Why the Sahara Is Exactly Where It Is (and Why It Was Green)

The Sahara is almost as big as the United States, yet about seven thousand years ago a lake bigger than all five Great Lakes sat in what is now its southern edge. We follow the evidence on the map: the rain belt and the sinking air that pin the desert where it is, the rock art, buried rivers and old shoreline that prove it was green, the wobble in Earth's orbit that turned it green and back, and the dried lake floor in Chad whose dust still crosses the Atlantic to the Amazon. Chapters 0:00 A lake in the Sahara 0:39 Why is the desert there? 2:30 How do we know it was green? 4:18 What turned it green? 5:39 How fast did it end? 6:39 Where did the lake go? How we measured Lake Mega-Chad on screen is our own rough check: today's ETOPO 2022 elevation map filled to the 329 m shoreline that geologists mapped (with the Mayo Kebbi overflow gap plugged), about 371,000 km², close to the published 361,000 km² (Armitage et al. 2015). "About three quarters of the world's hot desert lies 15 to 30 degrees from the equator" and "a third of the Sahara is more than 1,000 km from the sea" are our calculations from the Köppen-Geiger map (Beck et al. 2023) and ETOPO 2022. The rain maps are CHIRPS means (1991-2020 annual; 2001-2020 for January and August). The seabed dust record is core GC68 off Mauritania (McGee et al. 2013). How much of the Amazon's dust comes from the Bodélé is still debated; we give the estimates and who made them. Sources Armitage, Bristow and Drake (2015), West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad, PNAS 112(28) Drake and Bristow (2006), Shorelines in the Sahara, The Holocene 16(6) deMenocal et al. (2000), Abrupt onset and termination of the African Humid Period, Quaternary Science Reviews 19 McGee et al. (2013), The magnitude, timing and abruptness of changes in North African dust deposition over the last 20,000 yr, EPSL 371-372 Tierney, Pausata and deMenocal (2017), Rainfall regimes of the Green Sahara, Science Advances 3 Larrasoaña, Roberts and Rohling (2013), Dynamics of Green Sahara Periods and Their Role in Hominin Evolution, PLoS ONE 8(10) Skonieczny et al. (2019), Monsoon-driven Saharan dust variability over the past 240,000 years, Science Advances 5 Kutzbach et al. (1996), Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing, Nature 384 Claussen et al. (1999), Simulation of an abrupt change in Saharan vegetation in the mid-Holocene, GRL 26(14) Kröpelin et al. (2008), Climate-driven ecosystem succession in the Sahara, Science 320 Kuper and Kröpelin (2006), Climate-controlled Holocene occupation in the Sahara, Science 313 Sereno et al. (2008), Lakeside cemeteries in the Sahara, PLoS ONE 3(8) McCauley et al. (1982), Subsurface valleys and geoarcheology of the eastern Sahara revealed by Shuttle Radar, Science 218 Washington et al. (2003, 2006) and Todd et al. (2007) on the Bodélé Depression and its low-level jet Koren et al. (2006), The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest, ERL 1 Ben-Ami et al. (2010), Transport of North African dust from the Bodélé depression to the Amazon Basin, ACP 10 Yu et al. (2015), The fertilizing role of African dust in the Amazon rainforest, GRL 42 (and NASA's summary) Hudson-Edwards et al. (2014), Solid-phase phosphorus speciation in Saharan Bodélé Depression dusts, Chemical Geology 384 Yu Y. et al. (2020); Barkley et al. (2022); NASA Earth Observatory (2021) on El Djouf Pham-Duc et al. (2020), The Lake Chad hydrology under current climate change, Scientific Reports 10 UNESCO / IUCN World Heritage Outlook: Lakes of Ounianga; UNESCO: Tassili n'Ajjer NASA: Earth's orbit, perihelion and aphelion; US Census Bureau and US EPA area figures Maps and data NASA Blue Marble Next Generation (public domain) · NOAA ETOPO 2022 (public domain) · CHIRPS v2.0 rainfall, UC Santa Barbara Climate Hazards Center (public domain) · Köppen-Geiger 1991-2020, Beck et al. 2023 (CC BY 4.0) · Natural Earth (public domain) · Contains modified Copernicus Sentinel data 2024-2025 · NOAA NCEI Paleoclimatology (McGee et al. 2013) · NASA Earth Observatory and NASA/JPL imagery · photographs credited on screen (Wikimedia Commons, Pexels) #geography #sahara #maps