I think there are two separate claims running in this thread - that methane can't meaningfully reach the surface, as proved by many scientific papers; and that the scale of the methane seepage does not have a drastic effect on the global scale.
Based on Dr. Semiletov's evidence, the first claim is false. The second is true for now.
Oren,
You can't say the first claim is false. There have been many field expeditions to the Arctic to test whether the atmosphere above the methane seeps is higher in methane concentration than background and they've' found that there are slightly elevated readings near the seeps, but the methane concentration quickly fades to the background levels within a few 100 meters of where the methane breaks the water surface.
Here's one from 2016, reporting on measurements taken in the Laptev and Eastern Siberian Seas (over the ESAS) in 2014:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL068977The methodological approach in this paper (high‐resolution simultaneous continuous measurements in surface waters and in the air) allows us to test two recently proposed hypotheses about sea‐air exchange of CH4 across the ESAS. The proposed extensive bubble flux from subsea CH4 seeps to the atmosphere could not be validated for the middle and outer ESAS waters >35 m depth. Neither widespread strong enhancements of ambient CH4 nor strong near‐surface gradients could be identified. Our high‐resolution data set rather suggests a spatially limited effect of gas seep bubbles on atmospheric CH4 mixing ratios. Instead, diffusive fluxes alone can explain observed atmospheric mixing ratios that are slightly elevated in some areas but much less than those reported for shallow inshore areas.
Note that this is consistent with the results from the paper I posted upthread that studied the Beaufort Sea. They found no elevated methane levels in the atmosphere where the ocean depth was greater than 30 m.
Our annual flux, not accounting for impermeable ice cover periods, is in rough agreement with previously reported regional fluxes based on measurements in open waters of the LS and ESS, 1–4.5 Tg yr−1 [Shakhova et al., 2005], though later work noted similar fluxes with additional contribution from subsea CH4 seeps [Salyuk and Semiletov, 2010]. A more recent study focused on shallower portions of the LS and ESS (6–24 m depth) and found an average flux of 287 mg m−2 d−1, based on bubble fluxes derived from sonar [Shakhova et al., 2014]. This same study also extrapolated shelf‐wide fluxes as well, reporting a flux from subsea CH4 seeps to the atmosphere of 9 Tg CH4 yr−1, and a total flux (including diffusive fluxes) of 17 Tg CH4 yr−1 for the LS and ESS, by estimating that their study accounted for 10% of extant ESAS seeps. Finally, we note that our in situ results (along with all previous in situ results) are higher than a recent model of CH4 release (bubbling + diffusion) from the Siberian continental shelves of 0.42 Tg yr−1 [Archer, 2015] but are similar to a recent modeling estimate based on Pan‐Arctic CH4 measurements from long‐term monitoring stations on land [Berchet et al., 2016].
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Note the comparisons to modeled results and results posted by S&S.
As noted above, our seawater CH4 measurements do not exclude bubbles, and over several days of continuous measurements crossing the seep regions, our seawater CH4 values certainly incorporate some bubble‐delivered CH4 in the surface waters. There is a large difference in transport success of bubble‐contained CH4 in 6 m deep water and 40 m deep water [McGinnis et al., 2006]; rapid exchange and loss of CH4 to the generally deeper water column in our study alone may account for some of this difference between our and previous studies closer to shore [Sergienko et al., 2012; Shakhova et al., 2014], in shallower depths than studied during SWERUS‐C3 (>35 m). Assuming an average CH4 concentration for the entire water column, we calculated the necessary sustained fluxes to raise the atmospheric concentrations by various amounts. We assumed a very shallow 200 m mixing height (very low inversions were observed during SWERUS‐C3) [Tjernström et al., 2015], a 35 m water column, and an 80 ppb atmospheric CH4 enhancement, raising atmospheric CH4 from apparent background levels of about 1.87 ppm to 1.95 ppm. This requires a sustained CH4 flux of about ~12 mg m−2 d−1, similar to the fluxes we observed during SWERUS in areas near subsea gas seeps on the ESAS (Table 2). An average flux near ESAS subsea seeps of 13 mg m−2 d−1 was reported previously [Sergienko et al., 2012]. Sustaining higher atmospheric CH4 concentrations is even more difficult: to sustain 2.1 ppm CH4 in the atmosphere (35 m water column, 200 m mixing height) would require a flux similar to a subarctic wetland (~36 mg m−2 d−1) Bartlett and Harriss, 1993] or an order of magnitude above the average fluxes we observed in the ice‐free ESAS. [(Various mixing scenarios' effects on atmospheric CH4 are shown in the supporting information Table S2.)
Again, note the comparisons with S&S results and comparisons with the emissions from subarctic wetlands (which I bolded above).
The study goes on to address shallower parts of the ESAS (but note that more than half of the ESAS, with an average depth of 50 meters, is deeper than the 30 to 35 meter depths where methance doesn't reach the surface):
Surveys conducted in shallower waters, closer inshore, have reported substantially higher atmospheric CH4, 2.97 and 2.66 ppm average in the LS and ESS, respectively, with spikes to 8.2 ppm [Semiletov et al., 2012; Shakhova et al., 2010a, 2010b, 2007]. Such average atmospheric mixing ratios of CH4 across large expanses of the LS and ESS require sustained regional CH4 fluxes of roughly 75–200 mg m−2 d−1, depending on local winds and atmospheric mixing. Such atmospheric enhancements are not sustainable by local diffusive CH4 fluxes alone but require a substantial bubble contribution, due to mixing limitations and the strong salinity gradient which acts as formidable barrier to mixing and transport in the ESAS, especially in the LS [Wåhlström et al., 2012]. Using an entrainment velocity model (supporting information), we note that even with sustained gale force winds of 20 m s−1, entrainment velocities in the LS are only 10 cm h−1, suggesting that a week is needed to mix from 15 m depth. Storms with winds >15 m s−1 occur on average only once or twice each summer in the ESAS [Proshutinsky et al., 1999], even less close to shore [Günther et al., 2015], though rare large storms have occurred [Simmonds and Rudeva, 2012]. Thus, during the ice‐free sea season, generally, only the upper 10 m of water is subject to surface layer turbulent mixing. This stratification likely contributes to limiting the diffusive transport of CH4 from deeper waters to the surface.
Here's what the researchers concluded (emphasis added):
Our measurements of CH4 in the atmosphere and surface water across the middle and outer ESAS during July and August 2014 show an average flux of CH4 from the Siberian shelf seas to the atmosphere (along the ice‐free portions of the cruise track) of 3.8 mg m−2 d−1. In a region of CH4 seeps in the LS, fluxes reached 14 mg m−2 d−1. Enhanced levels of CH4 were observed in below‐ice waters of the ESS; such CH4 would have to be stored for winter months and released with near‐100% efficiency after late summer or early autumn ice‐out, providing a short‐duration increase to the total flux, to reach annual fluxes of 2.9 Tg yr−1. Such short‐duration fluxes at ice‐out must be better quantified to constrain the total annual flux. We note that the below‐ice CH4 concentrations in the ESS were considerably below what would be expected if CH4 was collected over the entire ice‐covered months, suggesting overwinter loss processes and/or incorporation into sea ice. This, combined with a lack of knowledge of fluxes through the full ice‐free season, causes us to regard our annual estimates of ESAS sea‐air CH4 fluxes (2.9 Tg yr−1) as very likely high rather than low. Unquantified ice‐out fluxes could increase the annual ESAS CH4 flux above the estimated ice‐free season flux of 0.87 Tg yr−1. Although our estimated sea‐air CH4 fluxes for the ESAS far exceed fluxes reported for other shelf seas, they are roughly an order of magnitude below ESAS CH4 flux estimates reported previously. Reconciling these differences requires much better knowledge of the spatial extent of ESAS CH4 sources (including near‐shore areas not accessible during SWERUS‐C3 where riverine and terrestrial CH4 sources may play a greater role), especially the seemingly highly localized bubble sources, as well as quantification of stored CH4 released at ice‐out.
This study concluded that the then current (2014) estimate of methane release from the ESAS into the atmosphere of 2.9 Tg per year is too high.
Also, multiple studies have shown that methane released from depths of greater than 30m don't reach the surface (I think even S&S reached this conclusion, I forget whether it was in their 2010 or 2013 paper).