This is a question that has huge implications for our understanding of how dry areas get created. In coming decades, as temperatures continue to warm in places like the American Southwest and the Sahel on the Sahara's border, that understanding, and our ability to make accurate models based on it, will be crucial.

The Sahara—parts of which get virtually no rainfall—is the most arid region on Earth. But it was not always that way. There was a wet time, beginning around 10,000 years ago (called the African Humid Period), when large parts of the present-day Sahara looked more like the Serengeti plains in east Africa: tree-pocked grasslands which supported a diversity of animals such as giraffes, rhinos and roaming herds of wildebeest, as well as large human populations.

But then things began to change—the climate became progressively hotter and drier. This coincided with the founding of pharaonic civilization in Egypt 5,000 years ago, when human migration from increasingly inhospitable regions to the Nile Valley sparked the meteoric rise of ancient Egypt.

The Sahara Desert (with the Nile River snaking through) and the Red Sea, as seen from the International Space Station (Reuters)

Precisely how—and how quickly—the region dried up, however, has been a matter of scientific dispute. A 2008 study by Stefan Kröpelin of the Institute of Prehistoric Archaeology at the University of Cologne in Germany of Lake Yoa in northern Chad concluded that there had been a gradual transformation to a desert environment over a period of several millennia, as the north African monsoonal rains gradually moved south.

However, the information from the final research vessel to brave Somali pirates may have just turned the tables on this accepted scientific view. Jessica Tierney, of Woods Hole Oceanographic Institution, and deMenocal, of the Lamont-Doherty Earth Observatory at Columbia University used a newly developed technique, the analysis of the hydrogen and carbon isotopes in minute particles of leaf wax (the shiny outer coatings of leaves) in ocean sediments to get climate information for the last twenty thousand years. The ocean, which is not subject to erosion and other geological and chemical processes that effect deposits on the land, preserves the Earth’s continuous climate history intact (much as ice core samples do in the polar regions.)

What the scientists found was that, far from shifting gradually from wet to dry, the climate in the Horn of Africa changed in perhaps as little as 100 to 200 years, incredibly quickly in geological terms. The reason north Africa warmed up, they believe, was a cyclic change in Earth’s orientation toward the sun (called precession) which caused more sunlight to fall during the Northern Hemisphere's summer. But the precession cycle is slow, taking 23,000 years to complete. So why was the changeover in the Horn of Africa so quick?