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Doctorat
Israel
The climate effect of large-scale afforestation in semi-arid regions
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
Résumé
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
Page publiée le 11 mai 2022