17 December 2014

Increasing Greenhouse Gases Linked to Rains over Africa During the African Humid Period


The future response of African rainfall to increasing greenhouse gas (GHG) concentrations is a critical socio-economic issue, with implications for water resources and agriculture, but uncertainties among model projections remain. African hydroclimate changed substantially during the last deglaciation, the most recent time period during which natural global warming was associated with increases in GHG concentrations. Numerous proxy records from Africa indicate that dry conditions during the Last Glacial Maximum (~21,000 years ago) were rapidly replaced by a much wetter interval, commonly referred to as the African Humid Period, starting ~14,700 years ago over much of Africa. Explaining the abrupt onset and hemispheric coherence of this early African Humid Period has been challenging.



In a new study published in Science, a team of scientists from the National Center for Atmospheric Research and eight universities in the United States and China drew on analyses of sediments and other records of past climate and transient simulations with the CCSM climate model. They found that when the model included the orbitally-forced insolation changes, increasing greenhouse gases, and retreating ice sheets and associated meltwater release to the oceans, the temporal evolution of the simulated deglacial precipitation showed good agreement with individual proxy records. The model simulation found that as the ice sheets that covered large parts of North America and northern Europe started retreating from their maximal extent, meltwater release into the northern North Atlantic Ocean reduced the Atlantic meridional overturning circulation (AMOC) to near shutdown and cooled the North Atlantic sea surface temperatures. Cooling over the North Atlantic was rapidly transmitted eastward into Europe and Asia and over the western Indian Ocean through the atmosphere. Simulated precipitation was suppressed in northern Africa and southeastern equatorial Africa.



Once the AMOC reestablished, wetter conditions developed rapidly in northern Africa and southeastern equatorial Africa. Wetter conditions developed north of the equator in response to high summer insolation associated with orbital cycles and increasing GHG concentrations, whereas wetter conditions south of the equator were a response primarily to the GHG increase.


Normally climate simulations cover perhaps a century, or take a snapshot of past conditions. Transient simulations over the more than 10,000-year period evaluated in this study were completed on the Oak Ridge National Laboratory supercomputers allowing the researchers to dissect the transient evolution of the climate in Africa for the first time with a model of the capability of CCSM.

Bette L Otto-Bliesner
Otto-Bliesner, BL, JM Russell, PU Clark, Z Liu, JT Overpeck, B Konecky, P deMenocal, SE Nicholson, F He, and Z Lu.  2014.  "Coherent Changes of Southeastern Equatorial and Northern African Rainfall During the Last Deglaciation."  Science 346: 1223-1227.  https://doi.org/10.1126/science.1259531.

This research was supported by the Office of Science (Biological and Environmental Research), U.S. Department of Energy (DOE), the NSF Paleo Perspectives on Climate Change (P2C2) program, and NSF grant AGS 1160750. This research used resources of the Oak Ridge Leadership Computing Facility, located in the National Center forComputational Sciences at Oak Ridge National Laboratory, which is supported by the DOE Office of Science.