For feelings of timelessness, unboundedness, and permanence, nothing beats the Sahara Desert. Yet as recently as 14,800 years ago, vast reaches of it were green, as a stronger summer monsoon enabled lakes, wetlands, grass and shrubland to expand upwards from the Sahel. Then around 6,000 years ago, with increased incoming sunlight and a weakening monsoon, desertification set it. But was that process fast or slow? Is it a case in point for those sounding alarms about â''abrupt climate changeâ''â''change that takes place too fast for humans and ecosystems to adapt?
Research appearing tomorrow (May 9) in Science magazine, with an accompanying commentary by Jonathan A. Holmes of the Environmental Change Research Centre at Londonâ''s University College, finds that the change in fact was gradual. S. KrÃ¶pelin of the University of Cologne (KÃ¶ln) and colleagues studied sediments in Lake Yoa to extract information about pollens, salinity, and dustiness. â''The continuous and well-dated pollen record for this site shows no abrupt change in vegetation in the mid-Holocene,â'' comments Holmes. â''The rise in Lak Yoaâ''s salinity was rapid, but this was almost certainly a response to a local threshold being crossed as the lake changed from hydrologically open to hydrologically closed, rather than to abrupt climatic drying.â''
Last weekâ''s Science (May 2) contained a report by Kiel Universityâ''s Lothar Stramma and colleagues reporting a different kind of desertification. Studying intermediate-depth waters in selected tropical ocean regions, they constructed a 50-year history of oxygen concentrations. What they found was that huge underwater oxygen-starved deserts are rapidly expanding.