A million and a half years of climate history revealed

Posted by Kate Taylor

Scientists say they've made a major breakthrough in understanding Earth's climate machine by reconstructing highly accurate records of changes in ice volume and deep-ocean temperatures over the last 1.5 million years.

Until now, it's been hard to construct an accurate picture of how atmospheric changes caused past climatic shifts because the most readily available marine geological record of ice-ages - changes in the ratio of oxygen isotopes preserved in deep sea fossils called foraminifera - is compromised.

This is because the isotope record shows the combined effects of both deep sea temperature changes, and changes in the amount of ice volume, and separating the two has been near-impossible.

But the University of Cambridge introduces a new set of temperature-sensitive data, allowing them to identify changes in ocean temperatures alone and subtract that from the original isotopic data set.

"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," says Professor Harry Elderfield.

"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."

The new technique incolves looking at the ratio of magnesium to calcium in foraminifera. Because it's easier for magnesium to be incorporated at higher temperatures, larger quantities of magnesium in  imply that the deep sea temperature was higher at that point.

The results show 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 seen a very slow build-up of ice, with ice volume responding to orbital change far more slowly than ocean temperatures.

"Any uncertainty about 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," says Elderfield.
 
"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."