OFFICIAL11 What is the lecture mainly about?

[00:00.00]Narrator: Listen to part of a lecture in an environmental science class.
[00:06.25]FEMALE PROFESSOR: When land gets developed for human use, the landscape changes...[00:10.42] we don't see as many types of vegetation... trees... grasses... and so forth... [00:14.78]this in turn leads to other losses... the loss of animals that once lived there... [00:19.92]uh, but these are the obvious changes... But there are also less obvious changes, like the climate.
[00:26.57]One interesting case of this... uhh... of changes in the local land use causing changes in climate, specifically the temperature, is in Florida. [00:36.10]Now, what comes to mind when you think of the state of Florida?
[00:39.91]MALE STUDENT: Sunshine! Beaches... 
[00:42.11]FEMALE STUDENT: Warm weather, oranges... 
[00:44.77]FEMALE PROFESSOR: Yes, exactly. Florida has long had a great citrus industry—large groves of oranges, lemons, and the like.[00:51.96] Florida’s winter is very mild; the temperature doesn't often get below freezing. [00:57.00]But there are some areas of Florida that do freeze, [00:59.90]so in the early 1900’s farmers moved even further south in Florida to areas that were even less likely to freeze. [01:07.89]Obviously, freezing temperatures are a danger to the crops. [01:11.65]A bad bout of cold weather . . . a long spell of frosts... could ruin a farmer's entire crop.
[01:17.56]Anyway, before the citrus growers moved south, much of the land in south Florida was what we call “wetlands.”[01:25.25]Wetlands are areas of marshy... swampy land... areas where water covers the soil, or is present either at or near the surface of the soil, for a large part of the year.[01:36.95]Wetlands have their own unique ecosystems with plants and animals with special and interesting adaptations—[01:43.99]very exciting, but it’s not what we’re talking about today—[01:47.44]Ummm... where was I?
[01:49.16]MALE STUDENT: Farmers moved south?
[01:51.36]FEMALE PROFESSOR: Oh yes... farmers moved south, [01:54.02]but the land was not suitable for farming... [01:56.43]you can’t grow oranges in wetlands... so farmers had to transform the wetlands into land suitable for farming. [02:03.50]To do that, you have to drain the water from the land, move the water elsewhere, and divert the water sources, such as rivers.
[02:11.18]Hundreds of miles of drainage canals were built in the wetlands. [02:14.98]Now these areas... the new areas the farmers moved to... used to be warm and unlikely to freeze, [02:21.18]however, recently the area has become susceptible to freezes... [02:25.84]and we are trying to understand why.
[02:28.19]FEMALE STUDENT: Is it some global temperature change or weather pattern... like El Ni?o or something?
[02:34.10]FEMALE PROFESSOR: Well, there are two theories. [02:36.60]One idea, is as you suggest... that major weather patterns . . . something like El Ni?o... are responsible. [02:42.51]But the other idea, and this is the one that I personally subscribe to, is that the changes in the temperature pattern have been brought about by the loss of the wetlands... 
[02:52.33]MALE STUDENT: Well, how would loss of wetlands make a difference?
[02:55.78]FEMALE PROFESSOR: Well, think about what we've been studying so far. [02:59.17]We've discussed the impact of landscapes on temperature right? [03:02.46]What effects does a body of water have on an area?
[03:06.17]MALE STUDENT: Oh yeah, uh, bodies of water tend to absorb the heat during the day... and then they release the heat at night.
[03:12.91]FEMALE PROFESSOR: Yes, exactly! What you just said is what I want you all to understand. [03:17.93]Bodies of water release heat—and moisture back into the environment. [03:22.13]So places near large bodies of water are generally milder... uh, slightly warmer... than those without water.[03:28.95]And what I... and others think is that the loss of the wetlands has created a situation where the local temperatures in the area are now slightly different, slightly colder, than they were a hundred years ago before the wetlands were drained.
[03:42.80]FEMALE STUDENT: Hmmm... do we know what the temperature was like back then? 
[03:47.78]FEMALE PROFESSOR: Well we were able to estimate this: [03:50.45]We have data about south Florida's current landscape, uh, the plant cover, [03:55.31]and we were able to reconstruct data about its landscape prior to 1900. [04:00.59]Then we entered those data... information about what the landscape looked like before and after the wetlands were drained, [04:07.72]we entered the data into a computer weather model. [04:10.44]This model can predict temperatures... [04:13.10]and when all of the data were entered... an overall cooling trend was predicted by the model.
[04:17.96]FEMALE STUDENT: How much colder does it get now?
[04:20.26]FEMALE PROFESSOR: Well, actually, the model shows a drop of only a few degrees Celsius... [04:24.56]but this is enough to cause dramatic damage to crops. [04:27.89]If temperatures overnight are already very close to the freezing point, then, this drop of just a few degrees can take the temperature below freezing... [04:35.91]and freezing causes frosts, which kill crops. [04:39.15]These damaging frosts wouldn't happen if the wetlands were still in existence. Just a tiny temperature difference can have major consequences.