Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Wednesday 8 July - 17:30-19:00 UPMC Jussieu - Amphi 34

3330 (a) - Facing climate change in Sub-Saharan Africa

Parallel Session

Lead Convener(s): S. Janicot (Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN), Paris, France), A. Amani (UNESCO, Nairobi, Kenya)

Convener(s): B. Sultan (IRD, Paris, France), R. Cornforth (University of Reading, Reading, United Kingdom), A. Gaye (University Cheikh Anta Diop Dakar, Dakar, Senegal), C. Thorncroft (Professor, University at Albany, Albany, United States of America)

17:30

Observatories, a Key Tool to Tackle Climate Changes in Tropical Regions

T. Lebel (IRD, 38041 Grenoble Cedex, France), C. Peugeot (IRD, Montpellier, France), M. Grippa (Université de Toulouse, Toulouse, France), S. Galle (IRD, 38041 Grenoble Cedex, France)

Abstract details
Observatories, a Key Tool to Tackle Climate Changes in Tropical Regions

T. Lebel (1) ; C. Peugeot (2) ; M. Grippa (3) ; S. Galle (1)
(1) IRD, LTHE, 38041 Grenoble Cedex, France; (2) IRD, Hydrosciences montpellier (hsm), Montpellier, France; (3) Université de Toulouse, Get, Toulouse, France

Abstract content

For both physical and socio-economic reasons, tropical regions are highly sensitive to the impacts of climate change. At the same time, the model projections are more uncertain than for many other regions of the world, especially regarding the hydrologic cycle that is the key driver for water resources, agriculture and food security. The various components of the continental water balance display a strong variability over a large range of space and time scales that are not properly documented by the operational meteorological and hydrological networks. This natural variability of the water cycle challenges our ability to detect significant trends potentially linked to the global warming and/or other factors, such as land use changes or ecosystem evolutions. While detecting changes in the annual mean of precipitation, runoff or deep infiltration is not always self-evident, it is still much more challenging to assess significant changes in the extreme values, because the less frequent are the phenomena to observe, the longer should be the period of observations to detect a statistically significant non stationarity. This set of issues applies in much the same way to other key components of our environment, such as for instance, erosion, dust transportation, soil resources.  

Adequate observing systems allowing the documentation of both the climate evolution at regional scale and its impacts on the air quality, radiative budgets, hydrology and vegetation are thus a key tool for a fine characterization of climate change and for providing decision makers with the appropriate knowledge to be used for implementing and following ambitious public policies in order to mitigate its socio-environmental consequences. In the long run, observing systems are also crucial for improving climate models, especially when it comes to the simulation of variables –such as rainfall, river flows, dust transportation – that display the largest interannual variability.

This will be illustrated through the history and achievements of the AMMA-CACTH and other observing systems (IDAF, PHOTONS, Dust-Transect, GPS) providing the ground to debate the need for consolidating a collaborative strategy for tropical climate change observations and to discuss how to reinforce the science/policy interfaces in this area.

17:45

Climate Change Projections in the Sahel. The what and the why

M. Biasutti (Columbia University , New York, United States of America), M. Biasutti (Columbia University, Palisades, NY, United States of America)

Abstract details
Climate Change Projections in the Sahel. The what and the why

M. Biasutti (1)
(1) Columbia University, Lamont Doherty Earth Observatory, Palisades, NY, United States of America

Abstract content

This talk will review the projected 21st century changes in temperature and precipitation at seasonal and sub seasonal scale in Subsaharan Africa, with particular emphasis on the Sahel and the West African Monsoon region. 

 

Greenhouse-gas induced temperature increases are large compared to interannual variability and such forced seasonal temperature anomalies are decoupled from the occurrence of drought or pluvials; this results in a robust projection of unprecedented seasonal temperatures by mid-century.  

 

Precipitation anomalies remain more uncertain. In the Sahel,  although outlier models remain, the ensemble anomalies indicate lesser precipitation totals in the west and greater in the east, and a change in seasonality that is manifest in a delay in the beginning, peak, or demise  of the rainy season. 

 

Additionally, we present the projected changes in the characteristics of rainfall and in the occurrence of extreme events and we evaluate the robustness of such anomalies with respect to model uncertainty, natural variability, and the use of statistical downscaling to debias daily rainfall and temperature. 

 

Finally, we interpret  seasonal rainfall anomalies in terms of thermodynamic and dynamic forcings and in terms of regional and global modes of change. We emphasize the role of the large-scale circulation and the forcing from both the tropical and the midlatitude sea surface temperature anomalies. 

18:00

External forcing and Sahelian African climate anomalies - A study from the WAMME project

Y. Xue (Professor, UCLA, Los Angeles, United States of America), W. Lau, (2. University of Maryland, College Park, College Park, United States of America), A. Boone, (CNRM, Toulouse, France), S. I. Seidou (Centre Régional AGRHYMET/CILSS, Niamy, Niger, Republic of), W. Thiaw (5. National Centers for Environmental Prediction, College Park, United States of America), L. Druyan, (Goddard Institute for Space Studies/NASA and Columbia University, New York City, United States of America), D. Rowell (Met Office Hadley Centre, Exeter, United Kingdom)

Abstract details
External forcing and Sahelian African climate anomalies - A study from the WAMME project

Y. Xue (1) ; W. Lau, (2) ; A. Boone, (3) ; SI. Seidou (4) ; W. Thiaw (5) ; L. Druyan, (6) ; D. Rowell (7)
(1) Professor, UCLA, Dept. of geography & dept. of atmospheric and oceanic sciences, Los Angeles, United States of America; (2) 2. University of Maryland, College Park, College Park, United States of America; (3) CNRM, Toulouse, France; (4) Centre Régional AGRHYMET/CILSS, Niamy, Niger, Republic of; (5) 5. National Centers for Environmental Prediction, NOAA, College Park, United States of America; (6) Goddard Institute for Space Studies/NASA and Columbia University, New York City, United States of America; (7) Met Office Hadley Centre, Exeter, United Kingdom

Abstract content

The Sub-Sahara Africa is a diverse climatic and economically fragile region and dramatic change over the Sahelian Africa from wet conditions in the 1950s to much drier conditions in the 1970s-1980s and then to partial recovery from the 1990s represents one of the strongest interdecadal climate variability and the longest drought on the planet in the twentieth century. A significant climate feature in the Sahelian Africa is the West African monsoon (WAM), which variability dominants the climate variability there.  However, the CMIP5 coupled models underestimate the WAM decadal variability and the drought.   Although encouraging progresses have been achieved, many systematic and robust biases of the coupled and atmospheric models have not improved from CMIP3 to CMIP5. It is necessary to have comprehensive understanding on the past Sub-Sahara decadal variability and predictability to provide reliable assessment of future climate change and adequate strategy for mitigation and adaptation under changing climate.

In past several decades, the West African climate community has recognized the importance of external forcings: oceans, land processes including land cover and land use change (LULCC), aerosols, and greenhouse gases, on WAM variability, especially their roles in the Sahel drought. However, most of these studies only focused on one external forcing with one single model.  The West African Monsoon Modeling and Evaluation (WAMME) is a project comprised of both general circulation models (GCMs) and regional climate models (RCMs) with the objective to collectively provide best estimation of the relative importance of all those external forcing on WAM at seasonal to multi-decadal time scales. WAMME research activities are closely coordinated with those of AMMA, involving many African institutions.  The observational and other relevant datasets acquired from AMMA provide important benchmark for assessing the role of external forcing in regional climate variability and anomalies.

In this paper, we mainly present the latest results from the WAMME-2, which is designed to test how seasonal and decadal variabilities of WAM precipitation are associated with external forcings, and assess their relative contributions in producing/amplifying the WAM seasonal and decadal climate variability. The sensitivity of the WAM variability to those external forcings is also examined. The WAMME-2 strategy is to apply observational data-based anomaly forcing of SST, land surface and aerosols, i.e., "idealized but realistic" forcing, in GCM and RCM simulations with the specific purpose of estimating the relative impacts of each forcing and feedback mechanisms. 

In the SST experiment, in addition to the global SST effect, each ocean’s role is also evaluated. The preliminary results from most GCMs consistently indicate that SST has a maximum impact on the WAM decadal variability compared with other forcings, and that the effect of the Pacific Ocean is most dominant.  The models, however, differ in producing other oceans’ contribution. Moreover, the models with specified maximum SST anomaly forcing are still unable to produce full Sahel drought (only slightly above 50% of the full drought). In the LULCC experiment, a newly available land use change map is applied. A consistent change in the vegetation maps is imposed for each modeling group. The simulated LULCC impact is also substantial, compatible to but less than the SST forcing (about 40% of the drought). In the dust experiment, the direct impact of dust on the radiation budget and its influence to the Sahel rainfall are evaluated using the GOCART dust data and its effect also contributes to the drought (less than 20% of the drought). In addition, some preliminary results for impact of the greenhouse and global warming on the Sub-Sahara climate decadal variability will also be presented.

WAMME is the first attempt to use multi-GCMs and RCMs to collectively explore the roles of multiple external forcing in WAM variability. WAMME2’s achievement provide better understanding of relative importance of various forcing and possible feedback mechanisms, complementary to experiments under CMIP, which are focused more on impacts of emission scenarios, and CORDEX, which is focus on RCM downscaling ability.  The results from WAMME should provide useful information to analyze and understand the CMIP results for future climate change and help design impacts scenarios and plan adaptation options. 

18:15

Projected Climate Conditions over West Africa for the End of the 21st Century: Impact on Extreme Precipitation Events

M. B. Sylla (WASCAL Competence Center, Ouagadougou, Burkina Faso), D. Wisser (University of Bonn, Bonn, Germany), N. Elguindi (Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy), F. Giorgi (Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy)

Abstract details
Projected Climate Conditions over West Africa for the End of the 21st Century: Impact on Extreme Precipitation Events

MB. Sylla (1) ; D. Wisser (2) ; N. Elguindi (3) ; F. Giorgi (3)
(1) WASCAL Competence Center, Climate Modeling and Climate Change, Ouagadougou, Burkina Faso; (2) University of Bonn, Center for development research (zef), Bonn, Germany; (3) Abdus Salam International Centre for Theoretical Physics (ICTP), Earth system physics (esp) section, Trieste, Italy

Abstract content

Global warming resulting from increased anthropogenic greenhouse gas (GHG) forcing will substantially alter West African regional climate conditions. 21st century projections from both Global Climate Models (GCMs) and Regional Climate Models (RCMs) indicate lower warming along the Gulf of Guinea and orographic zones and greater warming in the Sahel and the Sahara desert. In addition, mean precipitation changes and trends exhibit more complex, seasonally varying spatial patterns from the Gulf of Guinea to the Sahel and from the orographic areas to the flatter regions.

Investigating how these combined temperature and precipitation changes will affect the climate of West Africa requires a multivariate approach. Climate classifications integrate the influence of energy and moisture in order to define the climate of a region, and are thus well-suited tools for this purpose. As climate is the major influence on biological life, such classification is critical for the region, particularly at the time during which the loss of major ecosystems and croplands is a fact. Information on projected climate types over West Africa can thus enable the impact community to develop mitigation strategies and adaptation measures for the most vulnerable areas.

In this context, recent studies identified also substantial increases of very high monthly precipitation and an amplification of daily precipitation extremes by the end of the 21st Century. These increases in extremes are considerably spatially variable over West Africa and mostly driven by an intensification of the local hydrological cycle. However, key sectors and activities in West Africa may be more vulnerable to the seasonal timing of the occurrence of extremes than its yearly average. Estimating and understanding such seasonal and sub-seasonal changes is important for the formulation of adaptation and mitigation strategies. For example if an increase in high intensity rainfall events is concurrent to the peak of the rainy season, this may result in widespread flooding requiring strong responses. As another example, in the case of pre-monsoon high intensity rainfall events, early deployment of flood control measures may be required.

In this study, the revised Thornthwaite climate classification is employed to investigate the shift of West African climatic zones in response to future anthropogenic climate change under two Representative Concentration Pathways (RCP4.5 and RCP8.5) from multiple data sources: ensembles of GCMs from CMIP5, RCM projections from CORDEX and higher-resolution RegCM4 simulations over West Africa. The use of multiple ensembles enables us to assess the robustness of the response, and in particular the extent to which the higher resolution experiments offer added regional details. In addition, the study also examines how such a shift impacts on the timing of extreme events. In particular, we focus on the response to increasing GHG concentrations of the annual cycle of high intensity precipitation events, specifically during the pre-monsoon (April-June), mature monsoon (July-September) and post-monsoon (October-December) seasons. This task is carried out through the analysis of a series of standard indices of precipitation extremes applied to the daily precipitation projections.

The results reveal that West Africa evolves towards increasingly arid and semi-arid regimes with the recession of moist and wet zones, thus adding another element of vulnerability to future anthropogenic climate change for the ecosystems and agricultural lands in the regions.

In addition, analysis of the changes in the annual of high intensity precipitation events indicate that the pre-monsoon season experiences the largest changes in daily precipitation statistics. These changes are particularly towards an i

18:30

Trends in West African floods: A comparative analysis with physiographic indices

N. N. Bernadette (International Institute of Water and Environment Engineering, Ouagadougou, Burkina Faso), H. Karambiri (2IE, Ouagadougou, Burkina Faso), O. Ludovic (UPMC, Paris, France), P. Ribstein (Université Pierre et Marie Curie, Paris, France), J.-E. Paturel (IRD, Abdijan, Ivory Coast)

Abstract details
Trends in West African floods: A comparative analysis with physiographic indices

NN. Bernadette (1) ; H. Karambiri (2) ; O. Ludovic (3) ; P. Ribstein (4) ; JE. Paturel (5)
(1) International Institute of Water and Environment Engineering, Kadiogo, Ouagadougou, Burkina Faso; (2) 2IE, Ouagadougou, Burkina Faso; (3) UPMC, Metis, Paris, France; (4) Université Pierre et Marie Curie, Laboratoire sisyphe, Paris, France; (5) IRD, HydroSciences Montpellier, Abdijan, Ivory Coast

Abstract content

After the drought of the years 1970 In West Africa, the variability of rainfall and land use changes affected mostly flow, and recently, flooding is said to be an increasingly common occurrence throughout the whole of West Africa. These changes aroused many questions about the impact of climate change on the flood regimes in west african countries.

This paper investigates whether floods are becoming more frequent or more severe, and to what extend climate patterns have been responsible of these changes. We analyze the trends in floods of 14 catchments within the main climate zone of West Africa. The methodology includes two types of sampling flood events, namely the AM (Annual maximum) method and the POT (Peak over threshold), and two perspectives of analysis are presented, precisely long term analysis based on two long time series of flood, and regional perspective involving 14 catchments with shorter length of series.

Mann Kendall trend test and Pettitt break test are used to assess the stationarity of the time series. The trends detected in flood time series are compared to the trends of rainfall indices in one hand and vegetation indices in the second hand using contingency tables, in order to identify the main driver of change in flood magnitude and Flood frequency. The dependency between flood index and physiographic index is evaluated through a Success Criterion and the CramerV criterion calculated from the contengecy tables.

The results point out the existence of trends in flood magnitude and flood frequency time series with two main patterns. sahelian flood show increasing trends, and some sudanian catchments present decreasing trends. for the overall studied catchments, the maximum five consecutive days rainfall index (Rx5d) seems to follow the trend of floods, while NDVI indices do not show significant link between with the trends of floods, meaning that this index has no impact in the behaviour of flood in the region.

 

Temperature in the Sahel: mean climate and multidecadal warming in observations and climate simulations CMIP5

F. Guichard (CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France), J. Barbier (CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France), C. Leauthaud (LSCE, CEA, Gif-sur-Yvette, France), L. Kergoat (GET (UMR CNRS 5563), Toulouse, France), D. Bouniol (CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France), F. Couvreux (CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France), B. Diallo (LMD (IPSL, CNRS, UPMC), Paris, France), F. Hourdin (LMD (IPSL, CNRS, UPMC), Paris, France), S. Janicot (LOCEAN (CNRS, IPSL, IRD, UPMC), Paris, France), R. Roehrig (CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France)

Abstract details
Temperature in the Sahel: mean climate and multidecadal warming in observations and climate simulations CMIP5

F. Guichard (1) ; J. Barbier (1) ; C. Leauthaud (2) ; L. Kergoat (3) ; D. Bouniol (1) ; F. Couvreux (1) ; B. Diallo (4) ; F. Hourdin (4) ; S. Janicot (5) ; R. Roehrig (1)
(1) CNRM-GAME (UMR 3589, CNRS and Meteo-France), Toulouse, France; (2) LSCE, CEA, Gif-sur-Yvette, France; (3) GET (UMR CNRS 5563), Toulouse, France; (4) LMD (IPSL, CNRS, UPMC), Paris, France; (5) LOCEAN (CNRS, IPSL, IRD, UPMC), Paris, France

Abstract content

In a companion study (Leauthaud et al.), we show with observations that the Sahel experienced a strong warming since 1950; it is stronger in Spring, which is already climatologically very hot, and more pronounced in nighttime than daytime temperature. This warming is more pronounced that further South in the tropical Soudanian region and increases with latitude from the Gulf of Guinea to the Sahara. Observations also indicate a decrease of the diurnal temperature range (DTR) which is more pronounced outside of the rainy monsoon season.

 

In this study, we address the ability of climate models to simulate these salient observed features. We use outputs from the CMIP5 archive, and in particular amip, control and historical simulations. We also use amip-type cfSites ouptuts, that allows a more in depth understanding of the physics underlying the simulation of temperature with high-frequency energy budgets (when available). Here, we evaluate the simulation of the annual and diurnal cycles, of the distribution of temperature, and their multi-decadal fluctuations and changes.

 

The observations used for evaluation include CRU, GHCN and BEST monthly-mean gridded datasets, but also results from several SYNOP stations located across the Sahel and providing daily minimum and maximum temperature, plus, for some of them, 3-h sampled data. The later are used to compare temperature distributions and distribution shifts in the last decades.

 

First, it appears that the simulation of the annual cycle of temperature is very challenging, especially outside of the monsoon season (cf. also Roehrig et al. J. Climate 2013). Simulations often display phase shifts of several weeks to a few months in their extrema (Spring and Autumn maxima and Summer and Winter minima). Biases typically reach a few to several degrees in their monthly-mean values. In numerous models, compensating errors arising from sub-diurnal to seasonal scales are involved in the representation of the annual cycle. In general, differences among models are also found to dominate over differences among scenarios (amip, historical...).

 

High-frequency observations of the surface meteorology and radiative budget, available over the last decade, point to strong physical couplings between radiative fluxes and thermodynamics, for instance between longwave fluxes, DTR, water vapour, clouds and precipitation. Strong couplings are also found in simulations, but they are quantitatively distinct.

 

The causes and implications of these simulation biases for the study of the Sahelian climate and its future evolution are discussed.