Characterizing Carbon-Dioxide Fluxes from Oceans and Terrestrial Ecosystems

• Main Author: Krakauer, Nir Yitzhak
• Format: Thesis
• Language: English
• Published: California Institute of Technology 2006
• Subjects: interannual variability; carbon sink; climate change; biogeochemistry; air-sea gas exchange; Geochemistry; FOS Earth and related environmental sciences; greenhouse gases; Southern Ocean
• Online Access: https://dx.doi.org/10.7907/8b12-1g19; https://resolver.caltech.edu/CaltechETD:etd-05262006-111949

Understanding the processes that change the amount of carbon stored in the ocean and in the land biota, with their implications for future climate and ecology, is a fundamental goal of earth-system science. I have developed, refined, and applied several approaches that combine data analysis and modeling to better understand processes affecting carbon fluxes. (1) Using a database of tree-ring widths from some 40,000 trees, I looked at the impact of large volcanic eruptions in the past millennium on tree growth globally. I found a decline in growth north of 45° N lasting for several years after eruptions, presumably due to eruption-associated cooling, and no significant impact at lower latitudes. This argues against the hypothesis that the increased diffuse-light fraction due to volcanic aerosols greatly increased plant carbon uptake after the 1991 Pinatubo eruption, suggesting that other explanations are needed for the slow increase in atmospheric CO2 levels in the early 1990s. (2) I applied generalized cross-validation (GCV) to the problem of estimating a regional CO2 source/sink pattern consistent with observed geographic variation in atmosphere CO2 levels. I showed that GCV works for selecting data and regional-flux uncertainty levels to assume for this inverse problem; these have usually been estimated rather arbitrarily, though they can have a large impact on the solution. (3) The air-sea gas transfer velocity determines how fast the surface ocean adjusts to a change in atmospheric composition, and hence is important for understanding ocean CO2 uptake. By modeling the ocean’s adjustment to fluctuations in atmospheric carbon isotope composition and analyzing a variety of atmosphere and ocean bomb-14C and 13C measurements, I estimated regional and global mean gas transfer velocities, concluding that there may be less latitudinal variation in the gas transfer velocity than usually thought – implying, for example, relatively low CO2 uptake in the Southern Ocean.