A geologist's findings in Africa challenge the way scientists think about the threat of desertification
By GAUTAM NAIK
May 30, 2014 6:42 p.m. ET THE WALL STREET JOURNAL
Stefan Kropelin, a 62-year-old geologist at the University of Cologne, is one of the world's preeminent explorers of the Sahara desert. He's created a storm by challenging the extent to which man-made global warming is having on the region. See video
At the end of the last ice age, about 11,000 years ago, northern Africa became a grassland, home to fish, elephants and thousands of people. Then, 5,000 years ago, scientists say, it abruptly turned into an enormous wasteland—the vast desert that we now call the Sahara.
The shift from savanna to sand was the result of natural climate change, triggered by a cyclical alteration in the sun's orbit. The decline in rainfall pushed residents south and east and perhaps contributed to the rise of the Egyptian civilization. Today, scientists say, the region is again feeling the heat, this time from man-made climate change.
USING SAMPLES from Chad's Lake Yoa, above, Dr. Kröpelin can see the origins of the Sahara
Climate models for north Africa often come to contradictory conclusions. Nonetheless, mainstream science holds that global warming will typically make wet places wetter and dry places drier -and at a rapid clip [recently falsified]. That is because increased greenhouse gases trigger feedback mechanisms that push the climate system beyond various "tipping points." In north Africa, this view suggests an expanding Sahara, the potential displacement of millions of people on the great desert's borders and increased conflict over scarce resources.
One scientist, however, is challenging this dire view, with evidence chiefly drawn from the Sahara's prehistoric past. Stefan Kröpelin, a geologist at the University of Cologne, has collected samples of ancient pollen and other material that suggest that the earlier episode of natural climate change, which created the Sahara, happened gradually over millennia—not over a mere century or two, as the prevailing view holds. That is why, he says, the various "tipping point" scenarios for the future of the Sahara are overblown.
The 62-year-old Dr. Kröpelin, one of the pre-eminent explorers of the Sahara, has traveled into its forbidding interior for more than four decades. Along the way he has endured weeklong dust storms, a car chase by armed troops and a parasitic disease, bilharzia, that nearly killed him.
Six of those expeditions were to Lake Yoa in Chad, hundreds of miles from civilization. It is a singular desert lake not only because it is deep and therefore undisturbed but also because it has been around for millennia, despite the intense evaporation in the region.
Over that time, pollen from Saharan flora has landed on its surface, sunk to the bottom and become part of the sediment. In 2010, when Dr. Kröpelin pulled up a sedimentary core from 50 feet below the lake bottom, he had a largely pristine record of the Saharan climate going back 11,000 years. It took nearly a year of preparation and field work to obtain the core and then transport it back to Cologne.
Now 27 of those cores, each encased in a 3-foot-long Plexiglas tube, sit in a freezer at his lab. Each millimeter of sediment corresponds roughly to a year's climate record.
For the past four years, Dr. Kröpelin and his team of some 20 researchers have been using a spectrometer, an electron microscope and lasers to analyze the cores and build a climatic history of the Sahara. "It is geological treasure," he says.
Many scientists contend that about 5,000 years ago, as rainfall declined due to natural cycles, the verdant grasslands of north Africa abruptly changed to lifeless sand in a mere century or two. This was the conclusion of a 2000 study by Peter B. deMenocal of Columbia University and others, which analyzed ocean cores drilled off the northwest African coast. They found that a large amount of dust had blown off the continent over that time, suggesting a rapid shift to desert.
A 2013 study found evidence of a similar abrupt shift in northeast Africa. Another published analysis of core samples from northwest Africa echoed the general view. "It suggests some kind of tipping point" in the Sahara, brought on by gradual climate change, says David McGee, a paleoclimatologist at the Massachusetts Institute of Technology, who co-wrote the study on northwest African sediments.
But Dr. Kröpelin's analysis of the Lake Yoa samples suggests that there was no tipping point and that the change was gradual. He says that his argument is also supported by archaeological evidence. Digs in the Sahara, conducted by various archaeologists over the years, indicate that the people of the region migrated south over millennia, not just in a few desperate decades. "Humans are very sensitive climate indicators because we can't live without water," he says. If the Sahara had turned to desert quickly, the human migration pattern "would have been completely different."
The dispute is unresolved for now, but Dr. Kröpelin's expertise on the Sahara means that even detractors are paying attention. Says Dr. deMenocal of Columbia University: "Stefan's is not a majority view, but if I could go back in a time machine, it may be that what he sees [in the geological record] is really what happened."