A photo of flooding in the Philippines in 2012. Photo: Mathias Eick/Wikimedia commons

Philippines island faces risk of heavy rainfall which is now 50% more intense, along with deadly landslides : WWA report

From January 28, Mindanao Island—which is among the most populous regions in the Philippines—was hit by heavy rainfall. While the rainfall was not a particularly extreme event, devastating landslides highlight the urgent need for improved early warning and land management 

According to a rapid analysis by an international team of leading climate scientists from the World Weather Attribution (WWA) group, heavy rainfall, like the event that led to deadly flooding and landslides on Mindanao Island in early February, now drops around 50% more rain in the region.

In January, Mindanao Island, the second-largest island in the Philippines, was hit by unseasonably heavy rainfall. On January 16, the island experienced two days of rainfall. From January 28, the island was hit by heavy rainfall caused by a trough of a low pressure area associated with the Northeast Monsoon causing widespread flooding in various regions along with other related impacts, including landslides, damage to infrastructure and agricultural land and power outages, before finally dissipating on February 3. 

The downpours led to floods and landslides that destroyed homes, cut off roads, displaced more than 350,000 people and killed at least 110 people, including 98 people who died in a landslide in Masara.

The event occurred after a series of smaller events that led to more localised flooding from mid-January onwards. Researchers from the Philippines, the Netherlands, Germany, Sweden and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the weather conditions at the time of the most impactful floods.

While increasing rainfall is expected because a warmer atmosphere can hold more moisture, limitations in climate models meant the scientists could not quantify the extent to which human-caused climate change is responsible for this increase.

The study was conducted by 16 researchers as part of the World Weather Attribution group, including scientists from universities and meteorological agencies in the Philippines, the United Kingdom, the Netherlands, and the United States. The study also highlighted that a lack of early warning systems and the presence of settlements in areas prone to landslides turned the heavy rainfall into a devastating event.

Effect of climate change on the heavy rainfall

To quantify the effect of climate change on the heavy rainfall, scientists analysed observed weather data and climate model simulations to compare how the event has changed between today’s climate, with approximately 1.2°C of global warming, and the cooler pre-industrial climate, following peer-reviewed methods. The study looked at the five-day maximum rainfall from December to February in the Caraga and Davao regions of eastern Mindanao where the impacts were most severe. 

An analysis of historical weather data showed that heavy rainfall events now produce around 50% more rainfall than in the past. The researchers found that in today’s climate, with 1.2°C of global warming, heavy five-day rainfall events in the December to February period on Mindanao Island are no longer rare events and are expected to occur about once every 10 years. 

(a) Total precipitation over the region encompassing Mindanao (4-11N, 120-130E) during 28 Jan-1 Feb, 2024. The study region comprising the Caraga and Davao regions is outlined in red. (b) same as (a) showing the anomaly w.r.t 1990-2020 period.

The researchers also analysed the possible influence of El Niño-Southern Oscillation (ENSO) and found that the current phase of El Niño typically makes the rainfall less intense. This means that if the event had occurred in a neutral or a La Niña phase, the rainfall would have been expected to be even heavier.

There is a wide range of scientific evidence showing that rainfall is becoming heavier in the Philippines as the climate warms. However, the climate models analysed did not capture the observed trend of increasing rainfall in the eastern region of Mindanao Island, probably due to local weather patterns that are not captured by climate models. Because of this limitation, the scientists could not quantify the influence of climate change on the event. Still, more frequent and intense downpours is exactly what is expected from climate change, since a warmer atmosphere can hold more moisture.

The study found that the high vulnerability of the population played a crucial role in the impacts. While the rainfall was not record-breaking, the death tolls from flooding and landslides were high for an event of this magnitude.

Finally, the analysis also identified several shortcomings in early warning systems in eastern Mindanao Island. Many automated rainfall and stream level sensors have not been recording data since 2019. Consequently, there are large gaps in the monitoring and warning of imminent flood and landslide risk. The researchers said forecasts that highlight the risk of specific impacts and warnings supplemented by local knowledge can help address these gaps.

Richard Ybañez, chief science research specialist, University of the Philippines, Resilience Institute, Quezon, Philippines said, “We can’t just blame the rain for the severe impacts. A range of human factors is what turned these downpours into deadly disasters. It’s well known that Mindanao Island is highly vulnerable to landslides due to high rainfall, frequent earthquakes and steep, hilly terrain along its eastern half. Evacuations from high-risk locations were carried out when the island was hit by the rainfall in late January. However, many people were still in harm’s way. It is critical that both early warning systems and assessment of landslide-prone areas are improved to avoid similar disasters in the future.”