The rainfall at the end of November in Sri Lanka, Indonesia, and Malaysia was heavier due to climate change, contributing to the scale of the flooding disaster. This is the conclusion of World Weather Attribution, an international team of scientists, in a rapid attribution study led by the KNMI.
Rainfall from tropical cyclones
Large parts of Southeast Asia were hit by heavy rainfall and flooding at the end of November (figure 1). Three tropical cyclones brought enormous amounts of rain in a short time to, among others, Sri Lanka and parts of Vietnam, Malaysia, and Indonesia, resulting in floods and landslides. More than 1600 deaths occurred, hundreds of thousands of people were displaced from their homes, and millions faced the consequences of this extreme weather. Was this extreme weather as severe without climate change? In a rapid attribution study, we examined this for the amount of rain associated with the cyclones Senyar and Ditwah.
What do the rainfall observations say?
Most rain over Sumatra and Malaysia associated with cyclone Senyar fell between November 23 and 27 (figure 2, left). We analyzed four different rainfall observation datasets and see an increase in extreme rainfall totals in this area related to the global average warming. On average, the 5-day rainfall totals in this area are currently 9 to 50 percent higher than they would have been without climate change.
Most rain over Sri Lanka associated with cyclone Ditwah fell between November 24 and 28 (figure 2, right). Like over Sumatra and Malaysia, we see that the 5-day rainfall totals in the four different observation datasets are currently about 28 to 160 percent higher than they would have been without climate change.
The Indian Ocean has warmed
The intensified rainfall is directly related to higher sea temperatures. Measurements show that the temperature of the Indian Ocean is now about one degree higher than a century ago.
What do the climate models say?
In every attribution study, climate models are used alongside observations. The effect of climate change can be studied with models by comparing calculations with and without an increase in greenhouse gases. However, a condition is that models can reproduce the observed climate well. This is not the case when we compare rainfall in both regions from models with observations. For example, the distribution of rainfall over the year is not well represented. Also, models generally have difficulty representing rainfall over islands well; they often perform better over large land areas. Therefore, we cannot rely on the models in this study.
Role of climate change
Observations show that extreme rainfall is increasing. We also know that atmospheric warming is accompanied by an increase in water vapor, about seven percent per degree of warming on average. More water vapor in the air feeds heavier rainfall, exactly as seen in the observations. Therefore, based on observations and this knowledge, we conclude that climate change has led to an increase in rainfall from both cyclones. Exactly how much cannot be determined because the models still do not reproduce the observed rainfall in both regions well.
What does this mean?
The flooded regions experience floods more often. But this time, they were heavier than usual, causing greater infrastructure damage and more severe consequences in these densely populated areas than in other years. This was mainly due to the large amount of rain in a short time, but also because rapid urbanization in flood-prone areas exposes many people to flood risks. Additionally, the infrastructure is vulnerable, and adequate early warning systems are lacking.
The Earth will continue to warm, increasing the chance of heavy rainfall further. Limiting warming by quickly stopping fossil fuels helps reduce future casualties and damage, alongside infrastructure adaptations and early warnings.
