Saturday, August 11th 2012, 8:52 AM EDT
A new study from the University of Cambridge Department of Earth Sciences has successfully reconstructed temperature from the deep sea to reveal how global ice volume has varied over the glacial-interglacial cycles of the past 1.5 million years.
The study, “Evolution of ocean temperature and ice volume through the Mid-Pleistocene Climate Transition,” reported in the journal Science, announces a major breakthrough in understanding Earth’s climate machine by reconstructing highly accurate records of changes in ice volume and deep-ocean temperatures. It also offers new insights into a decade’s long debate about how the shifts in Earth’s orbit relative to the Sun have taken the planet in and out of an ice-age climate.
Reconstructing ancient climate changes is a critical part of understanding current climate behavior and predicting how the planet might respond to future man-made changes, such as the injection of large quantities of carbon dioxide into the atmosphere.
Creating such an accurate picture in the past has been problematic. Previous efforts have been thwarted by the fact that the most readily available marine geological record of ice-ages, changed in the ratio of oxygen isotopes (Oxygen 18 to Oxygen 16) preserved in tiny calcareous deep sea fossils called foraminifera, is compromised.
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The isotope record shows the combined effects of both deep-sea temperature changes and changes in ice volume. Separating the two has been nearly impossible in the past, 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.
The research team seems to have resolved this problem by introducing a new set of temperature-sensitive data. This allows them to identify changes in ocean temperatures alone, subtract that from the original isotopic data set, and then build what they describe as an unprecedented picture of climatic change over the last 1.5 million years – a record of changes in both oceanic temperature and global ice volume.
This new picture of change includes a much fuller representation of what happened during the “Mid-Pleistocene Transition (MPT), which was a major change in the Earth’s climate system that took place sometime between 1.25 million and 600 thousand years ago. Prior to the MPT, the cycle period between glacial and interglacial periods was approximately 41,000 years. After MPT, however, the intervals extended to nearly 100,000 years, which is the cycle we are in now. It seems this change occurred with little or no orbital forcing.
“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 behavior of the climate system.”
In the past, over 30 different possible theoretical models have been developed to show how these features of the climate might have changed. The debate has endured for over 60 years since the pioneering work of Nobel Laureate Harold Urey in 1946.
The new study resolves some of these problems by introducing a new dataset to the picture – the ratio of magnesium (Mg) to calcium (Ca) in foraminifera. It is easier for magnesium to be incorporated at higher temperatures, so larger quantities of magnesium in the tiny marine fossils imply a rise in the deep-sea temperatures at that point in geological time.
Taken from fossil records contained in cores drilled on the Chatham Rise, and area of ocean east of New Zealand, the Mg/Ca dataset 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 temperatures were doing and what the ice sheets were doing across a 1.5 million year period,” Professor Elderfield explained.
The resulting picture shows that ice volume has changed much more dramatically than ocean temperatures in response to changes in orbital geometry. Glacial periods during the 100,000-year cycles have been characterized by a very slow build-up of ice, which took thousands of years, the result of ice volume responding to orbital change far more slowly than the ocean temperatures reacted. Ocean temperature change, however, reached a lower limit, probably because the freezing point of seawater put a restriction on how cold the deep ocean could get.
Additionally, the record shows that the transition from 41,000-year cycles to 100,000-year cycles of glacial to interglacial intervals was not as gradual as previously thought. In fact, the build-up of larger ice sheets, associated with longer glacial periods, appears to have begun quite suddenly, around 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.
The research team, which was supported by funding from the Natural Environment Research Council, The Royal Society, The Leverhulme Trust, The European Union & the University of Cambridge now plans to apply their method to the study of deep-sea temperatures elsewhere to investigate how orbital changes affected the climate in 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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