4.1 Possible Causes Behind Observed Trends

While we have shown that sea ice‐associated warming is an important driving force behind recent increases in methane emissions from northern wetlands, it is possible that earlier snowmelt has similarly led to raised temperatures when highly reflective snow gives way to the dark ground beneath. In turn, this warming feedback may lead to higher methane emissions but can also contribute to more sea ice melt—leading to even higher temperatures. This synergy complicates the interpretation of a correlation with sea ice during springtime when both sea ice decline and earlier snowmelt could cause a temperature‐related increase in emissions [Serreze and Barry, 2011; Screen et al., 2012]. It is, however, important to note that the interannual variation of sea ice in Hudson Bay is largely due to local atmospheric forcing, as there is little export or import of sea ice into the basin and few intrusions of warm water [Hochheim et al., 2011]. It is therefore possible that the close match observed in Figure 3 between sea ice extent and methane emissions in the area surrounding Hudson Bay from May to July is due, in part, to the same external influence that also promotes snowmelt. This may explain the low difference between sea ice‐affected and unaffected areas during spring, as shown in Figure 4. Earlier snowmelt may have occurred in both areas, resulting in concomitant higher methane emissions. However, sea ice is undergoing a long‐term thinning trend [Laxon et al., 2013] which implies that progressively less warming is needed to melt away the same area of sea ice [Rind et al., 2009], possibly enhancing the importance of the sea ice albedo effect on spring fluxes relative to snowmelt—although this remains difficult to detect at this time.

Spring is also a decisive time for the rest of the sea ice melt season since the area of melt ponds in this period has been shown to be an important predictor of the sea ice minimum [Schröder et al., 2014]. Due to the lower albedo, a higher melt pond area early in the melt season will lead to more absorption of solar radiation, and thus more sea ice melt during the summer. Springtime weather conditions can therefore explain much of the September sea ice extent [Kapsch et al., 2014]. This implies that weather conditions at the end of the melt season are of lesser importance to the state of the sea ice. On the contrary, the strong reduction in autumn sea ice conditions is one of the most important causes behind increased near‐surface air temperatures at that time [Lawrence et al., 2008; Serreze and Barry, 2011; Screen et al., 2012]. A good match between the interannual variation in sea ice and methane emissions during the later months in Figure 3 is, therefore, unlikely to be due to a similar response from an outside influence but rather a sea ice‐related temperature feedback. As a result, the connection between sea ice retreat and higher methane fluxes from terrestrial sources is most easily identified in that time of year.

All three methane models used in this study represent the state of the art in modeling methane emissions from northern wetlands and permafrost regions. Nonetheless, it is important to verify that model agreement is not due to similar assumptions. Surprisingly, however, spatial patterns and emission trends are most dissimilar between TEM6 and Peatland‐VU despite sharing some model components as proposed by Walter and Heimann [2000]. This may be because of different implementations for both hydrology and net primary production [Zhuang et al., 2004; Mi et al., 2014]. Moreover, the methane subroutine of LPJ‐GUESS was developed completely independently [Wania et al., 2010]. Despite these differences among the models, all three show a similar increase in methane emissions toward the autumn, which is likely due to comparable trends in the climate forcing as shown in Figure 4. In general, the models predict that higher temperatures will lead to higher methane emissions, which agrees with observations [Olefeldt et al., 2013]. A more detailed discussion of the similarity and differences between the models is included in the supporting information.

Another strong control on methane emissions is water table depth, since this determines where in the soil production and oxidation of methane take place. Although a strong link between precipitation and sea ice appears absent (see Figure S4), higher temperatures could arguably lead to drier conditions—suppressing methane emissions. However, sea ice retreat has been linked to increasing atmospheric moisture [Screen and Simmonds, 2010] and may increase precipitation in the long term [Bintanja and Selten, 2014]. This suggests that—in general—sea ice retreat will lead to wetter rather than drier conditions across the Arctic, although large differences may occur regionally [Watts et al., 2014]. We emphasize the continued need to improve methane models and the role of changing hydrology, whether associated to sea ice or not.