Water can shape history. It can make or break a king, it can be used as an agent of oppression or even as an instrument of war. In one of history's most bizarre collaborations, Leonardo da Vinci plotted with Machiavelli in 1503 to divert the Arno River away from Pisa, with which Leonardo's home city of Florence was at war.

The German historian Karl Wittfogel was one of the first to point out that water is power. He argued in the 1930s that Asian societies -- China in particular -- were 'hydraulic': despotic civilizations in which water was the currency of political might. A Marxist himself, Wittfogel gradually came to regard the ancient hydraulic societies of China and Russia as the precursors of the modern socialist dictatorships. Irrigation was the key to production and stability, demanding massive hydro-engineering works with an accompanying managerial bureaucracy. Social order, repression and political crises all revolved around the vicissitudes of the water that fell from the sky to irrigate the fields.

This is a message reflected in new research on the climate of east Africa, reported in Nature1. Dirk Verschuren of the University of Minnesota and colleagues say that periods of social unrest in this region have coincided with periods of drought since the Middle Ages.

Verschuren and colleagues have obtained a record of rainfall in east Africa over the past 1,100 years by digging into the muddy sediments of the submerged crater lake on Cresent Island in Lake Naivasha in Kenya. The crater lake provides an unusually sensitive indicator of climate, with a water level that fluctuates markedly in response to wet or dry spells.

These changes in the lake leave their imprint in the sediments that gather at its bottom. As the water level changes, the mud alters from being clay-rich to silt that is full of organic matter. If the lake starts to dry out, the water becomes enriched in dissolved salts; this produces more carbonate in the sediments. Changes in saltiness also affect the relative abundance of different species of diatom that live in the water, whose fossil remnants from hundreds of years ago can be discerned in the layers of mud.

These various types of evidence have allowed Verschuren and colleagues to draw up a record of lake depth stretching back to 900 AD, during which time it rose and fell by over 40 metres. The researchers say that the changes in water level reflect changes in rainfall over the entire east African region.

Comparison of Crescent Island Crater history with documented and reconstructed climate-proxy data. The depth and salinity of the lake is shown with the record of atmospheric 14CO2 production as a proxy for solar radiation, and the pre-colonial history of east Africa. Grey bars indicate evidence of drought-related political upheaval recorded in oral tradition, genealogically dated using a 27-yr dynastic generation. Stippled bars compile the evidence of severe drought events from various archival records including the (incomplete) record of Nile River discharge.

They find that the regional climate was drier than today between about 1000 and 1270, an episode known as the 'Medieval Warm Period' during which Europe is known to have been unusually warm. And the climate was wetter than today from around 1270 to around 1850. Over this period, called the Little Ice Age, Europe was colder than today.

But the wet centuries in east Africa were interrupted by three prolonged dry episodes: in about 1380--1420, 1560--1620 and 1760--1840. These three periods correspond precisely with times of famine, political unrest and large-scale migrations as recounted in the oral traditions of the region. The intervening wet years, meanwhile, match times of political stability and population growth: the 'first and second Age of Prosperity', according to local histories. Drought-induced famine, it seems, played a major role in destabilizing these pastoral societies.