2024/11/08-09 Colombia Floods


November 2024 floods in Colombia mostly exacerbated by human-driven climate change 

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Citation


Press Summary  (First published 2024/11/19)

Event Description

On November 8 and 9, 2024 Colombia’s Pacific coast faced severe flooding due to extreme rainfall driven by a set of meteorological factors. The heavy rain was linked to tropical depressions over the Caribbean Sea in combination with the Madden-Julian Oscillation, the dominant mode of intraseasonal variability in the tropics generating large-scale precipitation and circulation anomalies. These conditions were associated with an increase in the flow of the western trade wind over the tropical Pacific, tied to the developing La Niña. The interaction of the western flow, transporting humid air, with the Andes mountain range forced the air to lift and triggered intense precipitation. In addition, the Intertropical Convergence Zone’s (ITCZ) seasonal migration significantly influences the Colombian Pacific region every year during October and November, a time marked by increased rainfall as the region transitions into its rainy season. This year, the ITCZ also likely contributed to the extreme weather conditions observed on November 8 and 9, 2024. Rainfall intensity peaked during November 8 and 9, with up to 200 mm/day. This caused the Atrato, San Juan, and Baudó rivers to overflow. Chocó, a department in the Colombian Pacific with high poverty rates and fragile infrastructure, was the hardest hit. With 85% of its territory and 188,000 people directly affected, Chocó is now facing a severe humanitarian crisis. In response to this widespread devastation, the government declared a nationwide state of emergency, set to last for one year, to expedite aid and recovery efforts.  

The Surface Pressure Anomalies show a slightly positive anomaly, up to 1 hPa over the Colombian Pacific coast with Temperature Anomalies displaying modest changes on the domain analyzed. Precipitation data show high daily amounts of precipitation over the Colombian Pacific coast reaching up to 200 mm/day over the Chocó department. Wind speed Data indicates moderate winds over the Pacific Ocean and no winds in the area affected by the floods. We remind you that our analysis is based on MSWX data. This product does integrate some station observations, especially for rain data. The values reported here can be different from those observed at single weather stations.

Climate and Data Background for the Analysis

The IPCC AR6 highlights a substantial increase in flood risks for Colombia due to climate change. With a global temperature rise of 1.5°C, the population exposed to river flooding in Colombia, along with Brazil and Argentina, could increase by 100–200%. This risk intensifies further with a 2°C rise due to the greater frequency and intensity of extreme rainfall events (IPCC AR6 WGII, Chapter 12). Observations already show a significant increase in the intensity of heavy precipitation, a trend projected to worsen during the rainy seasons. Coastal areas, particularly along Colombia's Caribbean coastline, face additional challenges from sea level rise, projected to reach 0.4–0.8 meters by 2100 depending on emissions scenarios, exacerbating the risk of coastal flooding in low-lying regions (IPCC AR6 WGI, Regional FactSheet for Central and South America). Moreover, what were previously considered 1-in-100-year flood events are expected to occur with greater frequency, potentially as often as once every 50 years under high-emissions scenarios. The economic impact is already significant, with flood-related damages averaging $500 million annually, a figure that could double by mid-century if global emissions remain unchecked (IPCC AR6 WGII, Chapter 12). These findings underscore the critical need for Colombia to enhance flood resilience through adaptive measures such as improved floodplain management, early warning systems, and climate-resilient infrastructure. Chocó is one of the wettest regions in the world, with some areas receiving more than 13 meters (13,000 mm) of rainfall annually. The region's extreme rainfall is a result of its particular location between the Eastern Pacific Ocean and the Andes Mountain Range, which leads to an exceptional land-atmosphere interaction. Moist and warm air transported from the tropical Pacific Ocean to the continental land by the Chocó Low-Level Jet encounters the Andes Mountain Range, undergoes orographic uplift, and causes frequent and heavy rains (Poveda and Mesa, 2000). The annual precipitation regime of the Colombian Pacific is driven by the meridional migration of the Intertropical Convergence Zone, resulting in more intense and frequent rainfall during the periods from September to November and March to May.

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 largely unique in the data record.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the meteorological conditions leading to the Colombia Floods have changed in the present (2001–2023) compared to what they would have looked like if they had occurred in the past (1979–2001) in the region [-80 -74°E 3°N 7.5°N]. 

The Surface Pressure Changes show no significant differences in the present climate than what they would have been in the past on the Northern Pacific coast of Colombia. Temperature Changes show that similar events produce temperatures in the present climate that are between 1 ºC and 1.5 ºC warmer than what they would have been in the past, over the Chocó and Antioquia department. The Precipitation Changes show up to 37 mm/day (up to 40%) wetter conditions over the Northern Pacific coast of Colombia and 10 mm/day drier conditions inland. Windspeed Changes indicate windier conditions up to 5 km/h (up to 20%) offshore Colombia. We also note that Similar Past Events occur with similar seasonality in the past and present periods, although with a decrease in October and an increase in November in the present climate. The seasonality of the events follows the meridional migration of the Intertropical Convergence Zone, the main driver of the wet and dry seasons in the tropics. Changes in Urban Areas reveal that there are non-significant changes in the precipitation for the city analyzed.

Finally, we find that sources of natural climate variability, notably the Pacific Decadal Oscillation, and the Atlantic 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 similar to those producing Colombia Floods are up to 37 mm/day (an increase of up to 40%) wetter over the Northern Pacific coast of Colombia and 10 mm/day drier inland in the present than they have been in the past. Additionally, conditions are up to 5 km/h (up to 20%) windier offshore Colombia and up to 1.5 ºC warmer inland in the present compared to the past.  We interpret the Colombia floods as an event driven by very exceptional meteorological conditions. Natural variability alone cannot explain the changes in precipitation and wind associated with this very exceptional meteorological condition. We remark that, 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. 

Additional Information : Complete Output of the Analysis

The figure shows the average of surface pressure anomaly (msl) (a), average 2-meter temperatures anomalies (t2m) (e), cumulated total precipitation (tp) (i),  and average wind-speed (wspd) in the period of the event. Average of the surface pressure analogs found in the counterfactual [1979-2000] (b) and factual periods [2001-2022] (c), along with corresponding 2-meter temperatures (f, g),  cumulated precipitation (j, k), and wind speed (n, o).  Changes between present and past analogues are presented for surface pressure ∆slp (d),  2 meter temperatures ∆t2m (h), total precipitation ∆tp (i), and windspeed ∆wspd (p): color-filled areas indicate significant anomalies with respect to the bootstrap procedure. Violin plots for past (blue) and present (orange) periods for Quality Q analogs (q), Predictability Index D (r), Persistence Index Θ (s), and distribution of analogs in each month (t). Violin plots for past (blue) and present (orange) periods for ENSO (u), AMO (v) and PDO (w).  Number of the Analogues occurring in each subperiod (blue) and linear trend (black).  Values for the peak day of the extreme event are marked by a blue dot. Horizontal bars in panels (q,r,s,u,v,w) correspond to the mean (black) and median (red) of the distributions.