Heavy rain in September 2025 Bali floods mostly intensified by human-driven climate change

Contact Authors

• Davide Faranda, IPSL-CNRS, France  📨davide.faranda@lsce.ipsl.fr    🗣️English,French, Italian

• Tommaso Alberti, INGV, Italy  📨tommaso.alberti@ingv.it 🗣️ English, Italian

• Pascal Yiou, IPSL-CEA Saclay 📨pascal.yiou@lsce.ipsl.fr  🗣️French, English

• Marisol Osman, CIMA/ CONICET-UBA 📨osman@cima.fcen.uba.ar 🗣️Spanish, English 

• Gianmarco Mengaldo, NUS, Singapore 📨mpegim@nus.edu.sg  🗣️Italian, English

Citation

• Faranda, D., Alberti, T., Yiou, P., Osman, M., & Mengaldo, G. (2025). Heavy rain in September 2025 Bali floods mostly intensified by human-driven climate change. ClimaMeter, Institut Pierre Simon Laplace, CNRS. https://doi.org/10.5281/zenodo.17142832

Press Summary

• Meteorological conditions similar to that causing floods in Bali are up to 2 mm/day (up to 7%) wetter in the present than they have been in the past.

• This event was associated with very exceptional meteorological conditions.

• Natural variability alone cannot explain the increase in precipitation associated with the Bali floods.

Event Description

On 9–10 September 2025, Bali was struck by intense rainfall that triggered widespread flooding across Denpasar, Ubud, and Singaraja. Precipitation locally exceeded 60–80 mm/day, with the strongest accumulations in southern and central Bali. Flash floods overwhelmed rivers and urban drainage, damaging infrastructure and disrupting transport. At least 17 people have died and many more remain missing after Bali was hit by its worst flooding in more than a decade, with torrential rains dumping over 385 mm in 24 hours and triggering landslides that swept away homes, roads, and bridges. Hundreds were evacuated at Denpasar and surrounding areas were inundated, adding to the growing toll of climate-intensified disasters in Indonesia, where deforestation and rapid development have heightened vulnerability. The floods come as South Asia faces a brutal monsoon season: India is enduring its wettest start in over a century, with Punjab experiencing its worst floods since 1988 and hundreds killed nationwide, while in neighboring Pakistan, more than 900 lives have been lost and 2 million people evacuated since late June.

The meteorological conditions were characterized by negative surface pressure anomalies of up to –0.5 hPa, signaling a weak low-pressure system. Temperatures were slightly warmer than average, with anomalies down to +0.5 °C, precipitation exceeded 60 mm/day over Bali and surrounding regions, while wind speeds reached 20–30 km/h with strong onshore flow, enhancing moisture advection from the Java Sea and surrounding waters.

Climate and Data Background for the Analysis

The IPCC AR6 report projects that in Southeast Asia, both mean monsoon precipitation and pluvial flooding are expected to increase, with greater intensity and frequency of heavy rainfall events as global warming progresses. These changes are attributed to the increased atmospheric moisture content under warming and amplified convective activity. Southeast Asia is identified in the AR6 WGI Summary for Policymakers as a region where heavy precipitation events will intensify and become more frequent.

Observational studies over the Indonesian Maritime Continent — including Bali — have documented upward trends in extreme rainfall indices, especially in response to intraseasonal variability like the Madden-Julian Oscillation (MJO). During active MJO phases, western and central Indonesia can experience up to 70–80% increases in extreme precipitation probability. Embedded in a climate system that is becoming wetter, climate variability contributes to the intensification of extreme events. The Indian Ocean Dipole (IOD) and ENSO also modulate rainfall variability across Indonesia, including Bali, by influencing moisture advection and convection dynamics. Under anthropogenic warming, the Indo-Pacific warm pool has been expanding and warming, which strengthens moisture fluxes and amplifies extreme precipitation potential over maritime Southeast Asia.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event we have low confidence in the robustness of our approach given the available climate data, as the event is very exceptional in the data record.

ClimaMeter Analysis 

We analyze here (see Methodology for more details) how events similar to the meteorological conditions leading to the September 2025 Bali floods have changed in the present (1987–2023) compared to what they would have looked like if they had occurred in the past (1950–1986) in the region [113°E–116°E, 9°S–7.5°S]. Surface pressure changes show no relevant differences between the two periods, with near-zero anomalies over most of the affected region. Temperature changes show increases of up to +1 °C in southern Bali, particularly around Denpasar. Precipitation changes reveal a general increase in rainfall totals, with present-day conditions up to 2 mm/day wetter in the region. Wind changes display enhanced wind speeds of up to +2 km/h across the southern part of the island, favoring stronger moisture transport from the surrounding seas.

Similar past events do not show a clear seasonal shift, apart from a slight increase in September occurrences during the present period compared to the past. Changes in urban areas reveal that Denpasar and Ubud experienced significantly wetter conditions during this event compared to similar past situations, while Singaraja showed smaller changes. These results suggest that meteorological conditions similar to those of the September 2025 Bali floods are becoming more favorable for extreme precipitation, in line with what would be expected under continued global warming.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation and the Pacific Multidecadal Oscillation, may have influenced the event. This suggests that the changes we see in the event compared to the past may be due to human-driven climate change, with a contribution from natural variability.

Conclusion

Based on the above, we conclude that meteorological conditions leading to the September 2025 floods in Bali are up to 2 mm/day wetter (up to 7%) compared to similar past events. Natural variability alone cannot explain the changes in precipitation associated with this very exceptional meteorological condition.