Randall Carlson

So sometimes the earth is farther away from the sun

and sometimes it's closer to the sun.

You know, the average is about 93 million, but sometimes it's more than 94 million.

Sometimes it's closer to 92 million.

Also, the tilt of the Earth's axis changes by several degrees.

So you have changes in what's called the eccentricity of the orbit, and you have changes in the inclination of the Earth's orbit.

And another factor, there's actually three factors that interact.

So sometimes...

the planet will be receiving more solar radiation because of these changing geometries, other times less.

So it was believed that these forces called, after Malutin Milankovitch was his name, he was a mathematician who calculated all of this.

So for quite a while, this was a satisfactory explanation.

These changes in the orbital geometry between Earth and Sun

was what was responsible for the Earth sometimes cooling, and then the ice would grow, and then warming.

The problem was is that the rates of change we now know are way too fast for those forces.

They play out over 100,000 years, 40,000 to 100,000 years.

So in the early 50s, Willard Libby invented radiocarbon dating,

which could be used to identify how long something that had carbon in it, radioactive carbon, which is carbon-14, how long it had been in existence.

Because when a plant or any living thing grows and interacts with the environment and the atmosphere, it takes on the amount of carbon that's in the atmosphere.

What then happens is when the plant or the animal or the person, whatever it is, the organic thing dies, it stops bringing in that carbon.

Carbon has a half-life that's well-known, and what happens is that the radioactive portion of the carbon begins to decay, and by looking at the ratio of carbon-12 to carbon-14, you can tell how long something has been since it died.