CO 2 physiological effect can cause rainfall decrease as strong as large-scale deforestation in the Amazon
2021
Abstract. The climate in the Amazon region is particularly
sensitive to surface processes and properties such as heat fluxes and
vegetation coverage. Rainfall is a key expression of the land
surface–atmosphere interactions in the region due to its strong dependence
on forest transpiration. While a large number of past studies have shown the
impacts of large-scale deforestation on annual rainfall, studies on the
isolated effects of elevated atmospheric CO 2 concentrations (eCO 2 )
on canopy transpiration and rainfall are scarcer. Here, for the first time,
we systematically compare the plant physiological effects of eCO 2 and
deforestation on Amazon rainfall. We use the CPTEC Brazilian Atmospheric
Model (BAM) with dynamic vegetation under a 1.5 × CO 2 experiment and a
100 % substitution of the forest by pasture grasslands, with all other
conditions held similar between the two scenarios. We find that both
scenarios result in equivalent average annual rainfall reductions
(Physiology: − 257 mm, − 12 %; Deforestation: − 183 mm, − 9 %) that are
above the observed Amazon rainfall interannual variability of 5 %. The
rainfall decreases predicted in the two scenarios are linked to a reduction
of approximately 20 % in canopy transpiration but for different reasons:
the eCO 2 -driven reduction of stomatal conductance drives the change in
the Physiology experiment, and the smaller leaf area index of pasturelands
( − 72 % compared to tropical forest) causes the result in the Deforestation
experiment. The Walker circulation is modified in the two scenarios:
in Physiology due to a humidity-enriched free troposphere with decreased deep
convection due to the heightening of a drier and warmer ( + 2.1 ∘ C) boundary layer, and in Deforestation due to enhanced convection over the Andes and a
subsidence branch over the eastern Amazon without considerable changes in
temperature ( − 0.2 ∘ C in 2 m air temperature and + 0.4 ∘ C in surface temperature). But again, these changes occur through different
mechanisms: strengthened west winds from the Pacific and reduced easterlies
entering the basin affect the Physiology experiment, and strongly increased
easterlies influence the result of the Deforestation experiment. Although
our results for the Deforestation scenario agree with the results of
previous observational and modelling studies, the lack of direct field-based
ecosystem-level experimental evidence regarding the effect of eCO 2 on
moisture fluxes in tropical forests confers a considerable level of
uncertainty to any projections of the physiological effect of eCO 2 on
Amazon rainfall. Furthermore, our results highlight the responsibilities of
both Amazonian and non-Amazonian countries to mitigate potential future
climatic change and its impacts in the region, driven either by local
deforestation or global CO 2 emissions.
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