Glacial/interglacial cycles, at least those that have occurred over the past million years, have been considered to be largely the result of the natural increases and declines in solar radiation striking the higher latitudes on Earth (those regions closer to the North and South Poles). These variations occur because our Earth wobbles as it rotates about the sun. There are actually three different types of wobbling that occur and each occurs on a different time schedule. One is an 100,000 year cycle in the shape of the ellipse the Earth makes as it revolves around the sun. The second wobble is in the angle the Earth’s axis of rotation about itself makes with the plane of its orbit about the sun - a 40,000 year cycle. Finally, there is a 21,000 year cycle of changes in the angle of the Earth’s axis with respect to the long axis of its elliptical path around the sun. Collectively, these astronomical cycles are known as the Milanković cycles after their discoverer. Periodically these cycles coincide with each other in a manner that either reinforces the tendency toward cooling of the high latitudes and the development of a new ice age or, conversely, reinforces warming of the high latitudes, leading to a rise in sea level and atmospheric greenhouse gasses along with the retreat of the glaciers and ice fields. In the past million years this has resulted in ice ages occurring about every 100,000 years.
The most recent period of warming began about 20,000 years ago and marked the ultimate end of the last ice age. Over the last several decades scientists from a variety of disciplines have been gathering data from around the world on the details of this latest warming (or interglacial) period. For the more recent years, of course, we have actual temperature measurements and atmospheric gas levels from a large number of sites around the world. Putting all this data together with the expected solar radiation impacting the Earth at all latitudes over the past 12,000 years, as determined from the Milanković cycles described above, Shaun Marcott and his colleagues have given us a detailed graph of the Earth’s global mean temperature over this time period. This graph shows the global mean temperature rising for the first 2,000 years and then leveling off for the next 5,000 years. This was followed by a very gradual and irregular decline for the next 4,000 years. At that point, about 1,000 years ago, the cooling became more rapid, but still with brief spikes of warmer periods, culminating in what became known as the “Little Ice Age” chronicled in Europe 400 years ago.