Scientists probe Earth's last warm phase

The study used data from the new NEEM ice core drilled in Greenland

Scientists now have a fuller picture of what happened at the poles during the last warm phase on Earth.

Known as the Eemian, this time period extended from roughly 129,000 years ago to about 116,000 years before present.

The poles were known to have been a few degrees warmer than they are today.

But by pulling together more than 40 ice core and marine sediment records, researchers, led by the British Antarctic Survey (BAS), have obtained the most comprehensive assessment yet.

It confirms that the Antarctic emerged from Ice Age conditions first. The Northern Hemisphere followed.

"Interglacial conditions, warm conditions, were in place earlier in the Southern Hemisphere than in the Northern Hemisphere," explained Dr Emilie Capron from BAS.

"Eventually, the Northern Hemisphere catches up and then both poles are warmer than they are today.

"It's something we knew looking at a few records, but now we have more records showing exactly the same pattern," she told BBC News.

The researcher was speaking here in Vienna at the European Geosciences Union General Assembly.

The data synthesis has been completed as part of the UK iGlass programme and the Past4Future project - initiatives that seek clues about what will happen to the Earth's climate in the decades ahead from an understanding of its past behaviour.

Scientists will now use the information to test their computer models.

If their simulations can reproduce the variation in temperatures across the land and ocean surfaces during the Eemian there will be greater confidence in the models as they look forward in time.

This has already been done for one model, "and its simulations are on the right track," confirms Dr Capron.

For her analysis, the BAS researcher combined five ice cores and 39 marine sediment records.

These can be used to infer past temperatures.

Fore example, by studying the ratio of light to heavy molecules of water in the layers of the ice cores, it is possible to gauge the likely precipitation conditions, and therefore the prevailing temperatures, during the ancient snowfalls on Antarctica and Greenland.

And something similar can be done using the mud layers of marine sediments.

These contain the skeletons of microscopic organisms called foraminifera, and the chemistry of their hard parts is heavily influenced by the temperature of the surface waters in which they swam.

"But having the temperatures is not enough," explained Dr Capron.

The fine layers in the Baltic Sea cores are expected to give up important new information

"If you are going to compare the climate from one place to another, you need a common chronology for all the different records. And this was the great challenge in this study - to try to transfer all the palaeoclimatic records on to just one chronology, because we are working beyond the time where we can use radiocarbon dating."

One way to line up these types of records is to look for distinctive markers such as ash layers from major volcanic eruptions.

A set of marine sediment records that came too late to be included in the study is the newly retrieved cores that were drilled from the Baltic Sea at the end of last year.

Under the International Ocean Discovery Program, scientists took cores from seven locations that trace the history of the Baltic Sea back in time from the present, all the way to, and through, the Eemian.

Preliminary study of these cores reveals extremely fine layers that should throw up fascinating new insights on the climate history of the region.

"The sediments of the Baltic basin provide a link between the continental and marine records," Dr Thomas Andren, the expedition's co-chief scientist, reported here at the EGU meeting.

"The Baltic is complicated because it reflects both the inputs of freshwater precipitation over land and also the inflow of marine water. These new cores will allow us to pull apart these signals, to see the climate history of the Baltic in unprecedented detail."

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