/ 30 January 2022

Intense tropical cyclones could hit South Africa’s east coast in the future — study

Safrica Weather Cyclone Irina
Future shock: Huge waves at Durban seafront were caused by tropical Cyclone Irina which hit in 2012. Ancient sediment from the seabed off the city’s coast provide evidence of extreme tropical storms, and modelling suggests this is likely to happen again. Photo: Rajesh Jantilal/AFP

Thousands of years ago, intense tropical cyclones made landfall on the eastern coast of South Africa. And under projected climate change conditions, these damaging phenomena could happen again.

These are the findings of an international study, led by the University of KwaZulu-Natal (UKZN), in which a team of geologists used state-of-the-art techniques to investigate seabed sediments. 

By examining the sediment record from the seabed off the coast of Durban, they found there was a period, under higher sea levels, when storms were much more extreme than they are now. This period coincided with warmer sea temperatures in the Indian Ocean, allowing tropical storms to travel further south than they do presently. 

“We found distinctive sediments that were deposited by severe storms that struck the coast between approximately five and seven thousand years ago,” said Andrew Green, the head of the marine geology research unit at UKZN. 

“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.”

When preserved, these tempestites provide a standing record of the largest storms, particularly intense tropical cyclones, and set a benchmark against which contemporary and future storminess can be assessed, according to the study published in Nature Geoscience

The authors, who are also from the University of Stirling and University of Bremen, said tropical storms are usually confined to Mozambique but renewed ocean warming as a result of climate change could once again allow them to move south, with potentially disastrous implications for cities such as 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 Andrew Cooper, an honorary research professor at UKZN. “If such a storm were to happen now, beachfront infrastructure would be devastated and the rainfall associated with tropical cyclones would cause serious flooding.”

In their study, the authors describe how examinations regarding changes to tropical cyclone frequency and intensity over the southern Indian Ocean under a warming climate have been inconclusive and often contradictory. 

“However, an increasing trend in the intensity and duration of tropical cyclones associated with warming sea surface temperatures and upper ocean heat content in the southern Indian Ocean has been observed in the last two decades.”

Under high greenhouse gas emission scenarios, multi-model climate projections strongly indicate more frequent and more intense strong positive Indian Ocean Dipole (the difference in sea surface temperature between two areas or poles) events. This is driven by increased sea surface variability in the western Indian Ocean, according to the paper.

“Therefore, global warming will likely lead to enhanced storminess in Southern Africa, linked to strong positive [Indian Ocean Dipole] events associated with more intense and southward tracking tropical cyclones, of which the mid-Holocene deposits on the Durban shelf provide a clear analogue … 

“Higher sea surface temperature and strong positive Indian Ocean Dipole events due to global warming are likely to lead to more intense, frequent and southward tracking tropical cyclones, whose impacts will be significantly greater than those of the present and the historical past along the coast of Southern Africa.”

Given the rarity of tropical cyclone landfalls in South Africa, the authors said their findings require a revaluation of hazards in areas along South Africa’s east coast that are likely to become more vulnerable to landfalling tropical cyclones in the future.

The paper describes how geological evidence of past storminess is fundamental for contextualising long-term climate variability and investigating future climate. But while reconstructions of past storminess exist for the Atlantic and Pacific Oceans, the same cannot be said for much of the Indian Ocean, which makes the research important in helping to fill a knowledge gap to understand what could happen under changing climate conditions and rising sea levels. 

“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 Dr Carlos Loureiro, a lecturer in physical geography at the University of Stirling, who carried out modelling of the storm waves, analysing how current ocean trends and climate projections aligned with past climate conditions.

“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.”