Tag Archives: interglacial warm periods

Three Million Years of Climate Change

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.

Climate Reconstructions

Global temperature reconstructions for the last million years from oxygen isotope ratio analyses of ocean sediment cores (Bintanja), Antarctic ice cores (EPICA and Vostok), and a Greenland ice core (GISP2). Click on graph to enlarge.

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.

Climate Reconstructions

Global temperature reconstructions for the last 3 million years from oxygen isotope ratio analyses of ocean sediment cores (Bintanja), Antarctic ice cores (EPICA and Vostok), and a Greenland ice core (GISP2). Click on graph to enlarge.

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.

Climate Reconstruction

Global temperature reconstruction for the last 3 million years based on oxygen isotope ratio analyses of ocean sediment cores (Bintanja). Click on graph to enlarge.

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.

Interglacial Comparisons

 

Matching Ice Core and Ocean Sediment Core Climate Proxies

In my previous blog entry I showed the large discrepancies in the magnitude of reconstructed paleo climate estimates between the ice core and ocean sediment proxies based on oxygen isotope ratio analysis. I made two sets of adjustments to the proxy data to bring them into better alignment – a high grouping and a low grouping. The high grouping shows higher glacial period temperatures and the low grouping lower glacial period temperatures.

In the high grouping I multiplied the Greenland GISP 2 ice core analysis by 0.5 and the Bintanja ocean sediment analysis by 0.6 to reduce their offsets from our modern “normal” climate reference and thus bring them into better alignment with the Vostok and EPICA ice core analyses from Antarctica, as shown below (click to enlarge).

Climate Reconstructions Adjusted to Match

A comparison of adjusted climate reconstructions from a composite of many ocean sediment cores (Bintanja) versus three different ice core reconstructions (Greenland GISP2 and Vostok and EPICA from Antarctica). The ice core reconstructions have been adjusted by the factor indicated to better match the ocean sediment reconstruction.

For the low grouping I increased the offsets from the modern “normal” climate reference by multiplying the Vostok and EPICA ice core analyses by 1.6 to bring them into better alignment with the Bintanja ocean sediment analysis. In this case I also multiplied the GISP 2 analysis by 0.9 to bring it into better alignment with the Bintanja analysis. The result is shown below (click to enlarge).

Climate Reconstructions Adjusted to Match

A comparison of adjusted climate reconstructions from a composite of many ocean sediment cores (Bintanja) versus three different ice core reconstructions (Greenland GISP2 and Vostok and EPICA from Antarctica). The composite ocean core reconstruction and Greenland GISP2 ice core reconstruction have been adjusted by the factor indicated to better match the Antarctica Vostok and EPICA ice core reconstructions.

Notice that even though the patterns are very similar between the two adjusted groupings, the temperature scale is much different.  The high grouping shows the lowest glacial period global temperatures about 8 to 10 degrees Centigrade (C) below our modern “normal” while the low grouping shows global temperatures about 12C to 18C below.  Likewise, the high grouping shows maximum interglacial warm period temperatures as much as 1C to 4C above our modern “normal” while the low grouping shows peak interglacial temperatures 2C to 8C above our modern “normal”.

I find the high grouping to be more aesthetically appealing because it doesn’t look quite as noisy, but I’m not sure which of these two groupings might be more accurate. The differences between them reenforce the uncertainty of the magnitude of past global climate variations. The timing of major events is remarkably similar between these proxies, although that might result from similar methods in relating the proxie measurements to a time scale for the reconstruction.

I my next post I will make a paleo climate persistence forecast based on the EPICA ice core analyses by comparing our current interglacial period with the previous four interglacials.

Interglacial Comparisons