listen to part of lecture in Environmental science Class.

OK, so we have been talking about theories that deal with the effects of human activity on the climate.

But today I'd like to talk a little bit about other theories that can explain variations in climate.

And one of the best known is called Milankovitch Hypothesis.

Um, now What the Milankovitch Hypothesis is about?

It says the variations in the Earth's movements, specifically in its orbit around the Sun, these variations lead to differences in the amount of solar energy that reaches Earth.

And these differences in amount of energy that's reaching Earth from the Sun,it is what causes variations in Earth's climate.

OK, a lot of people think of earth orbit around the sun as being perfectly circular, as smooth and as regular as um, say the way the hands move on a well-made watch.

But it just doesn't work that way.

You are probably aware that the Earth's orbit around the Sun is not shaped like a perfect circle.

It's more of an oval, it is elliptical.

But the shape of this orbit is not consistent; it varies over time over a period of about a hundred thousand years.

Sometimes it is a little more circular, sometimes it's more elliptical.

And when earth's orbit is more elliptical, Earth is actually closer to the Sun during part of the year, which makes Earth, and in particular, the Northern Hemisphere warmer.

And why is that important?

Well, because most of the planet's glaciers are in the Northern Hemisphere, and if it gets too warm then glaciers will stop forming.

And we already talked about how that affects Earth's overall temperature.

The second movement involved in the Hypothesis has to do with axial tilt, the tilt of Earth's axis, that imaginary pole that runs through the center of the earth.

And depending on the angle it tilts at, the seasons can be more or less severe.

It makes winters cooler and summers warmer, or, what some might say is doing now, it makes summers less hot, and more importantly, the winters less cold, which just like what I mentioned before, can also stop, prevent glaciers from forming, or cause them to melt.

There is the third movement the Hypothesis covers called Precession.

Precession basically is the change in the direction of Earth's axis of rotation.

It would take me a million years to explain even just the basics of this movement as Precession is quite complex.

And all these details are way beyond our scope.

What's important for you to understand is that these three movements, well they are cyclical and they work together to form, to produce complex but regular variations in Earth's climate and lead to the growth or decline the glaciers.

Now when Milankovitch first proposed this theory in the 1920s, many of the colleagues were skeptical.

Milankovitch did not have any proof.

Actually, there would not be any evidence to support the hypothesis until the 1970s, when oceanographers were able to drill deep into the seafloor and collect samples, samples which were then analyzed by geologists.

And from these samples, they were able to put together a history of ocean temperatures going back hundreds of thousands years.

And this showed that the earth climate had changed pretty much the way like Milankovitch Hypothesis suggested it would.

So this evidence was pretty strong support for Milankovitch's hypothesis, and by the year 1980s, most of people accepted this theory.

However, in the late 1980s, some scientists works exploring Devil's Hole, which is basically an extensive water-filled cave, far from the ocean in Nevada, in western United States.

Over million of years, ground water left deposits of a mineral called calcite on the rock within Devils' Hole.

And by studying these calcite deposits, we could determine the climate conditions, the temperatures over the last half million years.

Well, the Devil's Hole findings contradicted the one obtained during the 1970s, so basically, the question was, were the ages of one or both of the samples wrong?

Or were scientists misunderstanding the significance of the evidences.

Well, um, in the 1990s, a new study was done on the two samples, and the ocean floor sample's were found to be correct, as were the samples from Devil's Hole.

And now it is generally believed that the samples from Devil's Hole correspond to the variations in local climate in the Western United's States rather than global climate changes.