Roughly 11,000 to 5,000 years ago, during the\u00a0African Humid Period<\/a>, the Sahara Desert was likely far wetter and greener. Geologic and archaeological evidence indicates that vegetation, wetlands, and perhaps\u00a0even large lakes<\/a>\u00a0covered areas that are now oceans of sand.<\/span><\/p>\n In September 2024, hints of this more watery past emerged after an extratropical cyclone dropped a\u00a0deluge of rainfall<\/a>\u00a0on parts of northern Africa.\u00a0Runoff<\/a>\u00a0from the storm has partially filled several ephemeral desert lakes in normally dry areas.<\/span><\/p>\n The\u00a0OLI-2<\/a>\u00a0(Operational Land Imager-2) on\u00a0Landsat 9<\/a>\u00a0(right) captured this image of Sebkha el Melah, an ephemeral lake in Algeria, on September 29, 2024. The lake is situated along a ridge of the\u00a0Ougarta Range<\/a>\u00a0and fed by the Oued Saoura, an ephemeral river (sometimes called a\u00a0wadi<\/a>) that enters from the southeast. The other image (left) shows the salt-encrusted lakebed on August 12, before the rain event. NASA satellites observed the lake beginning to fill in\u00a0mid-September<\/a>.<\/span><\/p>\n As of October 16, water covered 191 square kilometers (74 square miles) to a depth of 2.2 meters (7.2 feet), and Sebkha el Melah was roughly one-third full, said Moshe Armon, a senior lecturer at the Hebrew University of Jerusalem. Armon calculated these values using satellite images of the water\u2019s extent, like the one above, along with a\u00a0three-dimensional map<\/a>\u00a0of the lake\u2019s\u00a0bathymetry<\/a>\u00a0based on\u00a0ICESat-2<\/a>\u00a0observations. Since June 2000, only two other rain events have resulted in larger lake volumes\u2014one in\u00a02008<\/a>\u00a0and one in\u00a02014<\/a>, Armon said.<\/span><\/p>\n The filling of a Sahara Desert lake is a \u201crare, largely undocumented, transient phenomenon,\u201d noted Jo\u00eblle Rieder, a colleague of Armon\u2019s, in a\u00a02024 study<\/a>\u00a0that detailed the frequency of refilling events at the lake since 2000. This part of Algeria has few ground-based weather stations, so the researchers used rainfall data from NASA\u2019s Integrated Multi-satellite Retrievals\u00a0(IMERG<\/a>) and\u00a0ERA5<\/a>\u00a0weather\u00a0reanalysis data<\/a>\u00a0from the European Center for Medium-range Weather Forecasts (EMCWF<\/a>) to study the meteorological conditions required to fill the lake.<\/span><\/p>\n Of the hundreds of rainfall events that affected Sebkha el Melah\u2019s watershed since 2000, only six delivered enough water to start filling it. All of these involved long-lasting\u00a0extratropical cyclones<\/a>\u00a0that produced especially heavy rainfall as moist tropical air was pushed up and over mountains, a process called\u00a0orographic lift<\/a>.<\/span><\/p>\n Satellite observations indicate that when Sebkha el Melah fills, the water can stick around. After it filled in 2008, it took until 2012 to dry completely. \u201cIf we don\u2019t get any more rain events, a 2.2-meter depth, like we have now, would take about a year to evaporate completely,\u201d Armon said.<\/span><\/p>\n Armon and other scientists track lake-filling episodes in part because questions remain about both the region\u2019s past and future. Despite evidence indicating that the Sahara was wetter during the African Humid Period, just how wet remains a matter of scientific debate. To help resolve it, scientists look to desert lakes, like Sebkha el Melah, because they function a bit like giant \u201crain gauges\u201d that provide clues about past precipitation patterns, explained Armon.<\/span><\/p>\n One of the challenges for researchers who study this topic is that models that simulate prior climate conditions struggle to reproduce the rainfall required to fill as many Saharan lakes as geologists believe\u00a0were present during the African Humid Period. This has led some researchers to suggest that either the Sahara\u00a0wasn\u2019t actually as rainy and verdant<\/a>\u00a0as paleoclimate experts think or that the models are missing something, explained Armon.<\/span><\/p>\n \u201cWe\u2019re proposing a third option: that extreme rain events, like the one in September in the northwestern Sahara, might have been more frequent in the past,\u201d said Armon. \u201cGiven how long it takes lakes to dry up, these events could have been common enough to keep lakes partially filled over long periods\u2014even years or decades\u2014without frequent rainfall.\u201d<\/span><\/p>\n It\u2019s generally accepted by paleoclimatologists that\u00a0small orbital variations<\/a>\u00a0called\u00a0Milankovitch cycles<\/a>\u00a0were key drivers of the African Humid Period because they would have caused slight changes in the distribution of solar radiation and\u00a0shifts in the strength and position<\/a>\u00a0of the North African monsoon. Less clear is whether and how much the Sahara might green and support long-lasting lakes in future centuries and millennia as the impacts of\u00a0greenhouse gas emissions<\/a>\u00a0and\u00a0climate change<\/a>\u00a0are layered onto the cyclical effects of the Milankovitch cycles.<\/span><\/p>\n Projections<\/a>\u00a0from the\u00a0Intergovernmental Panel on Climate Change<\/a>\u00a0(IPCC) indicate that while parts of the Sahara may receive more rainfall as global temperatures increase, other parts may receive less. \u201cBut the uncertainties in these projections are larger than the projected changes,\u201d said Armon. \u201cWhat\u2019s going to happen in the Sahara remains very unclear, but we hope that we\u2019ll eventually develop a better understanding of the Sahara\u2019s future by studying these lake-filling events.\u201d<\/span><\/p>\n NASA Earth Observatory images by Michala Garrison, using Landsat data from the\u00a0U.S. Geological Survey<\/a>. Story by\u00a0Adam Voiland<\/a>.<\/i><\/span><\/p>\n![\"\"](\"https:\/\/jazairhope.org\/wp-content\/uploads\/2024\/11\/jh-39-300x208.jpg\")
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