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I'm currently in university for environmental science, and in a lecture on paleoclimate our professor showed some graphs of climate in the Holocene and Pleistocene periods (included below), then remarked that the Holocene is much more stable in temperature than the Pleistocene, and then carried out without elaborating.

Graph of temperature averages during the Pleistocene and Holocene periods.

Do we know why this is the case? Has there been any research trying to find out why this is?

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The Earth is currently in the midst of an ice age, and has been for 2.6 million years. (Some say 33 million years as that was when the ice sheet started forming over Antarctica.) For the last 2.6 million years, the Earth has alternated between being almost ice-free in much of the Northern Hemisphere (except for Greenland) and extensively covered with ice in over northern parts of the Northern Hemisphere. The former are called interglacials while the latter are called glaciations.

The period of the oscillations from one interglacial to the next used to be about 41000 years, but switched to 100000 years about a million years ago. The coldness and extent of the glaciations also increased at that time. Explaining this is the 100000 year problem. There are multiple hypotheses, none universally accepted.

Since that switch, the interglacials have tended to be much shorter in duration than are the glaciations. The previous interglacial, the Eemian, lasted about 15000 years. The glaciations suffer lots of ups and downs. Snowfall happens mostly at the ice sheet margins; even at 100% relative humidity, air at -60° C does not contain much moisture. Those margins are subject to fluctuation. As temperature rises, ice at the margins melts and thereby sends boatloads of nutrients to the oceans. Algae celebrate and drain carbon dioxide from the atmosphere, causing temperatures to drop.

It's only when July temperatures at 65° north latitude consistently rise above 0° C that melting becomes persistent. Once persistent melting has started, it can quickly becomes an unstoppable train of melting. That's what happened about 12000 years ago or so.

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