Abstract. Atmospheric variations of carbon dioxide (CO 2 ) 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 CO 2 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 CO 2 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 CO 2 mole fraction, t CO 2 , that is a crude proxy for changes in CO 2 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 t CO 2 . 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 t CO 2 we determine estimates for the timing of the onset of net uptake and release of CO 2 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 CO 2 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.
Abstract. Atmospheric variations of carbon dioxide (CO2) mole fraction reflect changes in atmospheric transport and regional patterns of surface emission and uptake. We report new estimates for changes in the phase and amplitude of observed high northern latitude CO2 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 time derivative of CO2 mole fraction, ΔtCO2, 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 ΔtCO2. 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 ΔtCO2 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 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% (p< 0.01) and 0.42 ± 0.34% (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, but changes in the peak uptake and release together with an invariant growing period length provides indirect evidence that high northern latitude ecosystems are progressively taking up more carbon.
Atmospheric transport of midlatitude pollutant emissions to the Arctic can result in disproportionate impacts on the receptor region. We use carbon monoxide (CO), a tracer of incomplete combustion, to study changes in pollutant transport to the Arctic. Using a wavelet transform, we spectrally decompose CO mole fraction measurements from three Arctic sites (Alert, Barrow, and Zeppelin) collected by NOAA over the past 20–25 years. We show that CO concentrations have decreased by −1.0 to −1.2 ppb/yr. We find that the dampened seasonal cycle (−1.2 to −2.3 ppb/yr) is mostly due to a reduction in peak concentrations (−1.5 to −2.4 ppb/yr), which we attribute to reduced source emissions. We find no evidence to support a persistent increase in hydroxyl radical concentration. Using the GEOS‐Chem global 3‐D chemistry transport model, we show that observed decreases are consistent with reductions in fossil fuel usage from Europe and North America.
Abstract. Observed variations of the atmospheric greenhouse gas methane (CH 4 ) over the past two decades remain the subject of debate. These variations reflect changes in emission, uptake, and atmospheric chemistry and transport. We isolate changes in the seasonal cycle of atmospheric CH 4 using a wavelet transform. We report a previously undocumented persistent decrease in the peak-topeak amplitude of the seasonal cycle of atmospheric CH 4 at six out of seven high northern latitude 5 sites over the past two to three decades. The observed amplitude changes are statistically significant for sites at Barrow, Alaska and Ocean Station M, Norway, which we find are the most sensitive of our sites to high northern latitude wetland emissions. We find using a series of numerical experiments using the TM5 atmospheric chemistry transport model that increasing wetland emissions and/or decreasing fossil fuel emissions can explain these observed changes, but no significant role for trends 10 in meteorology and tropical wetlands. We also find no evidence in past studies to support a significant role for variations in the hydroxyl radical sink of atmospheric CH 4 . Using the TM5 model we find that changes in fossil fuel emissions of CH 4 , described by a conservative state-of-the-science bottomup emission inventory, are not sufficient to reconcile observed changes in atmospheric CH 4 at these sites. The remainder of the observed trend in amplitude, by process of elimination, must be due to 15 an increase in high northern latitude wetland emissions, corresponding to an annual increase of at least 0.7%/yr (equivalent to 5 Tg CH 4 /yr over 30 years).
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