Climate change worsens rainfall that caused devastating floods in South Africa: Study

Containers that fell over at a storage facility following heavy rain and winds last month in Durban, South Africa. PHOTO: AFP

SINGAPORE - The type of extreme rainfall that triggered deadly floods in parts of South Africa in April is nearly twice as likely to occur because of climate change, a study by a team of scientists has found.

Using peer-reviewed methods, scientists from South Africa, the Netherlands, France, Germany, the United States and Britain carried out what is called an attribution analysis.

They estimated that the probability of a similar flooding event happening has nearly doubled due to climate change caused by greenhouse gas emissions heating up the planet.

The study by the international World Weather Attribution (WWA) initiative is part of a growing body of evidence that climate change is supercharging the world's weather and increasing the likelihood of extreme events.

WWA says that all heatwaves are unilaterally linked to global warming and that the record rainfall that caused deadly flooding across Germany and Belgium in July 2021 was made up to nine times more likely by the climate crisis.

The floods in South Africa killed at least 435 people and caused an estimated US$1.57 billion (S$2.2 billion) in property damage.

More than 350mm fell in two days in some places and caused extremely damaging floods in KwaZulu-Natal and Eastern Cape provinces. Durban, the nation's third largest city, was badly affected by floods and landslides that hit the poorest communities the hardest.

The WWA analysis focused on maximum rainfall between April 11 and 12 in the most affected region. The results showed that an extreme rainfall episode such as this one can now be expected to happen about once every 20 years.

Without human-caused global warming, such an event would happen only once every 40 years. This means a heavy rainfall event that can now be expected about once every 20 years is about 4 per cent to 8 per cent heavier than it would have been without climate change, WWA says.

"Most people who died in the floods lived in informal settlements. So again we are seeing how climate change disproportionately impacts the most vulnerable people," said Dr Friederike Otto, of the Grantham Institute at Imperial College London.

Dr Otto is one of the scientists leading the WWA initiative, which started in 2014 and whose work is helping to give much greater clarity to the question of how much weather events can be attributed to climate change.

In the past, it was hard to point the finger of blame with regard to a single event. The best scientists could say was that an extreme heatwave, flood or storm is the sort of event that climate change would eventually cause.

But with nearly 1.2 deg C of warming since pre-industrial times, the evidence is much stronger now, along with the increase in extreme events, from the recent and relentless heatwave in India and Pakistan to repeated flooding along Australia's east coast.

So how does WWA reach its conclusions?

In a briefing note to the media on its work and findings, the initiative explains that its scientists analyse weather data and computer simulations to compare the climate as it is today with the climate of the past following peer-reviewed methods.

The first step is to define the extreme event - for example, the specific location, duration and other parameters. They give the example of different ways to look at the same event - a heatwave over Britain - which might be described as three days of more than 30 deg C in London, or 10 days of more than 25 deg C across England and Wales. This choice affects the results of the attribution study.

"The modern approach is to use several definitions and calculate results for each," WWA says. "This gives scientists an idea of how event definition affects the results and enables them to tailor the study towards an aspect of the event that is most linked to the impacts."

Scientists also use a method that incorporates observations of weather data from the present and the past to see how the probability of similar events have changed.

In addition, they use climate models to simulate the climate from a historical date, say the year 1900, to the modern day, with slowly rising human-caused emissions.

"This enables scientists to detect trends in the extreme as well as calculate an overall probability change. Using several attribution methods, as well as different climate models, to assess the influence of climate change increases the reliability of the results," the note says.

The results mean scientists can then make probability statements about a particular event.

Since the first attribution study was conducted in 2004 - the European heatwave of 2003 - more than 400 have been conducted globally. Scientists, in some cases, are now able to reach preliminary conclusions within days of an extreme event using accepted methods.

This helps the public, policymakers and scientists get a clearer idea of how climate change is affecting the planet and communities, and also inform strategies to adapt, in addition to reinforcing the urgency to cut greenhouse gas emissions to limit global warming.

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