Is interannual fluctuation of atmospheric CO2 dominated by combined effects of ENSO and volcanic aerosols?



[1] The influence of ENSO, volcanic eruptions, and the North Atlantic Oscillation (NAO) on fluctuations of the atmospheric CO2 concentration were globally investigated on the ENSO timescale for the period 1958 to 1994. Two deseasonalized and detrended monthly time series of anomalous CO2 fluxes were generated: (1) modeled terrestrial biospheric CO2 flux anomalies calculated by the High Resolution Biosphere Model (HRBM) driven by realistic temperature and precipitation data but without considering radiation effects and (2) anomalous CO2 fluxes deduced from atmospheric measurements. While comparing the two time series, three types of periods could be distinguished: “a” periods with a phase shift close to zero between the two series, “b” periods with a phase shift of up to 11 months, and “c” periods with the two curves completely out of phase. During the “c” periods the modeled data show carbon release by the biosphere while the observed data show carbon uptake. The periods of type “c” are accompanied by major volcanic eruptions with considerable aerosol production. Enhanced atmospheric aerosol loading can increase the diffuse fraction of the solar radiation. Diffuse radiation in many cases penetrates better into plant canopies thus potentially enhancing photosynthesis. The resulting additional uptake of carbon may have overcompensated the carbon release caused by ENSO effects thus possibly bringing about a net uptake of CO2. The length variations of the time lags (“b” periods) may be attributed to impacts of vegetation fires. The influence of the NAO remained unclear.