TPO 9 Lecture 2
Environmental Science
Listen to a part of lecture in an environmental science class.
Lecturer: So since we're around the topic of global climate change and its
effects, in Alaska, in the northern Arctic part of Alaska, over the last thirty years
or so, temperature has increased about half a degree Celsius per decade, and
scientists have noticed that there've been changes in surface vegetation
during this time. Shrubs are increasing in the "tundra". Tundra is flat land with
very little vegetation. Just a few species of plants grow there because the
temperature is very cold, and there's not much precipitation. And because of
the cold temperatures, the tundra has two layers: top layer, which is called the
active layer, is frozen in the winter and spring, but thaws in the summer.
Beneath this active layer is the second layer called "permafrost", which is
frozen all year around, and is impermeable to water.
Female Student: So because of the permafrost, none of the plants that grow
there can have deep roots, can they?
Lecturer: No, and that's one of the reasons that shrubs survive in the Arctic.
Shrubs are little bushes. They're not tall and being low in the ground protect
them from the cold and wind. And their roots don't grow very deep, so the
permafrost doesn't interfere with their growth. OK? Now since the
temperatures have been increasing in Arctic Alaska, the growth of shrubs has
increased. And this is presented to climate scientists with a puzzle...
Male Student: I'm sorry, when you say the growth of shrubs has increased, do
you mean the shrubs are bigger, or that there are more shrubs?
Lecturer: Good question! And the answer is both. The size of the shrubs has
increased and shrub cover has spread to what was previously shrub-free
tundra. Ok, so what's the puzzle? Warmer temperatures should lead to
increased vegetation growth, right? Well, the connections are not so simple.
The temperature increase has occurred during the winter and spring, not
during the summer. But the increase in shrubs has occurred in the summer. So
how can increase temperatures in the winter and spring result in increased
shrub growth in the summer? Well, it may be biological processes that occur in
the soil in the winter, that cause increased shrub growth in the summer, and
here's how: there are "microbes", microscopic organisms that live in the soil.
These microbes enable the soil to have more nitrogen, which plants need to
live and they remain quite active during the winter. There're two reasons for
this: first, they live in the active layer, which, remember, contains water that
doesn't penetrate the permafrost. Second, most of the precipitation in the
Arctic is in the form of snow. And the snow, which blankets the ground in the
winter, actually has an insulating effect on the soil beneath it. And it allows the
temperature of the soil to remain warm enough for microbes to remain active.
So there's been increase in nutrient production in the winter. And that's what's
responsible for the growth of shrubs in the summer and their spread to new
areas of the tundra. Areas with more new nutrients are the areas with the
largest increase in shrubs.
Female student: But, what about run-off in the spring, when the snow finally
melts? Won't the nutrients get washed away? Spring thaw always washes
away soil, doesn't it?
Lecturer: Well, much of the soil is usually still frozen during peak run-off. And
the nutrients are deep down in the active layer anyway, not high up near the
surface, which is the part of the active layer most affected by run-off. But as I
was about to say, there's more to the story. The tundra is windy, and the snow
is blown across the tundra, it's caught by shrubs. And deep snow drifts often
form around shrubs. And we've already mentioned the insulating effect of
snow. So that extra warmth means even more microbial activity, which means
even more food for the shrubs, which means even more shrubs and more
snow around etc.. It's a circle, a loop. And because of this loop, which is
promoted by warmer temperatures in winter and spring, well, it looks like the
tundra may be turning into shrub land.
Female student: But will it be long term? I mean maybe the shrubs will be
abundant for a few years, and then it'll change back to tundra.
Lecturer: Well, shrub expansion has occurred in other environments, like
semiarid grassland, and tall grass prairies. And shrub expansion in these
environments does seem to persist, almost to the point of causing a shift. Once
is established, shrub land thrives, particularly in the Arctic, because Arctic
shrubs are good at taking advantage of increased nutrients in the soil, better
than other Arctic plants.
TPO 9 Conversation 2
Listen to a conversation between a student and a librarian employee.
S: Excuse me. Can you help me with something?
L: I'll do my best. What do you need?
S: Well, I've received a letter in my mailbox saying that I'm supposed to return
a book that I checked out back in January, it's call "Modern Social Problems".
But because I'm writing my senior thesis, I'm supposed to be able to keep the
book all semester.
L: So you signed up for extended borrowing privileges?
S: Yeah.
L: But we are still asking you to bring the book back?
S: En-hen.
L: Well, let me take a look and see what the computer says. The title was
"Modern Social Problems"?
S: Yeah.
L: OK. Oh, I see, it's been recalled. You can keep it all semester as long as no
one else
requests it. But, someone else has. It looks like one of the professors in the
sociology
department requested it. So you have to bring it back, even though you've got
extended borrowing privileges. You can check out the book again when it's
returned in a couple of weeks.
S: But I really need this book right now.
L: Do you need all of it or is there a certain section or chapter you're working
with?
S: I guess there is one particular chapter I've been using lately for a section of
my thesis. Why?
L: Well, you can photocopy up to one chapter of the book. Why don't you do
that for the chapter you're working on right now? And by the time you need the
rest of the book, maybe it will have been returned. We can even do the
photocopy for you because of the circumstances.
S: Oh, well, that would be great.
L: I see you've got some books there. Is that the one you were asked to return?
S: No, I left it in my dorm room. These are books I need to check out today. Is it
Ok if I bring that one by in a couple of days?
L: Actually, you need to return it today. That is if you want to check out those
books today. That's our policy.
S: Oh, I didn't know that.
L: Yeah, not a lot of people realize that. In fact, every semester we get a few
students who have their borrowing privileges suspended completely because
they haven't returned books. They're allowed to use books only in the library.
They're not allowed to check anything out because of unreturned books.
S: That's not good. I guess I should hand back onto the dorm right now then.
L: But, before you go, what you should do is fill out a form requesting the book
back in two weeks. You don't want to waste any time getting it back.
S: Thanks a lot. Now I don't feel quite so bad about having to return the book.
