"Lush", with vast green areas spreading across the southern strip of one of the hottest and most arid deserts in the world. This is how the Sahara, in the north of the African continent, appears in a satellite photograph taken on 12 September 2024 by Nasa's Modis satellite, the United States space agency, and compared with the same day one year earlier. This description immediately conjures up something positive, not least thanks to the now-famous Great Green Wall, a project to restore a degraded area of the Sahel that we have heard so much about in recent years. And yet, the news of a less arid and more "verdant" desert conceals a decidedly less appealing side.

Hidden within the northward expansion of lush vegetation — spilling out of the African tropical belt and spreading across the desert — are the extreme weather events that have struck several West and Central African states this year. The rainfall that has driven a "migration of the intertropical convergence zone" — as described in a study published in the scientific journal Nature Climate Change — did not fall as a gentle rain that soaked the sand and caused it to miraculously bloom. No, this rainfall hit the ground in the form of downpours, storms and floods that caused inundations and severe damage to the populations and territories of Nigeria and Cameroon, Niger and Chad, extending all the way to Sudan and Libya.

But what is the intertropical convergence zone? It is an area where a state of low pressure persists throughout the year, encircling the Earth along the equatorial belt. A kind of "girdle" where high temperatures and humidity guarantee abundant rainfall — especially over the African continent — as a result of the convergence, hence the name, of large tropical air masses. Depending on how this zone shifts (ITCZ, in the English acronym), it also determines whether the areas it affects experience drier or wetter seasons.

This year, the ITCZ appears to have undergone a considerable northward migration with visible impacts on the tropical climate and on the societies of the affected populations. A migration that appears to be driven, as has so often been the case in recent months, by two fundamental factors. The first is the transition from El Niño, which has upended average global temperatures in recent years, to La Niña, which should cause a cooling of ocean surface waters and thereby bring an end to a period that is, by definition, drier. The other, needless to say, is global warming. The increase in CO2 emissions and other greenhouse gases in the atmosphere caused by the use of fossil fuels, and the consequent rise in average global temperatures, appears to be pushing the ITCZ further north and, according to the study published in Nature Climate Change, this migration is expected to become increasingly frequent over the next two decades.

But the consequences are already plain to see. In Chad, a country that sits on the boundary between the ITCZ and the Sahara, between two and three centimetres of rain fall between mid-July and early September. This year, however, the same period brought between 8 and 20 centimetres of water, causing devastating floods with nearly 1.5 million people affected and 340 deaths. Sudan suffered the same fate at the end of August, with at least 132 deaths and more than 12,000 homes destroyed, as did Nigeria, where the situation in recent days has been decidedly out of control. In the state of Borno, in the northeast of the country, the floods have been described by the United Nations High Commissioner for Refugees (UNHCR) as the worst in thirty years, marking the lives of hundreds of thousands of people. To date, tens of thousands of children have no access to clean water or healthcare, according to reports from the NGO Save the Children. They are obviously not attending school, and many of them no longer have a roof over their heads. "The situation here is terrible. Half the city has been submerged and the roads are impassable," said Chachu Tadicha, deputy director of Save the Children in Nigeria and head of humanitarian operations, who is currently in Maiduguri, at the heart of the affected area.

The migration of the intertropical convergence zone is something that needs to be studied thoroughly and carefully. And it certainly has its origins in several factors, including global warming, as mentioned above. The long-term effects of this phenomenon will also need to be studied, so as to avoid easy comparisons or — worse — conclusions regarding the project of a green corridor, such as the Great Green Wall along the Sahel, which began more than fifteen years ago and whose purpose, on the contrary, is to halt the advance of the desert southwards, through the restoration and cultivation of around one hundred million hectares of land.