EMPIRICAL TEMPERATURE-BASED ESTIMATES OF VARIABILITY IN THE OCEANIC UPTAKE OF CO2 OVER THE PAST 2 DECADES

Abstract

[ 1] We infer the year-to-year variability of net global air-sea CO2 fluxes from observed interannual changes in wind speed and estimated differences in CO2 partial pressure between surface seawater (pCO(2SW)) and the overlying atmosphere. Changes in pCO(2SW) are estimated from changes in sea surface temperature via seasonal algorithms that relate pCO(2SW) to sea surface temperature. Our diagnostic model yields an interannual variability of +/- 0.18 petagrams (1 sigma, Pg = 10(15) grams) of carbon per year for the period 1982 - 2001. El Nino Southern Oscillation - induced changes in the equatorial efflux contribute approximately 70% of the diagnostic modeled global variability. Regional flux anomalies for areas outside the equatorial Pacific are found to neither systematically reinforce nor counteract each other during times of transition from El Nino years to normal years. The interannual variability of +/- 0.18 Pg C yr(-1) obtained in the present work is at the low end of previous estimates that falls in the range of +/- 0.2 to +/- 0.5 Pg C yr(-1). Of the previous estimates, lower values are generally estimated from global ocean circulation - biogeochemical models, while higher values are derived from atmospheric inversion models constrained by atmospheric CO2 observations. Comparisons of our modeled results with two time series data sets and equatorial Pacific data suggest that our diagnostic model is not able to capture the full range of pCO(2SW) variations; this is probably due to the inability of the empirical model to fully account for changes in surface pCO(2SW) related to ocean biological and physical processes. The small interannual variability in our modeled fluxes suggests that observed year-to-year variations in the rate of atmospheric CO2 increase are primarily caused by changes in the rate of CO2 uptake by the land biosphere.