Strong regional climatic fluctuations in the tropics




GFZ-Scientists analyse sediments in the Challa-Sea, a crater lake at the Kilimandjaro. - Credit: GFZ
GFZ-Scientists analyse sediments in the Challa-Sea, a crater lake at the Kilimandjaro. – Credit: GFZ

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.

Big freeze plunged Europe into ice age in months

In the film, ‘The Day After Tomorrow’ the world enters the icy grip of a new glacial period within the space of just a few weeks. Now new research shows that this scenario may not be so far from the truth after all.

William Patterson, from the University of Saskatchewan in Canada, and his colleagues have shown that switching off the North Atlantic circulation can force the Northern hemisphere into a mini ‘ice age’ in a matter of months. Previous work has indicated that this process would take tens of years.

Around 12,800 years ago the northern hemisphere was hit by a mini ice-age, known by scientists as the Younger Dryas, and nicknamed the ‘Big Freeze’, which lasted around 1300 years. Geological evidence shows that the Big Freeze was brought about by a sudden influx of freshwater, when the glacial Lake Agassiz in North America burst its banks and poured into the North Atlantic and Arctic Oceans. This vast pulse, a greater volume than all of North America’s Great Lakes combined, diluted the North Atlantic conveyor belt and brought it to a halt.

Without the warming influence of this ocean circulation temperatures across the Northern hemisphere plummeted, ice sheets grew and human civilisation fell apart.

Previous evidence from Greenland ice cores has indicated that this sudden change in climate occurred over the space of a decade or so. Now new data shows that the change was amazingly abrupt, taking place over the course of a few months, or a year or two at most.

Patterson and his colleagues have created the highest resolution record of the ‘Big Freeze’ event to date, from a mud core taken from an ancient lake, Lough Monreach, in Ireland. Using a scalpel layers were sliced from the core, just 0.5mm thick, representing a time period of one to three months.

Carbon isotopes in each slice reveal how productive the lake was, while oxygen isotopes give a picture of temperature and rainfall. At the start of the ‘Big Freeze’ their new record shows that temperatures plummeted and lake productivity stopped over the course of just a few years. “It would be like taking Ireland today and moving it up to Svalbard, creating icy conditions in a very short period of time,” says Patterson, who presented the findings at the European Science Foundation BOREAS conference on humans in the Arctic, in Rovaniemi, Finland.

Meanwhile, their isotope record from the end of the Big Freeze shows that it took around two centuries for the lake and climate to recover, rather than the abrupt decade or so that ice cores indicate. “This makes sense because it would take time for the ocean and atmospheric circulation to turn on again,” says Patterson.

Looking ahead to the future Patterson says there is no reason why a ‘Big Freeze’ shouldn’t happen again. “If the Greenland ice sheet melted suddenly it would be catastrophic,” he says