Simon Armitage, Royal Holloway University, Charlie Brislow, University of London, Nick Drake, Kings College, London, UK.: North Africa was wetter 15,000–5,000 years ago than today. We reconstruct the lake-level history of Lake Mega-Chad, when it was the largest African lake, and demonstrate that this humid period ended abruptly 5,000 years ago, indicating that the African monsoon exhibits a nonlinear response to insolation forcing. The northern basin of Lake Mega-Chad, currently the world’s greatest dust source, became dry around 1,000 years ago.