The Azimuth Project
Glacial cycle (Rev #6)


Starting around the Pliocene Epoch, 5.3 - 1.8 million years ago, the Earth’s temperature has been getting every more jittery as it cools. Something is making the temperature unstable! And as the graph above suggests, these fluctuations are gradually taking longer, as well as slowing.

These temperature fluctuations are not really periodic, despite the optimistic labels on the above graph saying “41 kiloyear cycle” and “100 kiloyear cycle”. And beware: the data in the above graph was manipulated so it would synchronize with the Milankovitch cycles. For details, see:

δ 18\delta^{18}O is a climate proxy: something we can measure now, that we believe are correlated to features of the climate long ago. It is the change in the amount of oxygen-18 (a less common, heavier isotope of oxygen) in carbonate deposits dug up from ancient ocean sediments. These deposits were made by foraminifera and other plankton. The more oxygen-18 there is, the colder we think it was.

Here’s a graph that shows more clearly the noisy nature of the Earth’s climate in the last 7 million years:

Note that in the usual style of paleontology, the present on the left instead of on the right.

By the beginning of the Pleistocene Epoch, 1.8 - .01 million years ago, the Earth’s jerky temperature variations became full-fledged glacial cycles. In the last million years there have been about ten glacial cycles, though it’s hard to count them in any precise way—it’s like counting mountains in a mountain range:

Now the present is on the right, but up means cold, or at least more oxygen-18. This graph is copied from:

• Barry Saltzman, Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, Academic Press, New York, 2002, fig. 1-4.

We can get some more detail on the last four glacial periods from the change in the amount of deuterium in Vostok and EPICA ice core samples, and also changes in the amount of oxygen-18 in foraminifera (that’s the graph labelled ‘Ice Volume’):

As you can see here, the third-to-last glacial ended about 380,000 years ago. In the warm period that followed, the first signs of Homo neanderthalensis appear about 350,000 years ago, and the first Homo sapiens about 250,000 years ago.

Then, 200,000 years ago, came the second-to-last glacial period: the Wolstonian. This lasted until about 130,000 years ago. Then came a warm period called the Eemian, which lasted until about 110,000 years ago. During the Eemian, Neanderthalers hunted rhinos in Switzerland! It was a bit warmer then that it is now, and sea levels may have been about 4-6 meters higher.

The last glacial period started around 110,000 years ago. This is called the Winsconsinan or Würm period, depending on location; a more neutral name is simply last glacial period.

A lot happened during the last glacial period. Homo sapiens reached the Middle East 100,000 years ago, and arrived in central Asia 50 thousand years ago. The Neanderthalers died out in Asia around that time. They died out in Europe 35 thousand years ago, about when Homo sapiens got there. The oldest cave paintings are 32 thousand years old, and the oldest known calendars and flutes also date back to about this time. It’s striking how many radical innovations go back to about this time.

The glaciers reached their maximum extent around 26 to 18 thousand years ago. There were ice sheets down to the Great Lakes in America, and covering the British Isles, Scandinavia, and northern Germany. Much of Europe was tundra. And so much water was locked up in ice that the sea level was 120 meters lower than it is today.

Then things started to warm up. About 18 thousand years ago, Homo sapiens arrived in America. In Eurasia, people started cultivating plants and herding of animals around this time.

There was, however, a sudden return to colder weather 12,700 years ago: the Younger Dryas episode, a cold period lasting about 1,300 years. The Younger Dryas ended about 11,500 years ago. The last glacial period, and with it the Pleistocene, officially ended 10,000 years ago.

Carbon dioxide concentrations in ice cores are closely correlated to other climate proxies, though sometimes they lead or lag. The blue curve here shows CO2 concentrations as measured from the Vostok ice core over the last 420,000 years:

The carbon dioxide data from the Vostok ice core can be found here:

category: climate