Influences of the hydrological cycle on observed interannual variations in atmospheric CO18O

• Published in: Journal of Geophysical Research: Biogeosciences 2011-10, Vol.116 (G4), p.n/a
• Main Authors: Buenning, Nikolaus H., Noone, David C., Riley, William J., Still, Christopher J., White, James W. C.
• Format: Article
• Language: English
• Subjects: Air temperature; amount effect; Atmospheric sciences; Biosphere; Carbon dioxide; Climate change; Earth; ecosystem fluxes; Hydrologic cycle; Hydrology; isotope hydrology; Relative humidity; Solar radiation; Tropical environments; tropical precipitation
• ISSN: 0148-0227; 2169-8953; 2156-2202; 2169-8961
• DOI: 10.1029/2010JG001576

The global increase in atmospheric CO2 since the preindustrial era has not been accompanied by a long‐term trend in the CO18O/CO2 ratio (denoted as δCa), though many monitoring stations around the world observed a downward excursion in δCa (of about 0.1‰ y−1) after 1992/1993 that lasted for 5–6 years. A number of studies have suggested that the interannual variation, seasonal cycle, and spatial structure of δCa depend on terrestrial ecosystem fluxes. The δCa budget is evaluated to identify meteorological variables that could potentially influence the observed interannual δCa variations. Using global data sets, time series of relative humidity, 18O composition of precipitation (δWP), air temperature, precipitation, and solar radiation were correlated with Mauna Loa δCa variations. Observed δCa negatively correlated with relative humidity in tropical and middle‐ and high‐latitude regions, and we estimated that observed relative humidity variations drove a 0.14‰ decrease in δCa during the mid‐1990s. Similar variations in precipitation rates were also found within the tropics that suggest positive correlations between δWP and δCa consistent with an amount effect (δWP decreases as precipitation increases). The decrease in δWP during the 1990s was estimated to decrease δCa by as much as 0.48‰. In contrast to previous work, little evidence was found that would suggest carbon flux anomalies as the primary driver of interannual δCa variations. We conclude that interannual δCa variations were driven primarily by isotope hydrology and relative humidity, and that δCa can be exploited as a new constraint on hydrological cycle variations at a variety of scales. Key Points Interannual δCa variations were driven by isotope hydrology and RH No evidence of carbon flux anomalies driving interannual δCa variations was found