The regional impact of Land-Use Land-cover Change (LULCC) over West Africa from an ensemble of global climate models

Land surface processes have been identified as having a significant influence on the West African climate. Numerous studies have reported a significant increase in anthropogenic pressure on the already limited natural resources in this region, notably in terms of land use conversion and degradation, so there is a pressing need to understand potential feedbacks with the atmosphere. Several recent multi-global climate model (GCM) efforts have been initiated to better understand the possible impacts of LULCC. However, considerable inter-model differences in terms of the response exist, the most likely reasons are differences in the land surface model physics, the strength of the land-atmosphere coupling and how LULCC impacted a particular model's parameters. One of the main goals of the West African Monsoon Modeling and Evaluation project (WAMMEII) is to provide basic understanding of LULCC on the regional water cycle over West Africa, and to evaluate the sensitivity of the seasonal and decadal variability of the WAM to LULCC. The strategy is to apply observational data-based anomaly forcing in GCM and regional climate model (RCM) simulations over a 5-year period.  To a first order, degradation decreases radiative forcing via an increase in albedo which reduces net surface radiation leading to a reduction in the surface flux of moist static energy thereby impacting the lower level circulation and moist convection. The results reveal that by imposing a maximum 30 % LULCC within an approximate east-west band covering the Sahel (10-20N), there is a substantial and relatively consistent overall drought effect characterized by a 20-40% reduction across the models in annual rainfall. In terms of the dynamic response, the low level African Easterly Jet  is shifted towards the south and intensified  for all of the models averaged over the monsoon season.  The multi-model monsoon season average surface latent heat flux is decreased by upwards of 20 W m-2 along the southern extent of the Sahel (10N), with weaker increases in sensible heat flux (5 to 10 W m-2) in the same region. Compared to previous studies, the impact of LULCC is found to be stronger and a more consistent between the models. This is likely related to our emphasis on prioritizing a consistent impact on the land surface characteristics over a consistent change in land use. The relative consistency among models suggests that a similar strategy as used herein to impose land degradation be adopted in future multi-model LULCC experiments.

Aaron Boone

 

Con traducción

<