An assessment of the carbon balance of Arctic tundra: comparisons among observations, process models, and atmospheric inversions

[摘要] Although Arctic tundra has been estimated to cover only 8% of the globalland surface, the large and potentially labile carbon pools currently storedin tundra soils have the potential for large emissions of carbon (C) under awarming climate. These emissions as radiatively active greenhouse gases inthe form of both CO₂ and CH₄ could amplify global warming. Giventhe potential sensitivity of these ecosystems to climate change and theexpectation that the Arctic will experience appreciable warming over the nextcentury, it is important to assess whether responses of C exchange in tundraregions are likely to enhance or mitigate warming. In this study we comparedanalyses of C exchange of Arctic tundra between 1990 and 2006 amongobservations, regional and global applications of process-based terrestrialbiosphere models, and atmospheric inversion models. Syntheses of fluxobservations and inversion models indicate that the annual exchange ofCO₂ between Arctic tundra and the atmosphere has large uncertaintiesthat cannot be distinguished from neutral balance. The mean estimate from anensemble of process-based model simulations suggests that Arctic tundra has actedas a sink for atmospheric CO₂ in recent decades, but based on theuncertainty estimates it cannot be determined with confidence whether theseecosystems represent a weak or a strong sink. Tundra was 0.6 °Cwarmer in the 2000s compared to the 1990s. The central estimates of theobservations, process-based models, and inversion models each identifystronger sinks in the 2000s compared with the 1990s. Some of the processmodels indicate that this occurred because net primary production increasedmore in response to warming than heterotrophic respiration. Similarly, theobservations and the applications of regional process-based models suggestthat CH₄ emissions from Arctic tundra have increased from the 1990s to2000s because of the sensitivity of CH₄ emissions to warmertemperatures. Based on our analyses of the estimates from observations,process-based models, and inversion models, we estimate that Arctic tundrawas a sink for atmospheric CO₂ of 110 Tg C yr⁻¹ (uncertaintybetween a sink of 291 Tg C yr⁻¹ and a source of 80 Tg C yr⁻¹)and a source of CH₄ to the atmosphere of 19 Tg C yr⁻¹(uncertainty between sources of 8 and 29 Tg C yr⁻¹). The suite ofanalyses conducted in this study indicate that it is important to reduceuncertainties in the observations, process-based models, and inversions inorder to better understand the degree to which Arctic tundra is influencingatmospheric CO₂ and CH₄ concentrations. The reduction ofuncertainties can be accomplished through (1) the strategic placement of moreCO₂ and CH₄ monitoring stations to reduce uncertainties ininversions, (2) improved observation networks of ground-based measurements ofCO₂ and CH₄ exchange to understand exchange in response todisturbance and across gradients of climatic and hydrological variability,and (3) the effective transfer of information from enhanced observationnetworks into process-based models to improve the simulation of CO₂ andCH₄ exchange from Arctic tundra to the atmosphere.

[学科分类] 地球化学与岩石