Marine sediments unlock secrets about climate change in South Africa

Ocean sediments in South Africa provide evidence of climate variation going back 270,000 years. Rogan Ward/Reuters

Climate change projections suggest that droughts will intensify in most parts of South Africa by the end of the 21st century. This is due to reduced precipitation and the loss of water through the combination of evaporation and plant transpiration. The majority of climate models predict that most southern African countries will warm more than the global mean by 2-3°C. This warming will simulate stronger reductions in precipitation.

But these changes might not happen uniformly across South Africa. The southwestern Cape and Limpopo province seem to be the regions affected most from rainfall reductions. On the other hand, wetter conditions are projected in the southeast and along the Drakensberg mountain range.

While seasonal fluctuations in rainfall are normal, rainfall that is substantially below or above average can have serious negative effects. For example, during the 2007 El Niño-related drought in southern Africa, rainfall averaged 50mm-200mm below normal rainfall season levels during the most critical period for crops. This caused significant crop damage.

Future precipitation projections indicate that both these extremes – droughts as well as flooding – may become more frequent in the future. But do these climate predictions deviate from past natural climate variability? And is the projected change within the range of historical natural variability?

A silent witness of climate change

Marine sediments – solid, natural elements that are broken down by processes of weathering and erosion, and collect on the ocean floor – provide evidence of climate variation over time.

Studies show that the South African climate changed periodically between long-term droughts and wet conditions approximately every 23,000 years. Margit Simon

These sediment cores offer a journey through time: the longer the sediment core, the longer you are able to go back in time. For example, analysis of sediments delivered into the southwest Indian Ocean from rivers flowing off eastern South Africa can provide evidence of climate variability going back as far as 270,000 years.

Several rivers make their way through KwaZulu-Natal and the Eastern Cape provinces in South Africa before entering the Indian Ocean. During the rainy season they transport more water – and hence more sediment – into the ocean. The composition of the sediment – for example, the amount of iron it carries – also differs between wet and dry periods. In tropical and subtropical humid regions, high precipitation promotes intense chemical weathering of bedrocks. This results in highly weathered soils whose geo-chemical signature, rich in iron, is transferred to marine sediments.

Our analysis of marine sediment cores found that South Africa experienced rapid climate transitions toward wetter conditions at times when the Northern Hemisphere experienced extremely cold conditions during the last glacial cycle. These cold phases were associated with slow-downs of the global ocean circulation, which transports warm water from the tropics northwards in the Atlantic Ocean.

Warm tropical and subtropical waters remained longer in the Southern Hemisphere, allowing warm and wet conditions to prevail in South Africa for centuries to thousands of years at a time between 100,000 and 40,000 years ago. The Agulhas Current adjacent to South Africa warmed during that time. This potentially provided heat and moisture for additional rainfall on land.

In addition, our most recent study of a 10m-long sediment archive off the KwaZulu-Natal coast tells the story of climate variation over the past 270,000-odd years. The study, of sediment washed into the Indian Ocean via the Thukela River, reveals that climate changed periodically between long-term droughts and wet conditions approximately every 23,000 years. These cycles were caused by changes in the amount of solar radiation reaching subtropical South Africa.

The reason for this is that every 23,000 years the summer season in southern Africa occurs when the earth is closer to the sun during its orbit. So it receives slightly more solar radiation, warming the land more intensely. This creates changes in winds blowing over the Indian Ocean towards eastern South Africa, bringing more intense rainfall.

Climate change and human evolution

There is a striking correspondence between the archaeological record of South Africa and the timing of the abrupt climate change, as derived from marine sediments.

Bifacial points recovered from Blombos Cave, South Africa. The Middle Stone Age tools are made of silcrete and finished by pressure flaking. Christopher Henshilwood/University of the Witwatersrand

Modern humans, Homo sapiens, first evolved in Africa about 200,000 years ago. South Africa offers an unprecedented variety of archaeological sites that provide compelling evidence for the emergence of modern human behaviour. One example is the use of personal ornaments and the development of complex adaptive strategies during the Middle Stone Age, about 280,000 to 30,000 years ago.

What shaped human cultures during this time is an ongoing debate and the subject of many research activities.

Comparing the history of hydrological changes in the region with artefacts from the Middle Stone Age showed a striking correspondence. Climate-driven pulses in southern Africa were probably fundamental to the origin of key elements of modern human behaviour in Africa. But also to the subsequent dispersal of Homo sapiens.

One of the reasons for this is that humans need water, plants need water and so, too, do the animals that humans hunt and eat. These conditions are favourable for population growth. As human population density increases, people are able to network more readily, share ideas and invent technologies.

Looking ahead, many of the projected changes in climate are within the range of historical natural variability. But there are also significant changes that exceed the range of natural climate variability. The main difference between the climate change happening now and that of the geological record is the timing in which these changes occur: climate changes today are occurring at an unprecedented rate.

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