Six thousand years ago, the Sahara was lush and green. The Earth’s precession (wobbling) has been blamed as the main cause for the vast climate changes between arid desert and green land in the Sahara. Accordingly, it will be another 10,000 years before the Green Sahara is predicted to return. Nevertheless, this book reveals that the Earth’s precession only serves as a perturbation to trigger the transitions. The main control is the stability of the water cycle in the Sahara. This is governed by the non-linear relationship between precipitation and evaporation, and, in turn, depends on surface conditions and atmospheric circulation. About 5700 years ago, the Green Sahara suddenly started to wither. The wilting began in the standalone Great Chotts Basin, which is in the strong rain shadow of the Atlas Mountain ranges. When the water cycle stability in this basin was broken, the Great Chott Lakes dried quickly and the rain shadow effect became fully active, triggering desert formation. The desert expanded east and south, like a spreading wildfire powered by the westerlies and trade winds. This led to desertification and aridification in North Africa, West Asia, and the Mediterranean which have continued synchronously until this day. This process spans and affects most of human history as we know it, including the prosperity and desolation of Mesopotamia, the rise and fall of Egypt, and the civilization shifts from east to west across the Mediterranean and the Atlantic. This book will explain the step-by-step spread of the Sahara desert and its impact on West Asia and the Mediterranean regions. Archaeological records and paleoclimate data corroborate this new insight. As an independent validation, the desertification in Northwest China has followed a very similar process and control mechanism. Based on the new understanding, this book reveals methods to revive deserts in Northwest China and Australia, and to turn the Sahara back to green.
