Using reconstructions of global temperature based on oxygen isotope ratio analyses of ocean sediment cores and polar glacial ice cores we can look back at the Earth’s climate for about 800,000 years in considerable detail. To go farther back in relatively high temporal detail, we have to rely on the ocean sediment core analyses that provide data back to five million years.
The three previous blog posts looked at the Earth’s climate over the last half million years using these same proxies for global temperature. During that period the Earth’s climate was dominated by five intense glacial periods each lasting about 60,000 to 100,000 years, alternating with much shorter interglacial warm periods lasting about 2,000 to 25,000 years. The Earth is currently in the most recent interglacial period that first reached near modern “normal” temperatures about 12,000 years ago. The glacial-interglacial cycle time was about 80,000 to 120,000 years during this period.
Looking back another half million years to a million years ago, the glacial periods were about the same duration but progressively weaker going backward in time as can be seen in the graph below.
The interglacial warm periods were similar in length to the most recent half million years, but were weaker and did not quite reach the modern “normal” temperature. Consequently, the average global temperature was about the same as for the most recent half million years, a little over 5 degrees Centigrade (C) below our modern “normal” temperature. Thus, over the last million years, the Earth has averaged a little over 5C colder than our current modern “normal” temperature. This estimate is based on adjusting the ocean sediment core reconstruction to match the Antarctic ice core reconstructions from EPICA and Vostok. The average is even lower in the unadjusted ocean sediment reconstruction as will be shown later.
Another difference is that the range in temperature during each cycle was only about 8C during the during the earlier half of the last million years, as compared to about 14C during the most recent half of the last million years.
Looking back all the way to three million years ago, the adjusted ocean sediment core reconstruction shows that global temperature progressively dropped from levels much closer to the modern “normal” around three million years ago to the much colder average of the last million years. Thus, the start of our current ice age was about three million years ago. The cycling between cold glacial periods and warm interglacial periods was present throughout the last three million years but each cycle was only about 40,000 years duration prior to one million years ago compared to 100,000 years duration in the most recent million years as can be seen in the graph below.
Without adjustment, the ocean sediment core reconstruction shows even larger swings in amplitude of both warming and cooling with each cycle as seen in the graph below. The large difference between the adjusted and unadjusted global temperatures is an indication of the uncertainty involved in making these reconstruction estimates.
Note that in the unadjusted ocean sediment core reconstruction the global average temperature is estimated to have been over 7C warmer than our modern “normal” for a period of several thousand years as recently as a little over 2.9 million years ago. With temperatures that warm there would have been much less permanent ice than today and sea levels would have been substantially higher.
One of the challenges to understanding the Earth’s climate is to determine what caused this gradual trend into our current ice age. Another is to understand what caused the 40,000 year cold to warm cycle to increase fairly abruptly around a million years ago to about 100,000 years. And yet another challenge is to determine what caused the increase in amplitude of the cycles about half a million years ago.
One of the leading hypotheses as to what started this ice age is that it may have resulted from the connection of North America to South America by the uplifting that created the Isthmus of Panama and blocked ocean currents from passing between the Atlantic and Pacific.
The 40,000 year cycling corresponds well with orbital/rotational mechanics of the Earth that induce changes of solar radiation at the poles. But what caused the shift to 100,000 year cycling and a greater amplification is more difficult to explain. Until our climate models can reproduce these past changes in climate, I have little confidence that they will be able to accurately predict the future climate. In the mean time, extrapolating past climate cycles is probably our best estimate of the future climate changes we can expect, as was attempted in the previous blog post linked below.