Analysis of CO₂ mole fraction data: first evidence of large-scale changes in CO₂ uptake at high northern latitudes

Abstract. Atmospheric variations of carbon dioxide (CO₂) mole fraction reflect changes in atmospheric transport and regional patterns of surface emission and uptake. Here we present a study of changes in the observed high northern latitude CO₂ seasonal cycle. We report new estimates for changes in the phase and amplitude of the seasonal variations, indicative of biospheric changes, by spectrally decomposing multi-decadal records of surface CO₂ mole fraction using a wavelet transform to isolate the changes in the observed seasonal cycle. We also perform similar analysis of the first derivative of CO₂ mole fraction, Δ_tCO₂, that is a crude proxy for changes in CO₂ flux. Using numerical experiments, we quantify the aliasing error associated with independently identifying trends in phase and peak uptake and release to be 10–25 %, with the smallest biases in phase associated with the analysis of Δ_tCO₂. We report our analysis from Barrow, Alaska (BRW), during 1973–2013, which is representative of the broader Arctic region. We determine an amplitude trend of 0.09 ± 0.02 ppm yr^-1, which is consistent with previous work. Using Δ_tCO₂ we determine estimates for the timing of the onset of net uptake and release of CO₂ of −0.14 ± 0.14 and −0.25 ± 0.08 days yr^-1 respectively and a corresponding net uptake period of −0.11 ± 0.16 days yr^-1, which are significantly different to previously reported estimates. We find that the wavelet transform method has significant skill in characterizing changes in the peak uptake and release. We find a trend of 0.65 ± 0.34 % yr^-1 (p < 0.01) and 0.42 ± 0.34 % yr^-1 (p < 0.05) for rates of peak uptake and release respectively. Our analysis does not provide direct evidence about the balance between uptake and release of carbon when integrated throughout the year, but the increase in the seasonal amplitude of CO₂ together with an invariant net carbon uptake period provides evidence that high northern latitude ecosystems are progressively taking up more carbon during spring and early summer.