The findings run counter to a prevailing view that the change happened abruptly, within a few centuries, about 5,500 years ago, marking the end of the “African Humid Period” when monsoon rains poured down on the region. That view arises from ocean sediment cores drilled off the coast of Africa, to the west of Mauritania. In 2000, analysis of the cores by researchers led by Peter B. deMenocal of Columbia University’s Lamont-Doherty Earth Observatory showed a sudden rise in the dust blown off Africa at that time.

Dr. Kröpelin did not dispute the ocean core data, but said it had been “overinterpreted.”

Data about what was happening on land is sparse, because sands blow around and do not preserve a clear geological record the way lake sediments do. But at Lake Yoa, the water that filled underground aquifers during the humid period, which began 14,800 years ago, is still flowing out into the 80-foot-deep lake. The groundwater is enough to offset the six meters of water that evaporates out of the lake every year, Dr. Kröpelin said. Only a few millimeters of rain fall a year.

Dr. Kröpelin said he hoped to return to Lake Yoa next year to drill a deeper core that could trace the climate history back 12,000 years.

Dr. deMenocal praised Dr. Kröpelin’s research. “I think it’s a very good body of work,” he said. “It’s really the only thing of its kind from the arid interior.” But he wondered, he said, whether the pollen might have come from the area just around immediately surrounding the lake and not the larger Sahara.

“On the face of it, it’s puzzling,” said Jonathan A. Holmes, director of the Environmental Change Research Center at University College London in England. Dr. Holmes said both sets of research had been carefully done, and the challenge will be to put together a more complex history of the Sahara’s climate.