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The study, published in the American Meteorological Society journal Weather, Climate and Society, bridges atmospheric science with African history. It also shares lessons for today amid a warming future that threatens to exacerbate human conflict and migrations.
The study found that El Niño can be used as a proxy -- much like tree rings and corals -- for historical rainfall and temperature patterns in West Africa. The authors used reconstructed El Niño indices and the Slave Voyages dataset to examine the relationship between El Niño and the trans-Atlantic slave trade.
Historians have suggested, based upon qualitative assessments of journals, documents and chronicles, that droughts affected the trans-Atlantic slave trade. But they have not been able to quantify that relationship or to ascribe a mechanism for the droughts.
"The trans-Atlantic slave trade began in the 1400s, but instrumental rainfall data only goes back to around the 1800s," said lead author William Turner IV, a Ph.D. student at UC Davis in the Department of Land, Air and Water Resources. "To fill this data gap, we relied on the proxy association between El Niño and rainfall. We found that during El Niño, West Africa experiences drier conditions."
Delayed response
The authors found that El Niño-induced drier conditions are associated with a decrease in the number of enslaved people brought to the Americas, and it happened at a two-year lag. The lag is important, showing that El Niño-induced drier conditions caused a delayed response in the slave trade.
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