[A-CG09] Development and application of land and ocean biogeochemistry components of Earth system models

[ACG09-08] Current state of terrestrial CO₂ exchange estimations: progresses and remaining issues

*Masayuki Kondo¹, Kazuhito Ichii¹, Prabir Patra¹, Tazu Saeki¹ (1.Department of Environmental Geochemical Cycle Research, Japan Agency for Marine-Earth Science and Technology)

Keywords:Terrestrial CO2 exchange, Atmospheric CO2 inversion, Ecosystem model simulation

Terrestrial ecosystems play a critical role in formation of a feedback loop of carbon dioxide (CO₂) in atmosphere with atmospheric reservoir and climate, and thus directing a course of the future projection of climate change. The research community has spent significant efforts to understand behaviors of terrestrial ecosystems under a steady rise in atmospheric CO₂ concentration and temperature during the recent decades and deepen knowledge about the regional and global patterns of terrestrial CO₂ sinks and sources. estimate the terrestrial CO₂ exchange, while seeking consistency between simulated and observed CO₂ concentrations. The bottom-up approach estimates the terrestrial CO₂ exchange using ecosystem models, which simulate the ecosystem-scale carbon cycle by considering the internal biogeochemical mechanisms of carbon flows for each prescribed vegetation type and soil.
However, the current estimates of terrestrial CO₂ exchange by the bottom-up and top-down approaches remain inconsistent. As illustrated in the recent IPCC Assessment Report (AR5), the top-down approach tends to indicate stronger CO₂ sinks in temperate and boreal regions than the bottom-up approach does. Furthermore, the two approaches exhibited contrasting CO₂ sink–source patterns in the tropics; the bottom-up approach indicated CO₂ sinks and the top-down approach CO₂ sources. As illustrated by these inconsistencies, a consensus on the geographic distribution of the terrestrial CO₂ exchange has yet to be established among the research community.
In this study, we elaborate the current status and issues of terrestrial CO₂ flux estimations by the top-down and bottom-up approaches. Specifically, we compare the bottom-up estimate from dynamic global vegetation models that are forced by interannual variations of CO₂ concentration, climate and land use changes, with the top-down estimate from atmospheric CO₂ inversions. We show an improved level of agreement between the two estimates in relation to seasonal variability and, regional and global budgets, since the IPCC AR5. We also discuss the remaining issues causing inconsistency between the two estimates.

Acknowledgments
This research was supported by Environment Research and Technology Development Funds (2-1401) from the Ministry of the Environment of Japan and Asia-Pacific Network for Global Change Research (APN: grant#ARCP2011-11NMY-Patra/Canadell).