Abstract
Abstract. A global Earth system model is applied to quantify the impacts of direct anthropogenic aerosol effective radiative forcing on gross primary productivity (GPP) and isoprene emission. The impacts of different pollution aerosol sources (anthropogenic, biomass burning, and non-biomass burning) are investigated by performing sensitivity experiments. The model framework includes all known light and meteorological responses of photosynthesis, but uses fixed canopy structures and phenology. On a global scale, our results show that global land carbon fluxes (GPP and isoprene emission) are not sensitive to pollution aerosols, even under a global decline in surface solar radiation (direct + diffuse) by ∼ 9 %. At a regional scale, GPP and isoprene emission show a robust but opposite sensitivity to pollution aerosols in regions where forested canopies dominate. In eastern North America and Eurasia, anthropogenic pollution aerosols (mainly from non-biomass burning sources) enhance GPP by +5–8 % on an annual average. In the northwestern Amazon Basin and central Africa, biomass burning aerosols increase GPP by +2–5 % on an annual average, with a peak in the northwestern Amazon Basin during the dry-fire season (+5–8 %). The prevailing mechanism varies across regions: light scattering dominates in eastern North America, while a reduction in direct radiation dominates in Europe and China. Aerosol-induced GPP productivity increases in the Amazon and central Africa include an additional positive feedback from reduced canopy temperatures in response to increases in canopy conductance. In Eurasia and northeastern China, anthropogenic pollution aerosols drive a decrease in isoprene emission of −2 to −12 % on an annual average. Future research needs to incorporate the indirect effects of aerosols and possible feedbacks from dynamic carbon allocation and phenology.
Highlights
Terrestrial gross primary productivity (GPP), the amount of carbon dioxide (CO2) taken up every year from the atmosphere by plant photosynthesis, is the largest single flux in the carbon cycle and plays a major role in global climate change
The total effective radiative forcing (ERF) is computed in NASA ModelE2-Yale Interactive Terrestrial Biosphere Model (YIBs) as the arithmetic mean of all anthropogenic aerosol components
We isolated the role of pollution aerosol sources
Summary
Terrestrial gross primary productivity (GPP), the amount of carbon dioxide (CO2) taken up every year from the atmosphere by plant photosynthesis, is the largest single flux in the carbon cycle and plays a major role in global climate change. Terrestrial vegetation provides the main source of isoprene to the atmosphere, which controls the loading of multiple short-lived climate pollutants and greenhouse gases (ozone, methane, secondary aerosols). Isoprene production is closely linked to plant photosynthesis (Pacifico et al, 2009; Unger et al, 2013). Both GPP and isoprene emission may be influenced by a change in surface solar radiation (SSR, the sum of the direct and diffuse radiation incident on the surface) and surface atmospheric temperature (SAT). Aerosols may attenuate indirectly SSR by acting as cloud condensation nuclei, perturbing cloud cover and cloud properties (Rosenfeld et al, 2008)
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