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Scientists question Eemian period analogy

[Date: 2012-06-18]

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Researchers in Germany have shed new light on why the Eemian interglacial period should not be used as model for climate change in today's world. Presenting their study in the journal Geophysical Research Letters, the researchers discovered key differences between the Eemian and modern climatic conditions.

Geologists typically investigate the past to get a hold on what is going on now and will be the case in the future. They evaluate epochs that had conditions known to us today. Once the major climatic processes are determined, scientists simulate these processes with numerical models to further test the potential reactions of the systems on Earth.

Most experts agree that the Eemian period, a warm one that emerged some 125 000 years ago after the Saalian ice age, is a good one to consider for their studies. The planet's average temperatures during the Eemian period were higher than the temperatures we currently have. Even parts of the Greenland ice had melted, and the global sea level was higher than what it is today.

'Therefore, the Eemian time is suited apparently so well as a basis for the topical issue of climate change,' explains Dr Henning Bauch of the Academy of Sciences and Literature Mainz (AdW Mainz) at GEOMAR | Helmholtz Centre for Ocean Research Kiel in Germany.

But in this latest study, Dr Bauch, in cooperation with Dr Evgeniya Kandiano of GEOMAR and Dr Jan Helmke of the Institute for Advanced Sustainability Studies in Potsdam, show one important difference between the Eemian period and today's time: the development in the Arctic Ocean.

According to the researchers, in the current warm period, known as the Holocene, oceanic and atmospheric circulation delivers extensive amounts of heat northward into the high latitudes. The Gulf Stream and the North Atlantic Drift are key examples. Northern Europe has pleasant temperatures because of the currents, which go as far as the Arctic.

Past studies indicated that the oceanic heat transport to the Arctic has risen, while the Arctic Ocean's summer ice cover seems to be shrinking without any respite in sight. The researchers say there is a chance that such conditions also existed 125 000 years ago. No ice should have been present in the Arctic during the Eemian summers.

But an analysis of sediment cores from the seabed (from the Atlantic to the west of Ireland and from the central Nordic Seas to the east of Jan Mayen island in Norway) shows that they contain minute calcite tests of dead microorganisms (foraminifers).

'The type of species assemblage in the respective layers as well as the isotopic composition of the calcitic tests give us information about temperature and other properties of the water in which they lived at that time,' says Dr Bauch.

While the Atlantic-based samples showed typical Eemian temperature signals, topping those of the Holocene period, the Nordic Seas tests did not. 'The found foraminifers of Eemian time indicate comparatively cold conditions,' Dr Bauch points out. 'Major contrasts emerged between the ocean surfaces of these two regions. Obviously, the warm Atlantic surface current was weaker in the high latitude during the Eemian than today.'

Dr Bauch says the Saalian glaciations that preceded the Eemian time were of a much bigger extent in northern Europe than during the Weichselian, the ice age period before the present warm interval. 'Therefore, more fresh water from the melting Saalian ice sheets poured into the Nordic Seas, and for a longer period of time. This situation had three consequences: the oceanic circulation in the north was reduced, and winter sea ice was more likely to form because of lower salinity. At the same time, this situation led to a kind of 'overheating' in the North Atlantic due to a continuing transfer of ocean heat from the south.'

The results of this study provide fresh insight on the Eemian climate. Says Dr Bauch: 'Obviously, some decisive processes in the Eemian ran off differently, like the transfer of ocean warmth towards the Arctic. Models should take this into consideration if they want to forecast the future climate development on the basis of past analogues like the Eemian.'

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Category: Miscellaneous
Data Source Provider: Geophysical Research Letters
Document Reference: Bauch, H. A. et al., 'Contrasting ocean changes between the subpolar and polar North Atlantic during the past 135 ka', Geophysical Research Letters, 2012. doi:10.1029/2012GL051800
Subject Index: Climate change & Carbon cycle research; Coordination, Cooperation; Environmental Protection; Scientific Research

RCN: 34734