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5074282 
Journal Article 
Large but decreasing effect of ozone on the European carbon sink 
Oliver, RJ; Mercado, LM; Sitch, S; Simpson, D; Medlyn, BE; Lin, YanS; Folberth, GA 
2018 
Yes 
Biogeosciences
ISSN: 1726-4170
EISSN: 1726-4189 
15 
13 
4245-4269 
English 
WOS:000438539000001 
The capacity of the terrestrial biosphere to sequester carbon and mitigate climate change is governed by the ability of vegetation to remove emissions of CO2 through photosynthesis. Tropospheric O-3, a globally abundant and potent greenhouse gas, is, however, known to damage plants, causing reductions in primary productivity. Despite emission control policies across Europe, background concentrations of tropospheric O-3 have risen significantly over the last decades due to hemispheric-scale increases in O-3 and its precursors. Therefore, plants are exposed to increasing background concentrations, at levels currently causing chronic damage. Studying the impact of O-3 on European vegetation at the regional scale is important for gaining greater understanding of the impact of O-3 on the land carbon sink at large spatial scales. In this work we take a regional approach and update the JULES land surface model using new measurements specifically for European vegetation. Given the importance of stomatal conductance in determining the flux of O-3 into plants, we implement an alternative stomatal closure parameterisation and account for diurnal variations in O-3 concentration in our simulations. We conduct our analysis specifically for the European region to quantify the impact of the interactive effects of tropospheric O-3 and CO2 on gross primary productivity (GPP) and land carbon storage across Europe. A factorial set of model experiments showed that tropospheric O-3 can suppress terrestrial carbon uptake across Europe over the period 1901 to 2050. By 2050, simulated GPP was reduced by 4 to 9 % due to plant O-3 damage and land carbon storage was reduced by 3 to 7 %. The combined physiological effects of elevated future CO2 (acting to reduce stomatal opening) and reductions in O-3 concentrations resulted in reduced O-3 damage in the future. This alleviation of O-3 damage by CO2-induced stomatal closure was around 1 to 2 % for both land carbon and GPP, depending on plant sensitivity to O-3. Reduced land carbon storage resulted from diminished soil carbon stocks consistent with the reduction in GPP. Regional variations are identified with larger impacts shown for temperate Europe (GPP reduced by 10 to 20 %) compared to boreal regions (GPP reduced by 2 to 8 %). These results highlight that O-3 damage needs to be considered when predicting GPP and land carbon, and that the effects of O-3 on plant physiology need to be considered in regional land carbon cycle assessments.