The bank for a changing world

CPATEMP : Reconstructing the history of land-mass temperatures

  • 26.02.2015

A study of climate history can shed new light on current changes to our climate. But how can we look back to a time before the first meteorological measurements were taken, at the beginning of the 17th century? By analysing the indicators that successive climate changes have left behind, stored in the natural world. However, vital information on land-mass temperatures has so far been unobtainable, as we have not had to hand any reliable indicators enabling us to measure past continental temperatures.

Now however, with the aid of a new palaeo-thermometer, an international team led by the European Centre for Research and Education in Environmental Geosciences (CEREGE), located at Aix-Marseille University, is hoping to finally sort through the records of temperature evolution on the African continent over the last 20,000 years. These records will provide some perspective on climate sensitivity during a period close to our own era.

New biomarker for past climates

The natural archives on past climates are mainly stored in polar ice, but can also be found in marine and continental sediment. These libraries of the past contain indicators – the geochemical composition of elements, the presence of certain time-resistant chemical molecules, together with fossilised flora and fauna – all help to paint a picture of the weather patterns during the period when they were trapped.

However, very few these indicators actually provide quantitative data on past temperatures. There are some reliable markers in the marine environment and ice water molecules which enable us to make an assessment of the temperatures prevailing at the poles. But for continental environments, researchers have only been able to count on fossilised pollen as an indicator of local temperatures. This information is however imprecise and unreliable.Scientists were therefore very enthusiastic when a new biomarker – Glycerol Dialkyl Glycerol Tetraethers (GDGTs) – was discovered in the mid-2000s. These highly resistant molecules originate in the membranes of bacteria which live mostly in lakes. It is not yet known why, but the composition and density of these molecules varies according to the ambient temperature.

We think that these variations allow the micro-organisms to adapt the fluidity of their membranes when the temperature changes,” explains Guillemette Ménot, project coordinator at CEREGE, adding: ‟By studying the concentration of different types of these molecules, we can reconstruct the temperature.” 

Credits: Yannick Garcin

Lake sediment at past temperatures

As a first step, the researchers decided to focus their attention on the lakes in Central Africa.

 ‟There is still no data on temperature evolution in this part of the African continent over the last 20,000 years. However, this is a very interesting climatic zone, which underwent a wet period lasting about 8,000 years that remains more or less a mystery to us,” says Guillemette Ménot

The palaeoclimatologist is working on a large bank of archive material collected during earlier projects at the laboratory. But the archives had been nibbled away here and there and it was impossible to obtain data over a complete time series.

There is still no data on temperature evolution in this part of the African continent over the last 20,000 years.

So a new mission had to be sent out to obtain sediment samples. An initial expedition to Cameroon, in 2013, resulted in the extraction of a lake sediment core 20 metres long, representing around 17,000 years of climate data. From this long mud core, precious GDGTs can be extracted. The molecules are first purified and concentrated, and are then moved to an impressive piece of equipment, a chromatograph coupled with a mass spectrometer, which is able to measure the proportion of the different types of molecules.

It is this proportion which at the end of the day enables the researchers to determine the temperature at different depths of the lake, which means at different periods of time. A second sediment-retrieving mission to Chad took place in March this year, with a view to obtaining a regional overview of African climate history.

Past events shed light on the future

Spooling back through the temperature records on the African continent should help scientists to understand why the region entered upon what is known as the ‘African Humid Period’ around 12,000 B.C. and why that period came to an end. 

Was this episode triggered by a temperature change? 

Could these disruptions give us some clues to what is in store for the African continent, given current global warming? 

These measurements will be compared to the results from various climate reconstruction models. This should enable researchers to gradually adjust their modelling to the realities on the ground, and so forecast more accurately how the climate system will react to the various disturbances it is currently going through.

“At last we have access to a palaeo-thermometer which works for land masses. This will open up new prospects of understanding our planet’s climate mechanisms ”

Guillemette Ménot

of the European Centre for Research and Education in Environmental Geosciences (CEREGE), who is also a senior lecturer at Aix-Marseille University.

Crédits Photos : Guillemette Ménot et Yannick Garcin

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