New Interpretation of Antarctic Ice-Cores

Researchers at Alfred Wegener Institute expand prevailing theory on climate history

Bremerhaven, 2 March 2011. Climate researchers at the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association (AWI) expand a prevalent theory regarding the development of ice ages. In the current issue of the journal “Nature” three physicists from AWI’s working group “Dynamics of the Palaeoclimate” present new calculations on the connection between natural insolation and long-term changes in global climate activity. Up to now the presumption was that temperature fluctuations in Antarctica, which have been reconstructed for the last million years on the basis of ice cores, were triggered by the global effect of climate changes in the northern hemisphere. The new study shows, however, that major portions of the temperature fluctuations can be explained equally well by local climate changes in the southern hemisphere.

The variations in the Earth’s orbit and the inclination of the Earth have given decisive impetus to the climate changes over the last million years. Serbian mathematician Milutin Milankovitch calculated their influence on the seasonal distribution of insolation back at the beginning of the 20th century and they have been a subject of debate as an astronomic theory of the ice ages since that time. Because land surfaces in particular react sensitively to changes in insolation, whereas the land masses on the Earth are unequally distributed, Milankovitch generally felt insolation changes in the northern hemisphere were of outstanding importance for climate change over long periods of time. His considerations became the prevailing working hypothesis in current climate research as numerous climate reconstructions based on ice cores, marine sediments and other climate archives appear to support it.

AWI scientists Thomas Laepple, Gerrit Lohmann and Martin Werner have analysed again the temperature reconstructions based on ice cores in depth for the now published study. For the first time they took into account that the winter temperature has a greater influence than the summer temperature in the recorded signal in the Antarctic ice cores. If this effect is included in the model calculations, the temperature fluctuations reconstructed from ice cores can also be explained by local climate changes in the southern hemisphere.

Thomas Laepple, who is currently conducting research at Harvard University in the US through a scholarship from the Alexander von Humboldt Foundation, explains the significance of the new findings: “Our results are also interesting because they may lead us out of a scientific dead end.” After all, the question of whether and how climate activity in the northern hemisphere is linked to that in the southern hemisphere is one of the most exciting scientific issues in connection with our understanding of climate change. Thus far many researchers have attempted to explain historical Earth climate data from Antarctica on the basis of Milankovitch’s classic hypothesis. “To date, it hasn’t been possible to plausibly substantiate all aspects of this hypothesis, however,” states Laepple. “Now the game is open again and we can try to gain a better understanding of the long-term physical mechanisms that influence the alternation of ice ages and warm periods.”

“Moreover, we were able to show that not only data from ice cores, but also data from marine sediments display similar shifts in certain seasons. That’s why there are still plenty of issues to discuss regarding further interpretation of palaeoclimate data,” adds Gerrit Lohmann. The AWI physicists emphasise that a combination of high-quality data and models can provide insights into climate change. “Knowledge about times in the distant past helps us to understand the dynamics of the climate. Only in this way will we learn how the Earth’s climate has changed and how sensitively it reacts to changes.”

To avoid misunderstandings, a final point is very important for the AWI scientists. The new study does not call into question that the currently observed climate change has, for the most part, anthropogenic causes. Cyclic changes, as those examined in the Nature publication, take place in phases lasting tens of thousand or hundreds of thousands of years. The drastic emission of anthropogenic climate gases within a few hundred years adds to the natural rise in greenhouse gases after the last ice age and is unique for the last million years. How the climate system, including the complex physical and biological feedbacks, will develop in the long run is the subject of current research at the Alfred Wegener Institute.

Notes for Editors:  Your contacts at the Alfred Wegener Institute are Prof. Gerrit Lohmann (Tel: +49(471)4831-1758; e-mail: Gerrit.Lohmann@awi.de), Dr. Martin Werner,Tel: +49(471)4831-1882; e-mail: Martin.Werner@awi.de) and Dr. Thomas Laepple (Thomas.Laepple@awi.de). Your contact in the Communication and Media Department is Ralf Röchert (Tel: +49 (0)471 4831-1680; e-mail: medien@awi.de). 

The original title of the publication to which this press release refers is: Laepple, T., M. Werner, and G. Lohmann, 2011: Synchronicity of Antarctic temperatures and local solar insolation on orbital time scales. It will be published in the magazine “Nature” on 3 March 2011 (doi:10.1038/nature09825).

The Alfred Wegener Institute conducts research in the Arctic, Antarctic and oceans of the high and mid latitudes. It coordinates polar research in Germany and provides major infrastructure to the international scientific community, such as the research icebreaker Polarstern and stations in the Arctic and Antarctica. The Alfred Wegener Institute is one of the seventeen research centres of the Helmholtz Association, the largest scientific organisation in Germany.

Background:

Ice ages and Milankovitch cycles

The alternation of cold periods with extensive icing (glacials) and warm periods with shrinking glaciers (interglacials) is characteristic of the Quaternary geological period. The Quaternary encompasses the last 2 million years and is divided into the Pleistocene and Holocene epochs. These climate fluctuations are coupled with the build-up and decline of extensive ice sheets. The warm periods resemble the present in terms of climate and vegetation. During the last ice age 20,000 years ago the temperature in Europe was approx. 10-20 degrees lower than today.

The climatic alternation between cold and warm periods on the Earth (glacial-interglacial fluctuations) is linked to the change in insolation, changes in greenhouse gas concentrations (such as carbon dioxide) as well as internal interactions in the climate system. Although there is no generally recognised theory for the occurrence of ice ages, a hypothesis that states that fluctuations in the Earth’s orbit are responsible for glacial-interglacial cycles is considered to be the one best supported by data. Milutin Milankovitch was the first to elaborate this hypothesis on a mathematical basis in the first half of the 20th century. His theory is based on the variation of three parameters of the Earth’s orbit:

1. Eccentricity. The Earth’s orbit around the sun is not circular. The deviation of the orbital ellipse from a circle is described by its eccentricity. The most important periods of these fluctuations are around 100,000 and 400,000 years. The eccentricity fluctuates due to the gravitation of the other planets, primarily the two largest, Jupiter and Saturn.
2. Nutation, obliquity. The inclination of the Earth’s axis in relation to the plane described by the Earth’s orbit has a dominant period of around 41,000 years. This angle is currently 23.5 degrees and varied approximately between 22 and 24.5 degrees during the Quaternary.
3. Precession. The moon acts on the rotating Earth with a torque that attempts to shift the spin axis of the Earth perpendicular to the orbital plane of the moon. The Earth completes a precession of the equinoxes having a characteristic period of 21,000 years in interaction with the rotation of the Earth’s orbit.