Q7 Let me start by telling you
   an interesting fact that’s relevant to the rest of the lecture: the energy released
   by the impact of a large asteroid on our planet is equivalent to several million
   nuclear weapons. Tell me, why is this information important--from a historical
   perspective?
[Student A (male)] Well, if an impact like that can release as much energy as--as millions of nuclear
   weapons...well,Q7 that’s catastrophic enough to explain extinction events in
   Earth’s history.
[Professor] Exactly. Now by "extinction events" we’re referring to periods in the planet’s
   history when there’s a very...a very marked decrease in the number of species.
   Um, and it happens within a relatively short period of time--at least on a
   geological timescale. There’ve been a handful of extinction events during
   the past 540 million years, but--but the one that’s, um, generated the most
   interest--because it killed the dinosaurs--um, it’s the most recent one, the K-T
   extinction event...which happened 65 million years ago.
[Student B (female)] Q8 [Professor], what does "K-T" mean?
[Professor] Oh, "K-T." Well, "K" is a standard abbreviation for the Cretaceous Period. And
   "T" is the standard abbreviation for the Tertiary Period. Does anyone want to
   take a guess, then, about the meaning of the name "K-T"?
[Student A] Does it mean the extinction event occurred, um, during the Cretaceous and
   Tertiary Periods?
[Professor] Sort of. Of course, we aren’t exactly sure how long the K-T extinction event
   lasted.’Could’ve been a few years...could’ve been a few thousand years...or
   could’ve been even longer. But for a general figure, we date the K-T extinction
   event to around the end of the Cretaceous Period and the beginning of the
   Tertiary Period. It’s basically seen as the dividing line between the two. And
   Q6 what we’re going to talk about today is the reason why this extinction
   event happened. Which goes back to what I mentioned earlier--asteroid
   impacts can be extremely destructive.
   Uh, before I go on, Q11 I just want to emphasize that no one knows what
   caused the K-T extinction event...and there’re many hypotheses. The asteroid
   hypothesis is of course just one of many. Could’ve been volcanic activity,
   climate change, or sea-level changes...just to name a few theories.
   But, anyway, let’s get back to the asteroid hypothesis. OK. Now even before
   there was any, um, evidence for an asteroid impact causing mass extinctions,
   um, people were talking about the possibility. In 1980, though, we got our
   first glimpse of evidence that really backed the idea of an asteroid causing--
   causing the K-T extinction event. A group of researchers led by Luis Alverez
   discovered an interesting, um, stripe...uh, in the sedimentary layers of Earth’s
   crust that date back to the end of the Cretaceous Period and the beginning of
   the Tertiary Period. At the boundary of these two, um, layers--we call it the K-T
   boundary--there were deposits of a very rare element: iridium. Alverez and his
   team discovered that, um, the stripe of iridium was global...it appeared at the
   K-T boundary in sedimentary layers all over the world. Alverez and his team
   wondered about the cause of the iridium deposits and, um, suggested that the
   iridium might have been the result of an asteroid impact that occurred around
   that time.
   The theory was based on the fact that, uh, that certain types of asteroids contain
   a percentage of, um, iridium. So the Alvarez team made some calculations...
   assuming that the K-T asteroid had the, um, the usual amount of iridium. They
   came up with a size for the hypothetical asteroid--ten kilometers in diameter.
   That’s about the size of Manhattan. An asteroid that size would’ve hit Earth with
   a force about 2 million times greater than the most powerful bomb ever tested
   by humans! Unimaginable.
[Student B] That blast would’ve immediately killed life all over the planet?
[Professor] Well, not exactly like that. Q9(C)Q10 There’d be a huge dust cloud
   that would actually’ it would actually block out sunlight and prevent
   photosynthesis from happening on the surface of the planet. Bad news
   for plants...and for everything that eats plants...and for everything that eats
   things that eat plants, Q9(D) There also might’ve been, um, global firestorms.
   Produced by--by flaming debris from the blast. And if there were widespread
   fires, well, Q9(A) there would’ve been a lot of CO2 in the atmosphere...um, and
   as a result--a temporary greenhouse effect. So, um, some of the species that
   survived the initial blast would’ve been killed off by all that.
[Student A] Q11 An asteroid impact like that would’ve made a pretty huge dent
   in the Earth, wouldn’t it? I mean, is there any evidence of a crater from the K-T
   asteroid?
[Professor] Actually, there is. It’s on Mexico’s Yucatan coast. So there’s certainly a lot of
   evidence supporting the asteroid theory of the K-T extinctions. Still--no one
   knows for sure.