First Author

Data on historical climate changes around the globe are crucial to understanding how external factors such as long-term variations in incoming solar radiation have affected climate regimes at high and low latitudes. However, until now, few high-quality climate records had been identified for equatorial regions. Dirk Verschuren, a palaeoclimatologist at Ghent University in Belgium, and his colleagues have discovered a climate archive that provides information about the unique history of monsoon rainfall in equatorial east Africa (see page 637). Verschuren tells Nature more.

Why has it been difficult to find archives of purely equatorial climate dynamics?

In much of western and central equatorial Africa, climate history has been strongly influenced by environmental changes at high northern latitudes, such as the area covered by sea ice. These changes determined how much tropical heat could be exported to the poles by the Atlantic Ocean circulation, which, in turn, affected the amount of monsoon rainfall over the western side of equatorial Africa.

How did you choose your site?

To get around this strong high-northern-latitude influence on equatorial climate, we went to the Indian Ocean side of the African continent. Exploratory coring in Lake Challa on the border of Kenya and Tanzania had documented finely laminated sediments, which can yield a high-quality climate record. When probing the lake floor with seismic-reflection equipment, we found that it contains more than 250,000 years of sediment and that it could answer our questions about equatorial monsoon dynamics.

Was gathering samples a challenge?

At first, yes. In our initial visit we built a coring platform by affixing pieces of timber across two fishing canoes. But the canoes had been crafted from crooked local tree trunks. It was difficult to build a sturdy platform and row it to where we needed it. When we returned for deeper, older mud, we brought our own platform and an outboard engine.

Why is your finding important?

We've learned that when high-northern-latitude influences are modest, equatorial climate history is a hybrid of tropical monsoon dynamics in the northern and southern hemispheres. The Challa climate record also shows that tropical rainfall has always varied. Because natural background conditions are so variable, it's not easy to tell by how much anthropogenic greenhouse-gas emissions have affected tropical climate.