The positive phase of the Indian Ocean Dipole, high vulnerability of the population, and consequences of a three-year drought also contributed to the large impacts
The heavy rainfall that led to devastating flooding in Kenya, Ethiopia, and Somalia during October and November was made up to two times more intense by human-caused climate change, according to a rapid attribution analysis by an international team of leading climate scientists from the World Weather Attribution group.
The study found that the positive phase of Indian Ocean Dipole (IOD), a naturally occurring climate phenomenon, also doubled the intensity of the rainfall that hit East Africa, meaning a combination of climate change and the positive IOD made the rainfall an unusually extreme event.
From late October, extreme downpours have caused massive floods in Kenya, Ethiopia and Somalia. Millions have been displaced and at least 300 people have died. The heavy rain has also destroyed homes and infrastructure, disrupted transport and flooded a camp in Eastern Kenya where about 300,000 Somali refugees live. The rainfall is ongoing and follows a three-year drought in the region, which a previous World Weather Attribution study found was worsened by climate change.
The period between October and December is known in East Africa as the “short rainy season”. While rainfall is not unusual this time of the year, its frequency and intensity is highly variable and influenced by two natural climate phenomena: the El Niño–Southern Oscillation (ENSO) and the IOD. Both ENSO and the IOD alternate between positive, neutral and negative phases. In positive years, such as 2023, rains are more abundant in East Africa.
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 3-day and 30-day accumulated rainfall in an area encompassing parts of Kenya, Somalia and Ethiopia, over a period including October and November. The researchers also analysed the possible influences of ENSO and the IOD.
The researchers found that the heavy rainfall over East Africa between October and December was one of the most intense ever recorded in the region for the time-period of the data analysed. The analysis of historical data indicates that the IOD played an important role in these rains, increasing their intensity about two-fold. In today’s climate, such rainfall events are still rare, and are expected to happen only once every 40 years. On the other hand, the scientists could not detect a significant role for ENSO in this year’s rains.
Combining weather observations and climate models, the researchers found that climate change also contributed to the event, making the heavy rainfall up to two times more intense. However, they note that only a few models were able to reproduce the relationship between the IOD and precipitation in this region
and that while the climate change signal is very clear, model uncertainty means its exact contribution could not be confidently determined.
Because of these limitations, the study could not quantify the expected changes in likelihood due to global warming. However, the scientists note that as long as the planet continues to warm, heavy rainfall events such as this one will be more frequent in East Africa, highlighting the urgent need to phase out fossil fuels and reduce emissions to net zero.
The study also found that the high vulnerability of the population played a major role in the large impacts seen. Many communities were already reeling from a three-year climate change-driven drought, having endured livestock deaths, crop failure and food insecurity. The prolonged hardship caused by the drought meant many people struggled to cope with the devastating rainfall.
The researchers say that rapidly increasing risks from extreme weather has the potential to overwhelm the response of governments and humanitarian organisations in East Africa, highlighting the need for significant planning and review of existing response strategies.
The study was conducted by 10 researchers as part of the World Weather Attribution group, including scientists from universities and meteorological agencies in South Africa, the Netherlands, the United Kingdom, and the United States.
Quotes
Joyce Kimutai, researcher at the Grantham Institute – Climate Change and the Environment, Imperial College London, said:
“At just 1.2°C of warming, we are already seeing devastating climate impacts in Africa. With every fraction of a degree of warming, life in Africa will become more dangerous. We saw it in the past with the drought and we’re seeing it again now with the floods.
“We know that climate events such as the Indian Ocean Dipole or El Niño can greatly affect the weather in East Africa. But these are natural events which happen every few years. Climate change, on the other hand, is acting on the weather all the time, making these natural cycles more extreme and unpredictable.”
Simphiwe Stewart, Red Cross Red Crescent Climate Centre, the Hague, Netherlands, said:
“Attribution studies continue to shine a light on the complex crises faced by vulnerable groups who are simultaneously exposed to the impacts of climate and extreme weather events as well as the dynamics of displacement, migration, and various forms of conflict.
“Following extreme and yearslong drought, millions of people, critical infrastructure, and public services were already vulnerable to the impacts of drought across East Africa.
“Despite anticipatory action and early warning systems, the increasing frequency and intensity of weather disasters will strain government and humanitarian response capacity. Addressing the underlying dynamics of vulnerability including land use, urbanisation planning, peace and resilience building will be crucial in the Horn of Africa and elsewhere on the continent.”
Friederike Otto, Senior Lecturer in Climate Science at the Grantham Institute – Climate Change and the Environment, Imperial College London, said:
“Africa has contributed just 4% of global carbon emissions, but is disproportionately suffering losses and damages.
“At COP28, many developed nations are celebrating themselves for the insufficient finance they have pledged to the loss and damage fund.
“However, if the world does not agree on a fossil fuel phase out, floods and droughts like these ones will become even more severe, more people will die, and the finance to pay for the losses and damages won’t ever be enough.
“A failure to phase out fossil fuels at COP28 will put thousands more people onto the frontline of climate change in Africa.
“After 27 COP summits, we simply don’t have time for vague promises and agreements. We need fossil fuel phase out.”
After years of drought the short rainy season (October to December, OND) in the Horn of
Africa brought exceptionally heavy rains, particularly in November, leading to severe flooding
in the South of Ethiopia, Eastern Kenya and many regions in southern and central Somalia.
Weather stations reported between 200 and 500 mm more rain than usual this year, more
than a month before the end of the rainy season.
The ongoing floods are hitting vulnerable communities that were already suffering from loss
of livelihoods, malnutrition and hunger due to livestock deaths and crop failure in the context
of the drought that only ended with the ongoing heavy rains. The floods led to more than 300
reported deaths so far and displaced over a million people in Kenya and Somalia alone.
Researchers from Kenya, Ethiopia, South Africa, the United States of America, the
Netherlands, Germany and the United Kingdom collaborated to assess to what extent
human-induced climate change altered the likelihood and intensity of the heavy rainfall.
Across the region several individual heavy rainfall events of varying length led to flash
flooding as well as several rivers bursting their banks. To capture this range of flood-inducing
rainfall processes, we analysed different lengths of cumulative rainfall. However the results
are very similar independent of whether investigating a few days or a whole month. Thus we
choose maximum 30-day mean rainfall over OND as the event definition. The study region is
outlined in black in Fig. 1, and has a fairly homogenous climate with a distinct long (March to
May) and short rainy season (OND), characterised by arid and semi-arid climate.
Main Findings
● Historical drought and recent flooding compounded exposure and vulnerability of
populations and population sub groups to severity of flood-related impacts
● From the end of October throughout the rainy season up till now rainfall over the
region was very heavy, leading to exceptional amounts of precipitation accumulated
on several timescales from 1-day to 30-day, with 2023 showing either highest or
second highest events on record.
● The OND rainy season is known to be influenced by modes of natural variability,
including the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole
(IOD) as the dominant modes of variability. Both ENSO and IOD are in a positive
phase which has been shown to increase the likelihood of heavy rainfall in the wider
Horn of Africa OND season.
● For the region analysed in this study we find no significant role of ENSO but a
significant influence of the IOD. In the current IOD phase and under the current
climate conditions, an event of this magnitude is expected to happen more often than
under IOD neutral conditions, with a return period of approximately 1 in 5. In other
words, we would expect to see such an event occurring in every 5th positive
IOD-year in the present climate. Overall, taking into account the recent behaviour of
the positive IOD, the event would have a return period of around 1 in 40 years in the
current climate, that has been warmed by 1.2C due to the burning of fossil fuels.
● Based on three observation-based data products we find that due to the effect of the
IOD, the intensity of the rainfall this OND season was about twice what would be
expected in a neutral IOD year. Similarly we find that due to the warming of 1.2C up
till now, the magnitude of rainfall has also approximately doubled. Thus, climate
change and a positive IOD contributed approximately equally to the magnitude of the
event.
● It is important to highlight however that IOD is a natural phenomenon that oscillates
between neutral, positive and negative phases while the effect of human-induced
climate change will continue to increase until the burning of fossil fuels is stopped.
● Only very few models exhibit a correlation between the IOD and OND rainfall. These
models also show an increase in the intensity attributable to human-induced climate
change but smaller than the observations. Combining both observations and models,
we estimate that human-induced climate change increased the intensity of OND
rainfall by up to a factor of two.
● Due to the low number of models and short observed records we have however low
confidence in this quantification, but very high confidence in the overall result that
climate change increased the intensity of heavy rainfall in the OND season in the
Horn of Africa.
● Longstanding land use land cover (LULC) practices, unsustainable land management
in the face of rapid urbanisation, as well as systemic challenges in implementing
early warning early action in vulnerable communities increased community-level
exposure to extreme rainfall and subsequent flooding.
● Despite existing anticipatory action mechanisms and EWEA (=) protocols, the
increasing severity and frequency of extreme weather events in the HoA can
potentially overwhelm the operational response capacity of government,
development, and humanitarian actors. Ongoing review and reinforcement of
response, preparedness, and social protection systems will create a better prepared
Horn of Africa.
