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Weizmann Institute of Science (2015)

The climate effect of large-scale afforestation in semi-arid regions

Gil Yosef

Titre : The climate effect of large-scale afforestation in semi-arid regions

Auteur : Gil Yosef

Etablissement de soutenance : Weizmann Institute of Science

Grade : Doctor of Philosophy (PhD) 2015

Semi-arid regions are transition areas between the desert and temperate climates, and are sensitive to climate change. This study tested the hypothesis that large-scale afforestation in these semi-arid regions can result in changes in local and regional atmospheric circulation and, in turn, in the amount of precipitation. The parameterization of the afforested area was based on the extensive data from a 15-year study of the semi-arid Yatir Forest in Israel. We tested our hypothesis using the Global Climate Model (GCM) Ocean-Land-Atmosphere Model (OLAM), by simulating the effects of afforestation in the Sahel region (2.6 E6 km2 ) and North Australia (2.1 E6 km2 ). We carried out 17 years of simulations (1996-2012) with a 200 km horizontal grid scale and six years of simulations (2000-06) with a 50 km horizontal grid scale of afforestation scenarios. We chose the Sahel and North Australia (N-Aust) as our study regions because in these regions the summer solar heating leads to migration of the equatorial trough and the tropical convergence zones (ITCZ), leading to monsoon rains in some parts, and to a meridional surface temperature gradient that generates a low-level easterly jet that acts as a barrier to the penetration of moisture. The results focused on the summer rainy season (Sahel : Jul-Sep. and N-Aust : Jan-Mar) and showed that forestation of the native shrubland increased root depth, soil water content and evapotranspiration, and led to lower surface temperature over the forest (Sahel : 1.5˚C and N-Aust : 1˚C), and decreased the meridional temperature gradient. This led to weakening and displacement of the low-level thermal wind responsible for the generation of the easterly jet and, consequently, to increased precipitation (Sahel : 30 mm pre month and N-Aust : 15 mm per month) over most of the forest area, but also over a strip of 2.6 E6 km2 in the Sahel region and 0.5 E6 km2 in North Australia along the northern perimeter of the afforestation area. Ocean contribution to the precipitation over the Sahel region had positive feedback because of the higher sea-surface-temperature (SST) of the northern hemisphere during summertime. This study demonstrated the potential effects of afforestation in semi-arid regions on the local and regional hydrological cycle, and the importance of monsoon systems and consistent seasonality of the relevant ocean

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