SAHELP Sahara and Sahel vulnerability: lessons from the past

CONTEXT AND MAIN ISSUES The North African freshwater cycle and vegetation are very sensitive to the variability of the African Monsoon. Such changes are related to changes in sea surface temperatures, 30% of which being driven by external forcings (anthropogenic greenhouse gas increases, volcanism, dust). Current anthropogenic pressure and climate change may affect drastically Sahelian and Saharan ecosystems, but available predictions for the next decades still need to be improved due to high uncertainties in tropical rainfall estimates (amount and variability) from climate models; high uncertainties in hydrological system functioning and related vulnerability to climate; complex ecosystem processes coupling climate, water cycle, and vegetation types.

IMAGE

OBJECTIVES and METHODS In order to assess the vulnerability of Sahelian and Saharan ecosystems to global change, we need to understand how water and vegetation will be affected in the long-term. The aim of the SAHELP project is to analyze how ecosystem vulnerability in response to climate hazards has developed in the past, and how large time scale changes have impacted the ecosystem response initially induced by small time scale changes. Such analyses will allow us to determine the pertinent time scales (from season to century) that must be taken into account. Our research strategy is to analyze Holocene climate data to assess key climate parameters and to estimate critical thresholds for hydrological and vegetation component responses in order to examine African ecosystem vulnerability.

Contacts : Anne-Marie.Lezine@locean-ipsl.upmc.fr

HIGHLIGHTS

By focusing its attention on a key region whose recent history has demonstrated its extreme sensitivity to climate change SAHELP brought a new look at paleoenvironmental reconstructions and improved our understanding of the forcings (climate, aquifers, vegetation cover). We developed an extensive paleohydrological dataset for the Sahara and the Sahel from 15ky to the present and a pollen database for the sub-Saharan Africa. Case studies carried out at Lake Chad, at Lake Yoa, and in the Niayes regions led to detect and quantify the cooldown of hydro-systems and understand the role of wetlands and groundwater on the climate system. Data-model comparisons drew particular attention to the interactions of scale (both geographical (global general circulation models, regional aquifer scale, local scale) and temporal (long-term climate change, seasonal variability of rainfall, weather events..) needed to understand the vulnerability of the Sahara and Sahel.

SCIENTIFIC PRODUCTS Lézine, A.-M. et al. 2011. Sahara and Sahel vulnerability to climate changes, lessons from paleohydrological data. Quaternary science Reviews 3 0, 21-22, 3001-3012. Bastola, S., Delclaux, F. 2011. Temporal extension of meteorological records for simulation modeling of Lake Chad Basin (Africa) using satellite rainfall data and reanalysis datasets. Meteorological Applications, doi: 10.1002/met.257. Krinner, G., et al. 2012. A reassessment of lake and wetland feedbacks on the North African Holocene climate, Geophysical Research Letters, 39. Marzin, C. et al. 2013. Relative impacts of insolation changes, meltwater fluxes and ice sheets on African and Asian monsoons during the Holocene. Climate Dynamics. 41, 2267 2286. Zheng, W. P., Braconnot, P. 2013. Characterization of Model Spread in PMIP2 Mid-Holocene Simulations of the African Monsoon. Journal of Climate. 26: 1192-1210

PERSPECTIVES New scientific projects funded by the ANR (ex: EL PASO, C3A) to go deeper in our understanding of past climate change and ecosystem vulnerability throughhout the tropics, Increased International cooperation within PMIP

In Atei Paleolake, southern Algeria