School of Agricultural, Earth and Environmental Sciences

Cyclone Idai in 2019 was among the most recent intense tropical cyclones to make landfall along southern Africa.

Geologists Warn Tropical Cyclones Could Hit KZN

An international study led out of UKZN and published in the journal Nature Geoscience has used state-of-the-art techniques to investigate seabed sediments, revealing that severe tropical cyclones made landfall along the eastern coast of South Africa in the past and that under projected climate change conditions, these damaging phenomena could arise in the future.

Professor Andrew Green, Head of the Marine Geology Research Unit at UKZN’s School of Agricultural, Earth and Environmental Sciences led the research project with Honorary Research Professor Andrew Cooper and Ms Shannon Dixon of UKZN; Professor Matthias Zabel and Dr Annette Hahn of the University of Bremen’s Center for Marine Environmental Sciences in Germany, and Dr Carlos Loureiro of the University of Stirling in the United Kingdom.

While reconstructions of past storms exist for the Atlantic and Pacific Oceans, that is not the case for much of the Indian Ocean, making this research important in filling a gap in knowledge to help understand what could happen under changing climate conditions and rising sea levels.

The team of geologists examined the sediment record from the seabed off the coast of Durban and found that there was a period – under higher sea levels – when storms were much more extreme than they are now.

‘We found distinctive sediments that were deposited by severe storms that struck the coast between approximately 5 000 and 7 000 years ago,’ said Green. ‘These storms were much bigger than any storm that happened in the 4 000 years since. This has allowed the storm sediments, or tempestites, to be preserved just beneath the seabed.’

The period of increased storminess coincided with warmer sea temperatures in the Indian Ocean and this allowed tropical storms to travel further south than they do presently.

Loureiro, a lecturer in Physical Geography at the University of Stirling, carried out modelling of the storm waves and analysed how current ocean trends and climate projections aligned with past climate conditions.

‘This important work demonstrates that the past climate conditions that allowed very intense tropical cyclones to reach the South African coast are very similar to the ones projected now under climate change,’ said Loureiro.

‘By confirming that these conditions existed in the past, our work provides strong support to recent climate modelling studies indicating that tropical cyclones are likely to migrate poleward in response to global warming.’

At present tropical storms are usually confined to central Mozambique but renewed ocean warming because of climate change could once again allow them to travel south, with potentially disastrous implications for cities like Maputo, Durban and Richards Bay.

‘When these storms hit the coast there were no cities, buildings or roads and the coastline was free to adjust in a natural manner,’ said Cooper.

‘If such a storm were to happen now, beachfront infrastructure would be devastated and the rainfall associated with tropical cyclones would cause serious flooding.’

This research gives impetus to the need to evaluate hazards along South Africa’s east coast that will be more vulnerable to tropical cyclones making landfall.

This study also affirms the calibre of work coming out of UKZN in marine geology.

‘We have established UKZN as a global centre of excellence in marine geology and with our excellent facilities and international collaboration networks we are making major breakthroughs in understanding the coast and sea,’ said Green. ‘This is the first time this kind of evidence has been used to reconstruct past periods of storminess anywhere in the world.’

Words: Christine Cuénod

Photograph: MODIS image captured by NASA’s Aqua satellite, Public domain, via Wikimedia Commons