|Review of “Computation and analysis of atmospheric carbon dioxide annual mean growth rates from satellite observations during 2003-2016” by Buchwitz et al.|
The authors presented a new Level 3 XCO2 product, based on data from SCIAMACHY and GOSAT, and examined the atmospheric growth rate of CO2 captured by these data. They showed that the annual mean CO2 growth rate estimated from the XCO2 data is consistent with that estimated from the NOAA surface CO2 data. They also did an analysis to determine the relative contributions of ENSO and anthropogenic CO2 emissions to variations in the annual mean CO2 growth rate. The new Level 3 data will be a useful product for the community since working with Level 2 data can be challenging. The fact that the XCO2-based CO2 growth rate is consistent with that estimated from the surface data is reassuring. However, I cannot recommend the manuscript for publication in ACP. I do not believe that the manuscript contains sufficiently new scientific results to warrant publication in ACP.
In responding to the previous reviews, the authors described what new knowledge is contained in the manuscript. They stated that:
• "We present a new global total column CO2 (“XCO2”) data set (based on satellite data) covering 14 years
• We present a new method to compute annual mean XCO2 growth rates from this data set
• We present a new annual mean CO2 growth rate time series (covering the entire atmosphere, not only near-surface CO2) including a comparison with growth rates from NOAA based on surface CO2 observations; we find agreement within the reported uncertainty ranges and therefore consider our growth rates to be validated
• We present an answer to the question “Assuming that the variability of the CO2 growth rate is dominated by ENSO and by human emissions, which of the two considered causes dominates the growth rate variability given the satellite-derived growth rates and their uncertainty?” To answer this question we used a statistical analysis method, which we clearly explain. Our answer is given in the Conclusions section: “Our analysis also shows that the ENSO impact on CO2 growth rate variations dominates over that of human emissions throughout the period 2003-2016 (14 years) but in particular during the period 2010-2016 (second half of the investigated time period) due to strong La Niña and El Niño events. We estimate the probability that the impact of ENSO on the variability is larger than the impact of human emissions to be 63% for the time period 2003-2016. If the time period is restricted to 2010-2016 this probability increases to 94%.”
However, only the fourth bullet contains any new science results, and this is minimal. It is generally accepted that natural variability in the tropics is the main driver of the atmospheric growth rate, and ENSO is the dominant source of tropical variability. As noted in the IPCC AR5, “the causes of the year-to-year variability observed in the annual atmospheric CO2 accumulation … are estimated with a medium to high confidence to be mainly driven by terrestrial processes occurring in tropical latitudes as inferred from atmospheric CO2 inversions and supported by ocean data and models.” Of course, there is a need for attribution studies to better understand the processes driving interannual variability, but the simple analysis presented in this manuscript does not represent substantial new knowledge. It was suggested by Referee #2 that the authors consider publishing in Atmospheric Measurement Techniques (AMT), and I would agree with that suggestion. Indeed, the first three bullets describing the new knowledge in the manuscript suggest that the manuscript would be better suited for AMT. In its current form, I believe that the manuscript would be a good, short AMT paper. If the authors insist on publishing in ACP, they need to significantly expand the scope of the growth rate analysis, and perhaps include a model to help with the attribution analysis.
Page 4, line 23: This line mentions the “(ii) the spatial variability of the XCO2 within the selected region.” What region? Is this referring to the analysis of the different latitude bands that was removed?