Climate change does not unfold gradually or in a linear way. Peering back 11,000 years in our own Holocene era, Martin Claussen sketches a picture of abrupt and brutal shifts in the biosphere. His work involves modeling complex interactions among atmosphere, ocean, sea ice and vegetation. The Sahara Desert was once a green oasis, but dramatic disruptions in the last six thousand years led to its very rapid browning. Claussen’s models demonstrate that a slow evolution in the earth’s orbit and its tilt toward the sun triggered a reaction that led to swift loss of moisture and vegetation in North Africa. Claussen believes that interactions within Earth’s climate system -- specifically between vegetation cover and sea ice -- amplified the impact of the orbital shifts. “If the system gets a slight kick, it can jump from green to desert,” says Claussen. As a result of this change, humans may have been forced to migrate from the devastated Sahara region to the fertile Tigris, Euphrates and Nile River valleys, where new civilizations sprang up. Looking forward, Claussen notes that triggers such as an excess of human-generated carbon dioxide or deforestation might provoke similar dramatic climatic changes in global hotspots.