1.5 Million Years of Climate History Revealed

Scientists from the University of Cambridge Department of Earth Sciences have announced a major breakthrough in a decades old debate, the understanding of our planet’s climate machine, by reconstructing a highly accurate record of changes in ice volume and deep-ocean temperatures over the last 1.5 million years.

The results of this study offer insights into a decades-long debate that has raged in the scientific community over how the shifts in Earth’s orbit relative to the sun have affected our planet’s entry in to and exit out of ice-age climates.

A Problem Solved

Our ability to understand why the climate behaves as it does refers largely on our ability to understand climatic history. It also helps us begin to understand how our climate may begin to react to certain changes. However, scientists trying to recreate an accurate picture of how such changes have caused the shift in and out of ice-ages have been hindered by the fact that the most readily available marine geological record of ice-ages – that being the changes in the ratio of oxygen isotopes preserved in deep sea fossils – is compromised.

According to the University of Cambridge;

This is because the isotope record shows the combined effects of both deep sea temperature changes, and changes in the amount of ice volume. Separating these has in the past proven difficult or impossible, so researchers have been unable to tell whether changes in the Earth’s orbit were affecting the temperature of the ocean more than the amount of ice at the Poles, or vice-versa.

Enter this most recent study. Scientists from the University’s Department of Earth Science appear to have corrected this problem by introducing a new set of temperature-sensitive data into the mix which allowed them to identify changes in ocean temperatures alone. With this, they could then subtract the ocean temperatures from the original isotopic data set and build a record of changes in both oceanic temperature and global ice volume.

Explaining the Mid-Pleistocene Transition

Almost as a byproduct of this new study is a greater understanding of what happened during the Mid-Pleistocene Transition (MPT). The MPT took place between 1.25 million and 600 thousand years ago, and saw a dramatic shift in the way the planet’s climate worked. Prior to the MPT the alternation between glacial periods of extreme cold and warmer interglacials happened at intervals of approximately 41,000 years.

However, after the MPT this cycle was shaken, stretching out to intervals of 100,000 years.

“Previously, we didn’t really know what happened during this transition, or on either side of it,” Professor Harry Elderfield, who led the research team, said. “Before you separate the ice volume and temperature signals, you don’t know whether you’re seeing a climate record in which ice volume changed dramatically, the oceans warmed or cooled substantially, or both.”

“Now, for the first time, we have been able to separate these two components, which means that we stand a much better chance of understanding the mechanisms involved. One of the reasons why that is important, is because we are making changes to the factors that influence the climate now. The only way we can work out what the likely effects of that will be in detail is by finding analogues in the geological past, but that depends on having an accurate picture of the past behaviour of the climate system.”

Introducing a New Dataset

To explain the new dataset that was introduced to help solve the debate, we’ll let the University of Cambridge explain;

Researchers have developed more than 30 different models for how these features of the climate might have changed in the past, in the course of a debate which has endured for more than 60 years since pioneering work by Nobel Laureate Harold Urey in 1946. The new study helps resolve these problems by introducing a new dataset to the picture – the ratio of magnesium (Mg) to calcium (Ca) in foraminifera. Because it is easier for magnesium to be incorporated at higher temperatures, larger quantities of magnesium in the tiny marine fossils imply that the deep sea temperature was higher at that point in geological time.

The Mg/Ca dataset was taken from the fossil record contained in cores drilled on the Chatham Rise, an area of ocean east of New Zealand. It allowed the Cambridge team to map ocean temperature change over time. Once this had been done, they were able to subtract that information from the oxygen isotopic record.

“The calculation tells us the difference between what water temperature was doing and what the ice sheets were doing across a 1.5 million year period,” Professor Elderfield explained.

The Resulting Picture

At the end of it all, the study revealed that ice volume has fluxuated much more dramatically than ocean temperatures in response to changes in orbital geometry. Where glacial periods have been characterised by a very slow build-up of ice volume in response to orbital changes, ocean temperature change has reached a lower limit, probably because the freezing point of sea water puts a restriction on how cold the deep ocean can actually get.

Additionally, the new 1.5 million year record shows that the transition from 41,000-year cycles to 100,000-year cycles happened relatively quickly. The build-up of large ice sheets which are associated with longer glacials appears to have begun quite dramatically, approximately 900,000 years ago.

The pattern of the Earth’s response to orbital forcing changed dramatically during this “900,000 year event”, as the paper puts it.

What’s Next?

Up next for the research team is the application of their new method to different parts of the world.

“Any uncertainty about the Earth’s climate system fuels the sense that we don’t really know how the climate is behaving, either in response to natural effects or those which are man-made,” Professor Elderfield added. “If we can understand how earlier changes were initiated and what the impacts were, we stand a much better chance of being able to predict and prepare for changes in the future.”

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