Fifty years ago, long before it came to be known as a hotbed of Islamic terrorism, the world’s attention turned to the Sahel to bear witness to the devastation wrought by drought. With talk of climate change as a threat multiplier, to conflict, food insecurity, governance, migration, etc., one might ask: will the Sahel ever escape crisis?
On the surface it may look like history, crisis is bound to repeat itself in this most vulnerable of places. In fact, the nuanced understanding of the 20th century evolution of the Sahelian climate that has intervened in these 50 years lays the foundation for anticipating and managing future change, a situation that is in stark contrast to the unexpected onset of persistent drought in the late 1960s. Two elements are key. First of all, climate change has already happened. The persistence of drought in the 1970s and 1980s can be partially attributed to the influence of emissions from fossil fuel burning. Therefore, adaptation lessons can be learned from the past. Secondly, the situation that led to drought is unique to the past. Therefore, it is at best simplistic and at worst wrong to equate future change with drought. As observed in recent years, current change is consistent with a more variable climate. On time scales of days, it is the intensity in rainfall that has increased since the driest mid-1980s, not the number of rainy days. On time scales of years, the Sahel is characterized by alternation between abundant and deficient rainfall, not by the persistence of either condition.
Underlying apparent complexities are well understood processes. Above all is the dominance of oceanic over continental processes. Oceanic influence is synthesized in the contrasted roles of local and remote oceans. The remote oceans, the Indian and Pacific, because they comprise the regions of warmest sea surface temperature, set the global conditions that determine where it rains. In the Sahel, these conditions are met when the local ocean, the North Atlantic, supplies sufficient moisture to the monsoon.
The influence of human-caused emissions on sea surface temperatures is integral to this picture. In the western hemisphere (North America and Western Europe) reconstruction after World War II was powered by fossil fuel energy, leading to increases in greenhouse gas emissions and in pollution. Because it has an atmospheric lifetime of decades, if not centuries, carbon dioxide, the most abundant anthropogenic greenhouse gas, is well mixed in the atmosphere. The increase in its concentration associated with human activity has resulted in warming, including that of the global tropical oceans, which emerged in the early 1970s, and has continued unabated to this day. In contrast, pollution, especially the sulfate aerosols that are a byproduct of burning (the sulfur in) coal, because it is largely rained out in a matter of weeks, is not well mixed. It is this pollution that is critical in distinguishing future from past. In Western Europe and North America sulfate aerosol concentrations increased until the enactment of environmental legislation in the 1980s. Because they reflect incident sunlight, sulfate aerosols can mask warming regionally. Consequently, sea surface temperatures in the North Atlantic basin did not keep up with global tropical warming, and the moisture supply to the monsoon dwindled. Starved of moisture, the Sahel, the northern edge of the monsoon, suffered persistent drought. However, the footprint of pollution has changed in recent decades. Cleaner air around the North Atlantic has resulted in the emergence of warming.
The climate of the Sahel is now shaped by the competition between the warming of both local and remote oceans. This competition is at the heart of the increased variability since the mid-1990s. At the same time, an overall wetter monsoon climate, with potential for extreme rainfall events that trigger widespread flooding, is not unlike what is expected from warming.
In sum, the Sahel already knows climate change. We know what action(s) can be taken to anticipate impact in the short term and to increase resilience in the long term. In the short term, recognition that the envelope of possible outcomes is affected by human-caused emissions does not mean that the future of climate is predetermined. The year-to-year variability in Sahel rainfall that expresses the chaotic nature of climate still needs to be reckoned with. Confidence in climate prediction as a tool to manage climate risk is rooted in research to understand historical Sahelian drought. Current seasonal prediction systems, based on multi-model ensembles, have skill on the regional scale at lead-times up to 6 months ahead of monsoon onset. This skill is the basis for climate services being increasingly exploited by disaster responders such as national Red Cross/Red Crescent societies and the UN World Food Programme. The concept of forecast-based financing – investing in financial resources for humanitarian assistance based on a probabilistic forecast, in anticipation of a possible crisis, not after the crisis has hit – has gained ground. In the long term, precisely because it has suffered through persistent drought, the Sahel has already learned valuable lessons in adaptation (despite future climate change not equaling drought). Technologies based on local knowledge and aimed at reducing land degradation and soil erosion, and at making more efficient use of rainfall – for example the agro-forestry and soil & water conservation technologies that are involved in re-greening – are natural candidates to adapt to a more variable climate. Their impact on food security – indeed, the nexus of climate, food security and environmental change – is being actively investigated. If we harness the knowledge that has accumulated over the past 50 years, we can free ourselves of the crisis narrative, and look to climate change as an opportunity to act based on evidence, and build a sustainable future in the Sahel.