Climatic fluctuations close to the equator show a different pattern toclimate change in the Arctic and Antarctic. In the tropics distinct11500 year fluctuations between wet and dry periods can be clearlyidentified which do not occur in temperature reconstructions of polarice cores. The investigations of the climate of the last 25000 years intropical Africa show that dry phases prevailed during lower solarradiation in March and September, which caused the following rain periodto be less intensive. This emphasises the significance of hydrologicalvariations in regional climate change, as was formulated by a Europeanconsortium of earth scientists under the direction of Professor DirkVerschuren (University of Gent, Belgium) in the latest issue of thescience magazine "Nature" (Vol. 462, 7273).
Seasonally recurring rain periods are the decisive feature of thetropical climate, and are of existential importance for the life of thepeople there. In order to determine the reasons for the fluctuations inthe intensities of the rain periods, the European research team examinedthe 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 scientistsretrieved, for the first time in this region, annually laminatedsediment cores from the lake bottom, down to depths of 21 meters. "Withthis, the sediment core covers the last 25000 years" explains AchimBrauer. "Detailed microscopic and geochemical investigations of theindividual sediment layers deliver climatic information on a very exacttime scale". This world-wide first long profile of such lake depositsin the tropics is further supplemented with modern high-resolutiongeophysical data.
The results show that the changes from wet to dry phases vary on thesame temporal sample as fluctuations in the solar radiation, which arecaused by cyclic changes in the Earth's orbit around the sun. Inparticular the rotating of the Earth's axis at a rhythm of 23000 yearsbecomes obvious, which consequently leads to an alternating maximumsolar radiation every 11500 years in the southern tropics and innorthern tropics. These radiation maxima in turn steer the position andthe intensitiy of the inner-tropical convergence zone(ITCZ), therain-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 associatedregional fluctuations of solar radiation, even if these are relativelyweak, have a large influence on the climate at the equator. The questionas to whether these tropical climatic fluctuations have influenced theglobal climatic history still remains open.