The link between global warming and drought exists, but it is complex to establish. Global warming of the planet can indeed manifest itself differently depending on the region.
The summer of 2022 was marked by climatic extremes that affected the whole world. The Horn of Africa (Ethiopia, Kenya, Somalia) is experiencing one of the worst droughts of the last forty years, with dramatic consequences for more than 30 million inhabitants who suffer from hunger. This year is far from being isolated: over the past 50 years, Africa has recorded a total of 1 major climatic hazards (mainly floods, heat waves and fires, droughts) which have caused 695 deaths and economic losses of 731 US$.747 billion (World Meteorological Organization figures).
While droughts only account for 16% of these climatic hazards, they are responsible for more than a quarter of economic losses and above all 95% of all deaths.
Will these droughts worsen with anthropogenic global warming? We are tempted to affirm it, but some reservations are necessary because the link between drought and global warming is indeed more complex to establish than for the other climatic hazards that threaten the region.
There are droughts and droughts
Drought is a prolonged period of rainfall deficit leading to water shortages with negative repercussions on populations, ecosystems or sectors of activity such as agriculture, tourism, transport and/or energy. We are talking :
- Meteorological drought defined by a prolonged deficit of precipitation;
- Agricultural or ecological drought reflecting plant water stress and impacting agricultural production or the health of the ecosystem;
- Hydrological drought when water reserves themselves become deficient, the flow of rivers, the level of groundwater, lakes and reservoirs decrease to very low levels following a particularly long rainfall deficit, or a series of dry periods;
- Socio-economic drought when the demand for water for the various uses (domestic, agriculture, tourism, energy) is far greater than the available water.
The historical developments and future scenarios droughts differ greatly depending on the type of drought considered. Thus, there are few regions of Africa where a significant increase in meteorological droughts has been observed since the 1950s (in the West, Center and South-East of the continent) while almost the whole continent has experienced more severe ecological and agronomic droughts.
On the other hand, an increase in hydrological droughts could only be detected in West Africa. With warming reaching +2°C and a fortiori +4°C, all categories of droughts are increasing, in particular ecological and agronomic droughts in North and South Africa, under the effect of the increase temperatures which increase the transpiration of plants and act on evaporation and heat waves concomitant with droughts which strongly degrade the vegetation.
Nevertheless, it is possible that part of the negative effects of these increased droughts on crop production is offset by the effect of increased atmospheric concentrations of CO2, leading to improved water use efficiency in plants.
Significant regional disparities
The evolution of droughts in Africa is far from homogeneous. With warming reaching +2°C to +4°C, climate models simulate a significant worsening of droughts in South Africa and North Africa, and to a lesser extent in West Africa (particularly in Senegal, in Gambia and Mauritania).
This trend is not simulated in Central Africa or in the Horn of Africa, regions in which droughts could on the contrary decrease with a rise in global temperature of +2°C or +4°C. The Horn of Africa is, however, a region at the heart of concern for having been hit by deadly droughts in the past two years.
In fact, Kenya, Tanzania and Ethiopia are experiencing a significant decrease in long rains from March to May, which began in the 1980s. However, climate models, on the contrary, simulate an increase in these spring rains in the Horn of Africa as a result of the increase in greenhouse gases. This is what climatologists have called the East African paradox.
The difficulty of establishing the impact of global warming on recent droughts
In June 2021, southern Madagascar was severely affected by a extreme drought which caused more than a million people to suffer from hunger. The World Food Program (WFP) had described this serious food crisis as first famine due to global warming caused by human activities. This message was widely relayed in the media and by the Madagascan President during COP26 in Glasgow in November 2021.
However, the link between global warming and this drought has been denied by a recent attribution study which showed that the increase in greenhouse gas emissions did not make the occurrence of such a drought more likely. The attribution of extreme events, such as droughts, is a recent discipline, based on the observation and modeling of the climate, which seeks to know the causes of these phenomena. It has experienced rapid growth in recent years due both to new methodologies, new models, greater computing capacity, but also to growing demand from the general public and decision-makers wishing to know the responsibility of global warming in extreme events.
Le World Weather Attribution is a network of international scientists that carries out a large number of attribution studies on the most recent extreme events around the world (heat waves, intense rains, cold spells, droughts, storms). Of the 17 heat waves that have been studied since 2016, the impact of global warming on the probability and intensity of the event has been systematically demonstrated.
The World Weather Attribution also concludes that global warming is responsible for eight intense rainfall events out of the nine events analyzed.
On the other hand, the link between drought (whatever the type) and climate change is more uncertain, particularly in Africa. Indeed, it could not be established in almost all of the recent droughts in Africa (Horn of Africa, Madagascar), with the exception of those that occurred in South Africa.
In this region, a persistent drought lasting several years almost led to the “day zero” in Cape Town, that is to say the day when all the water reserves of the city will have been exhausted. It has thus been demonstrated that this drought has been made 5 to 6 times more likely by greenhouse gas emissions by the increase in emissions and that a new "zero day" will have an 80% chance of occurring if emissions continue to grow.
The need to strengthen the observation network and the quality of the models
The difficulties in identifying reliable trends on droughts and in identifying the impact of greenhouse gas emissions on the frequency and amplitude of these events in Africa are largely caused by a poor quality observation network compared to the other regions of the world.
Indeed, regular observation, over the long term, and well distributed in space is the key to monitoring and understanding climate change. In the global climate monitoring system, Global Climate Observing System Surface Network (GCOS GSN), the African continent largely stands out from the others in terms of the quality of the measurement network. It does not count in 2019 than 26% of resorts which meet the standards of the World Meteorological Organization, with 35% of stations not operational.
This lack is often compensated by the use of Remote sensing. This is essential for monitoring soil moisture, vegetation and rainfall, but it does not allow us to go back far enough in time to reconstruct historical trends on extreme events.
Due to this rare data, but also to the high natural variability of rainfall in Africa, it is indeed very difficult to assess the performance of climate models and to simulate the historical evolution of these droughts, which makes studies complex, if not impossible, attribution. In addition, the different climate models of the CMIP exercises on the future evolution of rainfall in many parts of Africa is not widely agreed upon. A better estimate of the evolution of droughts under the effect of global warming will necessarily require an improvement in the observation network and climate models.
An essential adaptation
Although the link between drought and climate change in Africa is far from obvious, the risk posed by a possible worsening of the frequency, intensity and extension of droughts is extremely high on the continent. This risk is combined with other proven hazards induced by anthropogenic warming that threaten the continent such as heat waves, intense rains and deadly floods.
For example, during 2021, the Sahel experienced both a severe drought in June-July - with dramatic repercussions on food security, already weakened by rising prices and security problems - and major flooding a month later in August. The damage caused by these cascading hazards is often amplified by the limited means to deal with them. Adaptation will undoubtedly be the key to the resilience of the African continent to the climate of today and tomorrow and at the heart of the debates of the COP27 which will take place… in Africa.
This article is published as part of the Fête de la science (which takes place from October 7 to 17, 2022 in mainland France and from November 10 to 27, 2022 overseas and internationally), and of which The Conversation France is a partner . This new edition will have as its theme: “Climate change”. Find all the events in your region on the site Fetedelascience.fr.
Benjamin Sultan, Research Director at the ESPACE-DEV laboratory (Montpellier), Research Institute for Development (IRD); Christine Raymond, Geographer, Paris 1 Panthéon-Sorbonne University, and Gilles Boulet, IRD researcher at the Center for Space Studies of the Biosphere (CESBIO), Research Institute for Development (IRD)