Abstract
Abstract. Diffuse light conditions can increase the efficiency of photosynthesis and carbon uptake by vegetation canopies. The diffuse fraction of photosynthetically active radiation (PAR) can be affected by either a change in the atmospheric aerosol burden and/or a change in cloudiness. During the dry season, a hotspot of biomass burning on the edges of the Amazon rainforest emits a complex mixture of aerosols and their precursors and climate-active trace gases (e.g. CO2, CH4, NOx). This creates potential for significant interactions between chemistry, aerosol, cloud, radiation and the biosphere across the Amazon region. The combined effects of biomass burning on the terrestrial carbon cycle for the present day are potentially large, yet poorly quantified. Here, we quantify such effects using the Met Office Hadley Centre Earth system model HadGEM2-ES, which provides a fully coupled framework with interactive aerosol, radiative transfer, dynamic vegetation, atmospheric chemistry and biogenic volatile organic compound emission components. Results show that for present day, defined as year 2000 climate, the overall net impact of biomass burning aerosols is to increase net primary productivity (NPP) by +80 to +105 TgC yr−1, or 1.9 % to 2.7 %, over the central Amazon Basin on annual mean. For the first time we show that this enhancement is the net result of multiple competing effects: an increase in diffuse light which stimulates photosynthetic activity in the shaded part of the canopy (+65 to +110 TgC yr−1), a reduction in the total amount of radiation (−52 to −105 TgC yr−1) which reduces photosynthesis and feedback from climate adjustments in response to the aerosol forcing which increases the efficiency of biochemical processes (+67 to +100 TgC yr−1). These results illustrate that despite a modest direct aerosol effect (the sum of the first two counteracting mechanisms), the overall net impact of biomass burning aerosols on vegetation is sizeable when indirect climate feedbacks are considered. We demonstrate that capturing the net impact of aerosols on vegetation should be assessed considering the system-wide behaviour.
Highlights
The Amazon rainforest is the largest expanse of tropical forest on Earth
The standard configuration of HadGEM2-ES that participated in CMIP5 had a global gross primary productivity (GPP) of the order of +140 PgC yr−1 for present-day conditions (Shao et al, 2013)
From our model experiments we concluded that the diffuse photosynthetically active radiation (PAR) fertilisation effect from biomass burning aerosols in HadGEM2-ES (Sect. 3.3) is comparatively modest, amounting to between +13 and +5 TgC yr−1 based on the result from the simulations BBAx1 and BBAx2
Summary
The Amazon rainforest is the largest expanse of tropical forest on Earth. It provides invaluable ecological services and plays a major role in the Earth system and climate (Malhi et al, 2008). The Amazon rainforest is a net sink of atmospheric CO2, drought frequency and intensity, which are expected to increase in the future, could have severe consequences for future forest resilience and potentially shift the Amazon rainforest from a sink to a net source of atmospheric CO2 Malavelle et al.: Aerosol radiative and climatic effects on the Amazon rainforest
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