Milankovitch cycles

[Lisiecki and Raymo (2005) used measurements of δ18O in benthic foraminifera from 57 globally distributed deep sea sediment cores, taken as a proxy for the total global mass of glacial ice sheets, to reconstruct the climate for the past five million years]

I read yesterday of a much grander cycle of climate change into which the current global warming episode can be inserted: a cycle which until about a million years ago oscillated at 40 thousand years and since then at 100 thousand years. A grander cycle about which we appear to know a great deal, plenty enough to run climate models. Data which is mostly derived from analysis of subtle changes in composition – in terms of chemicals or of remains of life – taken from cores of ice or cores of sediment. The point of these last two being that they give you a record over long periods of time.

The argument of the present paper (reference 1) is that there was a run of very cold glacial periods around a million years ago, cold enough to push hominins back from their foothold in Portugal. Hominins did not just arrive there from Turkey (or thereabouts) and stay, they had to have several goes at it.

Plus a certain amount of AMOC flip-flopping, the present possibility of which exercises global warming people.

[Part of Figure 1 from the present paper. Age estimates of European and SW Asian early hominin sites and paleoclimate context. 0.7mya left to 2.2mya right. a, Obliquity variations. b, benthic oxygen isotope record from Eastern Equatorial Pacific Site ODP677]

[Location of Site ODP 677]

Glossary

Atlantic meridional overturning circulation (AMOC). It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and oceanic (thermohaline) drivers.

Benthic oxygen isotope records are commonly used as a proxy for global mean surface temperatures during the Late Cretaceous and Cenozoic, and the resulting estimates have been extensively used in characterizing major trends and transitions in the climate system and for analysing past climate sensitivity.

Marine isotope stages (MIS), marine oxygen-isotope stages, or oxygen isotope stages (OIS), are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data derived from deep sea core samples. Working backwards from the present, which is MIS 1 in the scale, over 100 stages have been identified so far, going back some 6 million years.

Mid-Pleistocene transition (MPT). From 1–3 million years ago, climate cycles matched the 41,000-year cycle in the earth’s obliquity. Around one million years ago, there was a switch to the matching the 100,000-year cycle in eccentricity. The transition problem refers to the need to explain what changed one million years ago

Milankovitch cycles. The collective effects of changes in the Earth's movements on its climate over thousands of years. The Earth's rotation around its axis, and orbit around the Sun, evolve over time due to gravitational interactions with other bodies in the solar system. The variations are complex, but a few cycles are dominant. See reference 2.

Ocean Drilling Program (ODP) Site 677 provided excellent material for high resolution stable isotope analysis of both benthonic and planktonic foraminifera through the entire Pleistocene and upper Pliocene. Site 677 was cored using the Advance Piston Corer (ADC) at 1°12'N, 83°44'W in 3461m depth of water

Stadials and interstadials are phases dividing the Quaternary period, or the last 2.6 million years. Stadials are periods of colder climate, and interstadials are periods of warmer climate. Each Quaternary climate phase is associated with a Marine Isotope Stage (MIS) number, which describes the alternation between warmer and cooler temperatures, as measured by oxygen isotope data. Stadials have even MIS numbers, and interstadials have odd MIS numbers.

Vostok. Ice cores have been studied since the early 20th century and depths of over 400m were reached in the 1950s, a record which was extended in the 1960s to 2164m at Byrd Station in Antarctica. Soviet ice drilling projects in Antarctica included decades of work at the Vostok Station, with the deepest core reaching 3769m. Numerous other deep cores in the Antarctic have been completed over the years.

Other matters

I have also come across two more new-to-me words: cosplay and marplot. With cosplay being what people who like to dress up as characters – for example Batman or Mickey Mouse – in popular culture or in computer games. And with marplot being a person who mars someone else’s plot. Someone who interferes unhelpfully in other people’s business. See references 3 and 4. I think the first turned up in the Economist and the second in Henry James.

Conclusions

Once again I have been impressed both by the amount that we know about the past and by the amount of knowledge that is more or less freely available from the comfort of one’s armchair.

References

Reference 1: Extreme glacial cooling likely led to hominin depopulation of Europe in the Early Pleistocene – Vasiliki Margari, David A. Hodell, Simon A. Parfitt, Nick M. Ashton, Joan O. Grimalt, Hyuna Kim, Kyung-Sook Yun, Philip L. Gibbard, Chris B. Stringer, Axel Timmermann, Polychronis C. Tzedakis – 2023. My copy being an author manuscript rather than the finished paper.

Reference 2: https://en.wikipedia.org/wiki/Milankovitch_cycles. 

Reference 3: https://en.wikipedia.org/wiki/Cosplay.

Reference 4: https://en.wikipedia.org/wiki/Busybody.