NARRATOR：

Listen to part of a lecture in an environmental science class.

FEMALE PROFESSOR：

When land gets developed for human use, the landscape changes.

We don't see as many types of vegetation, trees, grasses and so forth.

This in turn leads to other losses: the loss of animal that once lived there.

Err... but these are the obvious changes, but there are also less obvious changes, like the climate.

One interesting case of this... of changes in the local land use causing changes in climate, specifically the temperature is in Florida.

Now what comes to mind when you think of the state of Florida?

MALE STUDENT：

Sunshine, beaches.

FEMALE STUDENT：

Warm weather, oranges...

FEMALE PROFESSOR：

Yes, exactly.

Florida has long had a great citrus industry—large growth of oranges, lemons and the like.

Florida's winter is very mild; the temperature doesn't often get below freezing.

But there are some areas of Florida do freeze.

So in the early 1900’s farmers moved even further south in Florida to areas that were even less likely to freeze.

Obviously, freezing temperatures are a danger to the crops.

A bad bout of cold weather... a long spell of frosts... could ruin a farmer’s entire crop.

Anyway, before the citrus growers moved south, much of the land in south Florida was what we call “wetlands.”

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.

Wetlands have their own unique ecosystems, with plants and animals with special and interesting adaptations.

Very exciting, but it's not what we are talking about today.

Emm... where was I?

MALE STUDENT：

Farmers moved south? [rising intonation]

FEMALE PROFESSOR：

Oh, yes. Farmers moved south. But the land was not suitable for farming.

You can't grow oranges in wetland, so farmers had to transform the wetlands into lands suitable for farming.

To do that, you have to drain the water from the land, move the water elsewhere, and divert the water sources such as rivers.

Hundreds of miles of drainage canals were built in the wetlands.

Now these areas, the new areas the farmers moved to, used to be warm and unlikely to freeze, however, recently the area has become susceptible to freezes.

And we are trying to understand why.

FEMALE STUDENT：

Is it some global temperature change or weather pattern like El NiNO or something?

FEMALE PROFESSOR：

Well, there are two theories.

One idea, is as you suggest... that major weather patterns... something like El Niño... are responsible.

But the other idea and this is the one that I personally subscribe to, is that the changes in the temperature pattern had been brought about by the loss of wetlands.

MALE STUDENT：

Well, how would the loss of wetlands make a difference?

FEMALE PROFESSOR：

Well, think about what we've been studying so far.

We discussed the impact of landscapes on temperature, right?

What affects does the body of water have on an area?

MALE STUDENT：

Oh, yeah. Bodies of water tend to absorb the heat during the day, and then they release the heat at night.

FEMALE PROFESSOR：

Yes, exactly.

What you just said is what I want you all to understand.

Bodies of water release heat—and moisture back into the environment.

So places near large bodies of water are generally milder, err... slightly warmer than those without water.

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.

FEMALE STUDENT：

Emm... do we know what the temperature was like back then?

FEMALE PROFESSOR：

Well, we were able to estimate this.

We have data about South Florida's current landscape, emm... the plant cover.

And we were able to reconstruct data about its landscape prior to 1900.

Then we enter those data, information about what the landscape look like before and after the wetlands were drained.

We enter the data into a computer weather model.

This model can predict temperatures.

And when all the data were entered, an overall cooling trend was predicted by the model.

FEMALE STUDENT：

How much colder does it get now?

FEMALE PROFESSOR：

Well, actually the model shows a drop of only a few degrees Celsius...

But this is enough to cause dramatic damage to crops.

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.

And freezing causes frosts, which kill crops.

These damaging frosts wouldn't happen if the wetlands were still in existence, just a tiny temperature difference can have major consequences.