Global Warming Tipping Point Models Are "Overblown"
(p. C3) 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. 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.
. . .
. . . 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."
For the full commentary, see:
Naik, Gautam. "Climate Clues in the Sahara's Past; A Geologist's Findings in Africa Challenge the Way Scientists Think about the Threat of Desertification." The Wall Street Journal (Fri., May 31, 2014): C3.
(Note: the online version of the commentary has the date May 30, 2014, and has the title "How Will Climate Change Affect the Sahara?; A geologist's findings in Africa challenge the way scientists think about the threat of desertification.")
One of the more recent Kröpelin papers arguing against the tipping point account is:
Francus, Pierre, Hans von Suchodoletz, Michael Dietze, Reik V. Donner, Frédéric Bouchard, Ann-Julie Roy, Maureen Fagot, Dirk Verschuren, Stefan Kröpelin, and Daniel Ariztegui. "Varved Sediments of Lake Yoa (Ounianga Kebir, Chad) Reveal Progressive Drying of the Sahara During the Last 6100 Years." Sedimentology 60, no. 4 (June 2013): 911-34.