The ‘monsoon rain’ that triggered deadly landslides in Kerala’s Wayanad district last month has been made 10% heavier by human-induced climate change, a new rapid attribution study has found.

The landslides followed an “exceptional period of monsoon rains” on July 30. They have killed at least 230 people, while more than a hundred people are still missing and rescue operations are underway.

An analysis by the World Weather Attribution (WWA) service found that the rainfall that hit Wayanad on July 30 was the third-heaviest on record in the region, surpassing even the extreme rainfall that led to flooding in Kerala in 2018.

The team of 24 researchers from India, Malaysia, the US, Sweden, the Netherlands and the UK found that rainstorms of this intensity have already become 17% heavier over the past 45 years.

In a world where average global temperatures are 2 degrees Celsius above pre-industrial levels, they estimate that extreme daily rainfall in Kerala could become another 4% more intense, potentially triggering even larger landslides.

The study also looks at other “mixed” factors that may have contributed to the high casualty rate and Wayanad’s “increased susceptibility” to landslides. These include a 62% loss of forest cover in the district and warnings that “many people remain unreached”.

Slippery slope

Wayanad is a mountainous district in northern Kerala in the Western Ghats of India, a mountain range older than the Himalayas that runs parallel to the country’s west coast.

With its high elevation and steep slopes – combined with a tendency to receive “long-term” rainfall and widespread changes in its natural vegetation – Wayanad is highly prone to landslides. It is the most landslide-prone district in Kerala, accounting for 59% of landslides in the country in 2015-22.

Since June 22, Wayanad has experienced “almost continuous” monsoon rainfall, the WWA study said – with some areas recording more than 1.8 metres of rain in just one month.

On July 30, Wayanad witnessed what study author Dr. Mariam Zachariah, a research fellow at Imperial College London’s Grantham Institute for Climate Change, called an “extreme burst” of more than 140mm of rain in a single day. This is equivalent to almost a quarter of the rain London receives in the entire year. The rain fell on loose, erodible soil already saturated by two months of monsoon rain.

The first landslide started at an altitude of 1,550 metres and hit Mundakkai village at midnight on July 30. Three more landslides followed within three hours, hitting Chooramala and Attamala villages.

Torrents of mud, water and rocks buried several neighborhoods, swept away victims and collapsed a main bridge, delaying rescue operations in the worst-hit areas.

While state authorities say the death toll stands at 231 at the time of writing, media reports suggest the actual number of lives lost to the landslides is more than 400. This disproportionately affects migrant workers who work on farms, resorts and tea plantations.

At a press conference, study author Prof. Arpita Mondal of the Indian Institute of Technology Bombay said that the “magnitude of the event was such that debris traveled a path of several kilometres”, and that “body parts have been recovered from rivers downstream as far as tens of kilometres from the landslide site”.

The incident, she said, was “particularly devastating for two villages – Mundakkai and Chooramala”, with an official telling News Minute that “I don’t think Chooramala district will exist anymore”.

Monsoon rain

To place the intense rainfall in Wayanad in a historical context and determine how unlikely it was, the authors analyzed a time series of maximum rainfall in a single day during the June–September monsoon season, focusing on northern Kerala.

They found that 140 mm of rain fell in northern Kerala on July 30, 2024, making it the third heaviest rainfall in a single day since 1901.

The intensity of these rainfalls even surpassed the “torrential” rainfall that hit large parts of Kerala in 2018, which killed more than 40 people and earned the floods the title of “Kerala’s worst floods in nearly a century”.

The map below shows the total rainfall on 30 July 2024 in northern Kerala, based on data from the Indian Meteorological Department. Dark blue indicates high total daily rainfall and yellow indicates low total rainfall. The study area is shown in red on the map.

Total rainfall on 30 July 2024, based on data from the Indian Meteorological Department. Dark blue indicates high total daily rainfall and yellow indicates low total rainfall. The study area is shown in red. Source: WWA (2024)

The authors conclude that in the current climate, such an intense, one-day rainfall event occurs once every 50 years.

In addition, using satellite observations, the authors found that heavy, one-day rainfall in northern Kerala has become about 17% more intense over the past 45 years. During that time, the global climate has warmed by about 0.85 °C.

Attribution

Attribution is a rapidly growing field within climate science that focuses on identifying 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 on the heavy rainfall event in northern Kerala on July 30, 2024.

To conduct attribution studies, scientists use climate models to compare the world as it is today with a “counterfactual” world, without the 1.3 degrees Celsius human-caused warming.

The authors conclude that climate change made the heavy rainfall on July 30 about 10% more intense.

“This may not sound like a lot, but when you look at this amount of rainfall, it really is a lot of extra rain,” Dr Claire Barnes, a research associate at the Grantham Institute for Climate Change at Imperial College London and an author of the study, told the news conference.

The authors note that Kerala is a mountainous region with “complex rainfall-climate dynamics” and explain that there is a high degree of uncertainty in the model results.

However, Zachariah told the press conference that the study’s findings are “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 investigate how rainfall intensity might change as the planet continues to warm. They found that if the planet were to warm by 2C above pre-industrial temperatures, rainfall intensity in northern Kerala would be expected to intensify by another 4%.

The study found that this increase in rainfall intensity “is likely to increase the potential number of landslides that may occur in the future”.

(These findings have yet to be published in a peer-reviewed journal. However, the methods used in the analysis have been published in previous attribution studies.)

Change in land use

The Western Ghats and their high mountain tropical forest ecosystems are internationally recognized as a biodiversity hotspot and influence India’s monsoon weather patterns.

Wayanad is known for its dense forests and rich biodiversity, but there is also significant deforestation and land use change.

While heavy rainfall was “a trigger” for the devastating landslides, human intervention “played a major role, there’s no doubt about that,” said Madhavan Rajeevan, India’s former secretary for earth sciences who was not involved in the study. He told Carbon Brief:

“In many interviews with locals they say that [large-scale] Construction was taking place in the areas that were hardest hit. And that construction [was done] by removing the local [Indigenous people] staying in the forest. But the landslide does not distinguish between rich and poor. If there had been no substantial human intervention in that area in the last four or five years, I am sure this landslide would not have happened.”

Between 1950 and 2018, Wayanad lost 62% of its forest cover, while the land under tea plantations grew by 1,800%, a study said. The district’s high slopes are also home to coffee, pepper, tea and cardamom plantations, and are dotted with luxury resorts.

At the same time, the increase in construction and building block mining in recent years has increased the demand for building blocks.[d] “concern” among scientists about the consequences for the stability of the slopes in the area.

On July 31, the day after the disaster, India’s climate ministry issued the sixth version of a notification to classify parts of the Western Ghats as Ecologically Sensitive Areas (ESAs), 14 years after experts recommended restricting development in the region.

Environmental lawyer Shibani Ghosh tells Carbon Brief that so far, 72,000 square kilometres of the Western Ghats identified by these experts “are not even within the purview of any proposed protection plan”.

While environmentalists still have “serious concerns” about the area that will be excluded from the Western Ghats ESA in the new draft, “if it were to be declared an area [even in its unsatisfactory form] “Today, environmentally damaging activities would already be regulated and the impact of these natural disasters would probably have been much less,” she adds.

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 many days and many hours, but the amount used to be very small: in millimeters per hour. But recent studies suggest that these shallow clouds are turning into deep convective clouds that drop very heavy rain in a very short period, and that could be attributed to the warming of the Arabian Sea.”

At the same time, the study also addresses the problem of forecasting. It points out that warnings did not reach many people and that the consequences were not specifically described.

After the landslides, the question of whether meteorological authorities warned of heavy rainfall became a subject of parliamentary debate. But Rajeevan points out that accurate rain warnings alone are not enough:

“Red and yellow alerts for the entire state or a few districts do not translate into a landslide warning. A district collector cannot translate them or take a decision. The Geological Survey of India has issued an alert but it was not alarming and a sophisticated, real-time landslide warning system needs a lot of money.

“The best solution is to identify and rehabilitate people living in landslide-prone areas, and not to hinder them by cutting down their forests.”