Twenty Nile Rivers escape the Mediterranean Sea – a giant water vapor spill boosting the July 2021 floods in Western Europe

Abstract

In summer 2021, severe drought and heatwaves hit the Western United States, Canada, and many other areas around the world. At the same time, record-breaking floods devastated Western Europe (WE) and Central China. Drought and flooding are a water imbalance problem, and heatwaves are always coupled with drought or originate from hot, arid areas. Global average evaporation and precipitation are balanced and steady. When some areas receive less precipitation, other areas receive more, often as heavy downpours. This study analyses one particular freshwater imbalance area – the Mediterranean Basin (MB), from a historical view and of recent trends. The net water vapor output from MB is equivalent to about 20 times the Nile River discharge. The north-south seesaw precipitation trends across Europe clearly indicate a water vapor transfer from MB to Western and Northern Europe. An upper low-pressure system and abundant water vapor supply from MB are an ideal combination for lingering heavy downpours and floods over WE, such as the case in July 2021. The root cause of MB freshwater imbalance is identified as the Sahara expansion. The breach of the green Sahara about 5700 years ago was the desiccation of the Atlas Basin. Based on water cycle stability a solution is suggested to restore the Sahara back to green.