Climate change impacts cyclone prevalence and citrus production

28th March 2014

South Africa could be at increased risk of being directly impacted by topical cyclones within the next 40 years, report researchers from the University of the Witswatersrand (Wits).

PhD candidate at the university’s school of geography, archaeology and environmental studies Jennifer Fitchett, states in her paper, titled A 66-year Tropical Cyclone Record for South-East Africa: Temporal Trends in a Global Context, which was published in the International Journal of Climatology in February, that increasing sea surface temperatures – caused by global warming – are causing an increase in the number of tropical cyclones.

“The big surprise came when we looked at where these storms have been taking place. As the oceans have warmed and the minimum sea surface temperature necessary for a cyclone to occur – 26.5 ºC – has been moving further south, storms in the south-west Indian Ocean have been moving further south too,” she explains.

The study found that in the last 66 years, there have been seven storms that have developed south of Madagascar and hit Mozambique head-on. Four of them occurred in the last 20 years.

“This definitely looks like the start of a trend, which could move further into South Africa, which is already feeling the effects of this shift. The cyclones that hit southern Mozambique in recent years caused heavy rain and flooding in Limpopo province.

“But the trend becomes even more concerning when one considers that the 26.5 ºC temperature line has been moving south at a rate of 0.6º latitude per decade since 1850. At current rates, we could see frequent serious damage in South Africa by 2050. This is not what we expected from climate change. We thought tropical cyclones might increase in number but we never expected them to move,” elaborates Fitchett.

However, her research also found that data for the south-west Indian Ocean over the last 161 years has confirmed results of previous studies, which found that there has not been an increase in the number of tropical cyclones.

“Much of the perceived change in numbers of tropical cyclones is a result of improved storm detection methods, where we have seen, since 1940, an increase in observations owing to aerial reconnaissance and satellite imagery,” she details.

Threats to Citrus Production

In a separate study, Fitchett and co-authors found that, while global warming is causing citrus fruit trees to flower as much as a month earlier than 50 years ago, changes in late season frost are not happening nearly as quickly.

The finding follows a study conducted on different types of citrus fruits, including oranges, lemons and tangerines, in Iran, where the existence of heritage gardens meant data was easily available.

“Before 1988, there were zero to three days between peak flowering and the last day of frost in the city of Kerman, Iran. Since then, the number has increased to zero to 15. The assumption is that, as temperatures get warmer, there will be less frost.

“But although the severity of the frost has decreased, the last day of frost hasn’t been receding as quickly as the advances in flowering. The result is that frost events are increasingly taking place during flowering and damaging the flowers. And essentially, no flowers means no fruit,” explains Fitchett.

According to the study, at current rates, it will take 70 years before it becomes a certainty that frost will occur during peak flowering in Kerman. Already, since 1988, frost has occurred during peak flowering in 41% of the years.

Fitchett adds that, as a producer of citrus, South Africa has been experiencing similar climate warming conditions to Iran. She concludes that, as local farmers do not record the flowering dates of their crops, it is hard to repeat the study in South Africa, but the threat is of concern.