The “monsoon rains” that triggered deadly landslides in Kerala’s Wayanad district last month were made 10% worse by man-made climate change, a new rapid attribution study says.
The landslides occurred after a spell of exceptional monsoon rains on July 30. They have killed at least 230 people, with more than a hundred still missing and rescue operations ongoing.
Analysis by the World Native climate Attribution (WWA) service shows that the rainfall that hit Wayanad on July 30 was the third-heaviest on record in the region, surpassing the extreme rainfall that led to floods in Kerala in 2018.
The team of 24 researchers from India, Malaysia, the US, Sweden, the Netherlands and the UK found that downpours of such intensity have become 17% heavier over the past 45 years.
In a world where average global temperatures are 2 degrees Celsius higher than pre-industrial times, they estimate that a single day of extreme rainfall in Kerala could be 4% heavier, potentially leading to even more catastrophic landslides.
The study also looked at other “mixed” factors that may have contributed to the high casualties and Wayanad’s “increased susceptibility” to landslides. These included the loss of 62% of forest cover in the district and warnings that “did not reach many people.”
Slippery Slope
Wayanad is a mountainous district in northern Kerala in the Western Ghats of India – a mountain chain older than the Himalayas that runs obliquely along the country’s western coast.
With its high elevation and steep slopes – combined with a tendency to receive “prolonged” rainfall and widespread changes to natural vegetation – Wayanad is highly susceptible to landslides. It is the most landslide-prone district in Kerala, accounting for 59% of the country’s landslides in 2015-22.
From June 22 onwards, Wayanad witnessed “near-continuous” monsoon rainfall, with some areas recording over 1.8 metres of rain in just one month, the WWA study said.
On July 30, Wayanad saw what study author Dr Mariam Zachariah – a research associate at Imperial College London’s Grantham Institute for Climate Change – called “an extreme outburst” of more than 140mm of rain in a single day. That’s nearly a quarter of what London gets in a year, and it hit loose, easily eroded soil that had already soaked through two months of monsoon rains.
The first landslide, which started at an altitude of 1,550 metres, occurred at Mundakkai village at midnight on July 30, followed by three more landslides within three hours, hitting Chooralmala and Attamala villages.
Mud, water and rock flows buried entire neighborhoods, swept away victims and collapsed a vital bridge, delaying rescue efforts to the worst-hit areas.
While the state government says the death toll at the time of writing is 231, media reports suggest the actual death toll from the landslides is more than 400 – disproportionately affecting migrant workers working in farms, resorts and tea plantations.
“The scale of the event was so large that debris was carried several kilometres downstream,” study author Professor Arpita Mondal of the Indian Institute of Technology Bombay said at a press conference, adding that “body parts were recovered from rivers downstream” tens of kilometres away from the landslide site.
The event “particularly devastated two villages – Mundakkai and Chooralmala,” she said, and one official told the Information Minute that “I don’t think Chooralmala ward will survive anymore.”
Monsoon rains
To put Wayanad’s intense rainfall into its historical context and determine how unlikely it was, the authors analyzed time series of maximum daily rainfall during the June–September monsoon season, focusing on northern Kerala.
They found that 140 mm of rain hit northern Kerala on July 30, 2024, ranking as the third-heaviest single-day rainfall in records stretching back to 1901.
The intensity of this rainfall even surpasses the “torrential” rainfall that hit large swathes of Kerala in 2018, which killed more than 400 people and was dubbed Kerala’s “worst floods in nearly a century.”
The map below shows total rainfall for July 30, 2024 in northern Kerala, based on data from the India Meteorological Department. Dark blue indicates high daily rainfall totals and yellow indicates low rainfall totals. The study area is shown in red on the map.
Total rainfall on 30 July 2024, based on data from the India Meteorological Department. Dark blue indicates high daily rainfall totals and yellow indicates low rainfall totals. The study area is shown in red. Source: WWA (2024)
The authors found that under today’s climate conditions, this type of intense rainfall is a once-in-50-year event.
Separately, using satellite observations, the authors found that one-day heavy rainfall in northern Kerala has become about 17% more intense over the past 45 years, during which time the global climate has warmed by about 0.85C.
Attribution
Attribution is a rapidly growing field of climate science that attempts to determine the “fingerprint” of climate change on extreme weather events, such as heat waves and droughts.
In this study, the authors investigated the impact of climate change, especially heavy rainfall in northern Kerala on July 30, 2024.
To conduct the attribution study, scientists used climate models to compare today’s world with a “counterfactual” world without 1.3C of human-caused warming.
The authors found that climate change made the intense rainfall on July 30 about 10 percent more intense.
“This may not sound like much, but actually, when you look at this rainfall, it’s a lot of extra rainfall,” said Dr Claire Barnes, a research associate at Imperial College London’s Grantham Institute for Climate Change and an author of the study, speaking at a press conference.
The authors note that Kerala is a mountainous region with “complex climate-rainfall dynamics” and explain that there is a high level of uncertainty in the model results.
However, Zachariah told the press conference that the research results were “consistent with the Clausius Clapeyron relationship”, which states that air can generally hold about 7% more moisture for every 1C increase in temperature.
The authors also looked at how rainfall intensity might change as the planet continues to warm. They found that if the planet warms by 2C above pre-industrial temperatures, rainfall intensity in northern Kerala is expected to increase by 4%.
This increase in rainfall intensity “is likely to increase the number of landslides that may occur in the future,” the study said.
(These findings have not yet been published in a peer-reviewed journal. However, the methods used in the analysis have been published in previous attribution studies.)
Land use change
The Western Ghats and the high-altitude rainforest ecosystem are internationally recognized as biodiversity hotspots and influence India’s monsoon weather patterns.
Wayanad is known for its dense forests and rich biodiversity, but it has also witnessed significant deforestation and land use change.
Madhavan Rajeevan, India’s former secretary of earth sciences who was not involved in the study, said that while heavy rainfall was a “risk factor” for the devastating landslides, human intervention “has played a major role, there is no doubt about that.” He told Carbon Transient:
“In many interviews with local people, they said that [large-scale] construction work was going on in the most affected areas. And that construction [was done] by eliminating the indigenous people in the forest. But landslides do not discriminate between rich and poor. “If there had not been significant human intervention in that area in the last four or five years, I am very sure this landslide would not have happened.”
According to one study, Wayanad lost 62% of its forest cover between 1950 and 2018, while tea plantations increased by 1,800%. The district’s steep slopes are also home to coffee, pepper, tea and cardamom, and are dotted with luxury resorts.
At the same time, the trend of construction and exploitation of construction stone in recent years has also “increased”.[d] Scientists are concerned about the impact on the stability of hillsides in the area.
On July 31, a day after the disaster, India’s climate ministry issued a sixth draft notification to classify parts of the Western Ghats as ecologically sensitive areas (ESAs), 14 years after experts recommended curbing development in the area.
Environmental lawyer Shibani Ghosh told Carbon Transient that, so far, the 72,000 square kilometers of Western Ghats identified by these experts “does not even fall within the scope of any proposed conservation plan.”
While environmentalists still have “serious concerns” about the area being excluded from the Western Ghats ESA in the new draft, “had it been declared [even in its unsatisfactory form],” she added, “the environmentally harmful activities would have been managed by now and perhaps the impact of these disasters would have been much less.”
Rajeevan also points out how the monsoon has changed in Kerala. He says:
“We know that the seasonal rainfall on the west coast is very high, it rains continuously for days and hours, but the rainfall used to be very small: millimetres an hour. But recent studies show that these shallow clouds are transforming into deep convective clouds that bring very heavy rain in very short periods of time, and that may be due to the warming in the Arabian Sea.”
At the same time, forecasting is another issue the study raises, drawing attention to the fact that warnings do not reach many people and impacts are not specifically stated.
In the wake of the landslide, whether the meteorological department had issued a heavy rain warning or not became a matter of debate in parliament. But Rajeevan pointed out that accurate rain warnings alone were not enough:
“Red and yellow alerts for the entire state or a few districts do not translate into landslide warnings. District collectors cannot interpret them or take decisions. The Geological Survey of India has issued warnings, but they are not alarming and a complex real-time landslide warning system requires a lot of money.
“The best solution is to identify and rehabilitate people living in landslide-prone areas and not cause them trouble by cutting down forests.”