Climatic fluctuations close to the equator show a different pattern to
climate change in the Arctic and Antarctic. In the tropics distinct
11500 year fluctuations between wet and dry periods can be clearly
identified which do not occur in temperature reconstructions of polar
ice cores. The investigations of the climate of the last 25000 years in
tropical Africa show that dry phases prevailed during lower solar
radiation in March and September, which caused the following rain period
to be less intensive. This emphasises the significance of hydrological
variations in regional climate change, as was formulated by a European
consortium of earth scientists under the direction of Professor Dirk
Verschuren (University of Gent, Belgium) in the latest issue of the
science magazine “Nature” (Vol. 462, 7273).
Seasonally recurring rain periods are the decisive feature of the
tropical climate, and are of existential importance for the life of the
people there. In order to determine the reasons for the fluctuations in
the intensities of the rain periods, the European research team examined
the climate of equatorial East Africa on long-time scales.
“To date there has been hardly any data on climate change in the tropics. Variations in the temperature do not a play a major role in comparison to hydrological changes”, explains Achim Brauer from the GFZ German Research Centre for Geosciences: His working group, together with his European colleagues, analyses the deposits in the Challa-Lake, a crater lake at the eastern foot of the Kilimandscharo.The GFZ scientists
retrieved, for the first time in this region, annually laminated
sediment cores from the lake bottom, down to depths of 21 meters. “With
this, the sediment core covers the last 25000 years” explains Achim
Brauer. “Detailed microscopic and geochemical investigations of the
individual sediment layers deliver climatic information on a very exact
time scale”. This world-wide first long profile of such lake deposits
in the tropics is further supplemented with modern high-resolution
geophysical data.
The results show that the changes from wet to dry phases vary on the
same temporal sample as fluctuations in the solar radiation, which are
caused by cyclic changes in the Earth’s orbit around the sun. In
particular the rotating of the Earth’s axis at a rhythm of 23000 years
becomes obvious, which consequently leads to an alternating maximum
solar radiation every 11500 years in the southern tropics and in
northern tropics. These radiation maxima in turn steer the position and
the intensitiy of the inner-tropical convergence zone(ITCZ), the
rain-rich cloud belt close to the equator. The ITCZ is strongest there,
where the radiation is intense and evaporation is high.
It can, thus, be proven that Earth’s orbit around the sun and associated
regional fluctuations of solar radiation, even if these are relatively
weak, have a large influence on the climate at the equator. The question
as to whether these tropical climatic fluctuations have influenced the
global climatic history still remains open.
This research work was supported within the framework of the
ESF-EuroCORES Programme “EuroCLIMATE”, with national funding through the
DFG, FWO Flandern, NWO and NERI.
Photos of Challa-Lake can be found here:
http://www.gfz-potsdam.de/portal/-?$part=CmsPart&docId=2517120
