The history of the climatic change over the last 1.5 million years has been revealed by scientists in a recent research.
The study, carried out by researchers in the University of Cambridge Department of Earth Sciences, offers new insights into a decades-long debate about how the shifts in the Earth's orbit relative to the sun have taken the Earth into and out of an ice-age climate.
Being able to reconstruct ancient climate change is a critical part of understanding why the climate behaves the way it does. It also helps us to predict how the planet might respond to man-made changes, such as the injection of large quantities of carbon dioxide into the atmosphere, in the future.
Included in this is a much fuller representation of what happened during the "Mid-Pleistocene Transition" (MPT) - a major change in the Earth's climate system, which took place sometime between 1.25 million and 600 thousand years ago.
Before the MPT, the alternation between glacial periods of extreme cold, and warmer interglacials, happened at intervals of approximately 41,000 years. After the MPT, the major cycles became much longer, regularly taking 100,000 years. The second pattern of climate cycles is the one we are in now. Interestingly, this change occurred with little or no orbital forcing.
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 Harry Elderfield, who led the research team, 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 characterised 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 sea water put a restriction on how cold the deep ocean could get.
In addition, the record shows that the transition from 41,000-year cycles to 100,000-year cycles, the characteristic changeover of the MPT, was not as gradual as previously thought. In fact, the build-up of larger ice sheets, associated with longer glacials, 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 now plan 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.
The research has been reported in the journal Science.