What is the lecture mainly about?

Listen to part of a lecture in a Geology class.
Professor: I’d like to continue our discussion of ice ages and what caused them. Well, we can’t answer this question for sure, but we know that a number of factors can produce conditions that lead to the formation of ice sheets and glaciers. The movement of the earth’s tectonic plates is probably one such factor. Because of tectonic plate movement the position of the continents can change, which can cause global ocean circulation patterns to change, which in turn, could be the cause of..., Jennifer?
Student: Climate change?
Professor: Right! ln fact, the ice age that began in the Arctic region of the northern hemisphere about three million years ago could have been the result of a climate change caused by plate movement, the movement that formed the Isthmus of Panama.
Student: The Isthmus of Panama? The little strip that connects North and South America? But, I mean, isn’t that in the tropics? I don’t see how that could have started an ice age in the Arctic.
Professor: Well, let’s take a look at the map. See, until about four million years ago, North and South America were not connected, but what did that mean? Well, it meant that the waters of the Atlantic and Pacific Oceans could intermingle freely, and this mixing created similar salinity levels in the two oceans. Now, roughly four million years ago, what happened? Well, two tectonic plates collide, one sliding under the other and the Isthmus of Panama rises up—connecting the two continents and separating the two oceans. Now, we think the initial result of this was actually that the northern hemisphere became warmer.
    You see, the Atlantic and Pacific Oceans were no longer connected, and this new land-bridge forced water northward, which intensified that flow of warm water along North America’s eastern seaboard, the ocean current we call the "Gulf Stream" today.
    But, eventually, the presence of the isthmus allowed the salinity levels of the Atlantic and the Pacific to diverge. Trade winds from Africa blowing west evaporated large amounts of the Atlantic surface water, making it more salty, They then transported that water vapor across the isthmus where it entered the Pacific Ocean as rainfall, which in turn lowered the salinity there. Now, the increasing salinity level in the Atlantic Ocean had some interesting effects on the Gulf Stream. lf water gets saltier, then it also gets... what?
Student: Denser.
Professor: Good. So the Gulf Stream carries warm salty water masses to the northern latitudes, and as it travels farther north, the water gets colder and denser. Eventually, the cold salty water becomes so dense that it sinks to the ocean floor before reaching the Arctic Ocean, and flows back to the south. That is how the Arctic Ocean became deprived of warmth; and its cooling might have cooled the whole arctic region.
    It is also conceivable that the Gulf Stream was transporting more atmospheric moisture into the northern hemisphere, moisture that fell as rain or snow into the Arctic Ocean, and this in turn promoted the formation of sea-ice. Since of course fresh water freezes at a higher temperature than salt water, the sea-ice would reflect sunlight, causing the arctic region to lose even more heat and causing even more ice to form, so a chain reaction and glaciation. Now, there may have been additional factors involved in the development of this ice age, factors like the reduction of heat trapping carbon dioxide in the atmosphere, which could have resulted in global cooling. And another factor to consider is fluctuation in the tilt of the earth’s axis. See, the tilt of earth’s axis towards the sun decreased slightly, about three million years ago, which could have reduced the amount of solar radiation reaching the northern hemisphere. Let’s now discuss these factors in more detail later.
    What amazes me is that something so basically simple as the emergence of that one tiny strip of land could have had such an impact on ocean circulation patterns. You see, the sinking of cold salty waters in the north Atlantic is the driving force behind a huge loop of ocean water moving around the globe.