WHY IS THE SAHARA A DESERT? Fossil pollen, rock art, and other hints indicate that the Sahara was much greener 6000 years ago in the mid-Holocene period. Neolithic peoples seemed to have hastened desertification at the northern and southern edges of the Sahara, but German geophysicists believe the main causes were natural. They point to the fact that precession (wobble) in the Earth's orbit causes changes in the timing of perihelion (closest Earth-Sun approach) and our planet's rotational tilt. These combined to promote a milder climate in most regions of the mid-Holocene northern hemisphere. Since then the climate has become cooler and more arid. The subtle alterations in northern hemisphere cooling, however, were amplified by a feedback between atmosphere and vegetation causing climate change in the Sahara region to be far more drastic than elsewhere. Indeed what occurred was "the largest change in land cover during the last 6000 years," according to Martin Claussen (Potsdam Institute for Climatology, claussen@pik-postdam.de, 011-49-331-288-2522). He and his colleagues have now confirmed this hypothesis with computer modeling. (Claussen et al., Geophysical Research Letters, 15 July; http://www.pik-potsdam.de/)