Lecture: The ice age: Narrator: Listen to part of the lecture in a geology class. Professor: So, let's return to 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 the conditions that lead to formation of ice sheets and glaciers. And the movement of the Earth, tectonic plates has probably one such factor. Because of tectonic plate movement, the position of cannons can change, which can cause global oceans circulation patterns to change, which in turn, to be the cause of ... Jennifer? Jennifer: Climate change? Professor: Right, in fact, we still be ice age that begin in artic region in the northern hemisphere about three million years ago, could've been the result of climate change caused by plate movement. The movement that from Isthmus of Panama. Jennifer: Isthmus of Panama? That little stream that connects North and South America? But, I mean, isn't that in the tropics? I don't see how back of sudden the ice age and Arctic? Professor: Well, let's take look of the map. You see, until about four million years ago. North and South America were not connected. What did that mean? Well, it meant the water in Atlantic and Pacific Oceans could intermingle freely. And this mixing created the similar salinity level in the two oceans. Now, roughly four million years ago, what happens? The two tectonic plates collided, one sliding another, and the Isthmus of Panama rises up, connecting the two continents and separating the two oceans. And we think the initial result of was actually the northern hemisphere became warmer. You see, the Atlantic and the Pacific Oceans were no longer connected and the new land bridge forced the water northward, which intensified that flow from water alone North America eastern seaboard, the ocean we called Gulf Stream today. But, eventually, the presence of Isthmus of Panamas allow the salinity levels of the Atlantic and the Pacific to diverge, treat winds from Africa blowing west, evaporated a large amounts of Atlantic surface water more salty. They don't transfer that water vapor across the Isthmus or enter the Pacific as rainfall, which in turn lower the salinity there. Now the increasing of salinity level in Atlantic Ocean has an interesting effect on the Gulf Stream. If the water gets saltier, then it also gets what? Jennifer: Denser? Professor: Good, so the Gulf Stream carries one salty water masses to the northern latitudes. And as the chalets farther north, the water gets colder and denser. Eventually, the cold salty water become so dense that it sinks to the ocean floor. The floor reaches the Arctic Ocean and flows back to the south. So that's how we think the Arctic Ocean became deprived worse and its cooling might have cooled the whole Arctic region. It also conceives one of gulf streams was transported more atmosphere moistures into the northern hemisphere, moistures that fell as rains or snows into the Arctic Ocean and this in turn promoted the formation of sea ice, since of course fresh water freezes at higher temperature than salt water. The sea ice will reflect the sunlight, causing the Arctic region loses more heat and causing even more ice to form. So a chain of reaction and glaciation. Now, there may be an additional factor involved in the development of this ice age, factors like the reduction of heat trapping carbon dioxide in the atmosphere which could result in the global cooling. Another factor to consider is fluctuation of tilts of the Earth axis. See the tilts of the Earth axis toward the Sun decrease slightly, about three million years ago, which could reduce the amount of solar radiation origin in the northern hemisphere. We'll discuss this sector in more detail later, but what amazes me is that something so, basically simple as the emergence of that one time mistral land could have such an impact on oceans' circulation patterns. You see, the seeking of the cold salty water in the Northern Atlantic is the driving force behind a huge loop of ocean water, moving around the globe.