Comment on essd-2020-307

<strong class="journal-contentHeaderColor">Abstract.</strong> <span id="page3610"/>Methane (CH<span class="inline-formula">₄</span>) emissions from natural landscapes constitute roughly half of global CH<span class="inline-formula">₄</span> contributions to the atmosphere, yet large uncertainties remain in the absolute magnitude and the seasonality of emission quantities and drivers. Eddy covariance (EC) measurements of CH<span class="inline-formula">₄</span> flux are ideal for constraining ecosystem-scale CH<span class="inline-formula">₄</span> emissions due to quasi-continuous and high-temporal-resolution CH<span class="inline-formula">₄</span> flux measurements, coincident carbon dioxide, water, and energy flux measurements, lack of ecosystem disturbance, and increased availability of datasets over the last decade. Here, we (1) describe the newly published dataset, FLUXNET-CH<span class="inline-formula">₄</span> Version 1.0, the first open-source global dataset of CH<span class="inline-formula">₄</span> EC measurements (available at <span class="uri">https://fluxnet.org/data/fluxnet-ch4-community-product/</span>, last access: 7 April 2021). FLUXNET-CH<span class="inline-formula">₄</span> includes half-hourly and daily gap-filled and non-gap-filled aggregated CH<span class="inline-formula">₄</span> fluxes and meteorological data from 79 sites globally: 42 freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drained ecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we (2) evaluate FLUXNET-CH<span class="inline-formula">₄</span> representativeness for freshwater wetland coverage globally because the majority of sites in FLUXNET-CH<span class="inline-formula">₄</span> Version 1.0 are freshwater wetlands which are a substantial source of total atmospheric CH<span class="inline-formula">₄</span> emissions; and (3) we provide the first global estimates of the seasonal variability and seasonality predictors of freshwater wetland CH<span class="inline-formula">₄</span> fluxes. Our representativeness analysis suggests that the freshwater wetland sites in the dataset cover global wetland bioclimatic attributes (encompassing energy, moisture, and vegetation-related parameters) in arctic, boreal, and temperate regions but only sparsely cover humid tropical regions. Seasonality metrics of wetland CH<span class="inline-formula">₄</span> emissions vary considerably across latitudinal bands. In freshwater wetlands (except those between 20<span class="inline-formula">^∘</span> S to 20<span class="inline-formula">^∘</span> N) the spring onset of elevated CH<span class="inline-formula">₄</span> emissions starts 3 d earlier, and the CH<span class="inline-formula">₄</span> emission season lasts 4 d longer, for each degree Celsius increase in mean annual air temperature. On average, the spring onset of increasing CH<span class="inline-formula">₄</span> emissions lags behind soil warming by 1 month, with very few sites experiencing increased CH<span class="inline-formula">₄</span> emissions prior to the onset of soil warming. In contrast, roughly half of these sites experience the spring onset of rising CH<span class="inline-formula">₄</span> emissions prior to the spring increase in gross primary productivity (GPP). The timing of peak summer CH<span class="inline-formula">₄</span> emissions does not correlate with the timing for either peak summer temperature or peak GPP. Our results provide seasonality parameters for CH<span class="inline-formula">₄</span> modeling and highlight seasonality metrics that cannot be predicted by temperature or GPP (i.e., seasonality of CH<span class="inline-formula">₄</span> peak). FLUXNET-CH<span class="inline-formula">₄</span> is a powerful new resource for diagnosing and understanding the role of terrestrial ecosystems and climate drivers in the global CH<span class="inline-formula">₄</span> cycle, and future additions of sites in tropical ecosystems and site years of data collection will provide added value to this database. All seasonality parameters are available at <a href="https://doi.org/10.5281/zenodo.4672601">https://doi.org/10.5281/zenodo.4672601</a> (Delwiche et al., 2021). Additionally, raw FLUXNET-CH<span class="inline-formula">₄</span> data used to extract seasonality parameters can be downloaded from <span class="uri">https://fluxnet.org/data/fluxnet-ch4-community-product/</span> (last access: 7 April 2021), and a complete list of the 79 individual site data DOIs is provided in Table 2 of this paper.