River response to climate change in the tropics: a three hundred thousand year history of the Nile

Abstract

The complex response of big tropical rivers to climate change is often hard to discern when the available instrumental records span only a few decades, so that the hydrological record from the past provides an invaluable guide to present and future climate change. The Nile is the longest river in the world with a total length of 6 670 km, spanning 35° of latitude between 3°S and 32°N and has the longest historical flood record of any river in the world. The Nile basin presently covers nearly three million km2 but was even larger as recently as 7 000 years ago. The Nile consists of three very distinct hydrological systems: the Ugandan lakes and the White Nile, the Ethiopian volcanic highlands and the Blue Nile, and the main or Saharan Nile. The Blue Nile provides much of the summer discharge and sediment load to the main Nile, but the White Nile provides over 80% of Nile discharge during the month of lowest flow and is responsible for maintaining perennial flow in the Nile during drought years in Ethiopia. During the last 300 000 years the summer monsoon has fluctuated in response to astronomically controlled changes in tropical insolation, with times of low and variable Nile river discharge coinciding with cold, dry glacial climates in the headwaters and enhanced flow with warm, wet interglacial climates. Over the past five centuries, at least, times of low flow coincide with El Niño events in Peru and times of high flow with La Niña years. Years of low Nile flow are synchronous with drought years in India, China, Java, Thailand and Australia, and conversely. This pattern is likely to persist into the future, albeit with subtle variations.