SUPPLEMENTARY INFORMATION

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1 Supplementary information for Large tundra methane burst during onset of freezing by Mastepanov et al. Supplementary Figures and Legends 8.86 (71%) 3.10 (41%) 3.36 (53%) 3.43 (55%) 1.68 (32%) 2.64 (46%) 4.43 (59%) 3.67 (29%) 2.93 (47%) 2.83 (45%) 3.52 (68%) 3.05 (54%) Supplementary Figure 1. Seasonal CH 4 emission from six individual chambers. A photograph showing the automatic chambers in the Zackenberg valley (muskoxen visible in the background). Superimposed are in Magenta (lower) part of the bars summertime methane fluxes (before 1 September), in blue (upper) part autumn fluxes (after 1 September). The numbers show total emission for the period, in g CH 4 -C/m

2 40-55N 70-85N MCF (ppt) 0.0 MCF (ppt) time (month) time (month) Supplementary Figure 2. Comparison of the simulated and observed multi-year mean seasonal cycles of MCF averaged over high (left panel) and mid (right panel) latitude sites of the northern hemisphere. Black, measurements with 1 sigma variations; Red, model simulations. See supplementary discussion CH 4 (ppb) month Supplementary Figure 3. Similarity in seasonal cycles of high latitude atmospheric methane over time. The black line with 2 sigma uncertainty levels is the same as Figure 2 (main text) i.e. atmospheric data from The red line represents the same procedure but here for NOAA data that were collected ten years earlier ( ). 2

3 Supplementary Table 1. Methane fluxes (mg CH 4 /m 2 /hr) and soil temperatures ( C) from Figure 1 (main paper). See supplementary notes. Date N Average StErr Soil temperature, daily Soil temperature, daily minimum maximum (2007) flux 5 cm 10 cm 15 cm 5 cm 10 cm 15 cm 24 Jun N/A N/A N/A N/A N/A N/A 25 Jun N/A N/A N/A N/A N/A N/A 26 Jun N/A N/A N/A N/A N/A N/A 27 Jun N/A N/A N/A N/A N/A N/A 28 Jun N/A N/A N/A N/A N/A N/A 29 Jun N/A N/A N/A N/A N/A N/A 30 Jun N/A N/A N/A N/A N/A N/A 1 Jul N/A N/A N/A N/A N/A N/A 2 Jul N/A N/A N/A N/A N/A N/A 3 Jul N/A N/A N/A N/A N/A N/A 4 Jul N/A N/A N/A N/A N/A N/A 5 Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Aug Aug Aug Aug Aug Aug Aug Aug 0 N/A N/A Aug 0 N/A N/A Aug 0 N/A N/A

4 11 Aug 0 N/A N/A Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Sep Oct Oct Oct Oct

5 5 Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Supplementary Table 2. Methane fluxes (mg CH 4 /m 2 /hr) for 2006 summer season (see also supplementary notes). Date Average N (2006) flux StErr 3 Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul Jul

6 27 Jul Jul Jul Jul 0 N/A N/A 31 Jul Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug 0 N/A N/A 14 Aug 0 N/A N/A 15 Aug 0 N/A N/A 16 Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug Aug

7 Supplementary Discussion The issue may be raised whether the soil storage of methane can be large enough to support the substantial emissions observed during the freeze-in period. The answer is not easy to provide as it is extremely difficult to estimate the capacity of methane storage in the peat layer. We have concentration profiles for dissolved CH 4 from the same site 24, but it is well known that the major storage reservoir in such systems is subsurface gas bubbles. Due to low methane solubility, even systems in equilibrium with equal volumes of water and gas bubbles at 0 ºC and 1 atm. pressure contains 95% of CH 4 in gas phase 25. Real gas bubbles in submerged soils are usually not in a good equilibrium with the water solution (because of low mobility of liquid phase and low diffusion rates in immobile liquid), and have a significant overpressure (due to elasticity in the peat structure). So the CH 4 concentration in individual bubbles can exceed 80% 26. If, just for mathematical exercise, we assume we have 25 cm deep peat, containing just 5% of bubbles (per volume) with a CH 4 concentration of 50%, then the methane storage in these bubbles is about 4.5 g CH 4 per m 2, which is equal to the whole of our summerseason worth of emissions. Examination of this storage in situ is incredibly difficult, as we should exactly measure size of each small bubble and its CH 4 concentration, which can vary significantly as shown for rice paddy soils 26. Regarding the comparison of observed and simulated methane concentrations it is well known that the seasonal cycle of methane at background sites is dominated by seasonal variations of the hydroxyl sink rather than variations of wetlands emissions. Therefore, potential inaccuracies in the simulation of methane oxidation by OH may seem a more likely explanation of the disagreement between the observed and simulated seasonal cycles than missing permafrost emissions. To further investigate this possibility we analyzed the results of TM5 simulations of methyl chloroform (MCF) conducted by M. Krol (manuscript in preparation). MCF is a powerful tracer to study OH, since several years after its emissions have been phased out its background concentration variations can almost exclusively be related to variations in OH. Supplementary Figure 2 shows a comparison of the modelled (red) and observed (black) mean seasonal cycles (simulated period was ). The averaging procedure was very similar to what was used for methane, except for the time window 7

8 and the locations of the stations. However, as mentioned earlier, the multi-year average seasonal cycle of methane is insensitive to the selected time window. Furthermore, like for methane, the selected MCF measurement sites can be considered representative of the specified latitudinal bands. If OH would explain the seasonality mismatch in CH 4 this should lead to a similar seasonality mismatch in MCF, which is clearly not the case as demonstrated by Supplementary Figure 2. There may be a minor impact of OH on the simulated CH 4 seasonality, but it certainly doesn't explain the disagreement between the observed and model simulated seasonal cycle of methane at the high northern latitudes. References: 24. Joabsson, A. & Christensen, T.R. Methane emissions from wetlands and their relationship with vascular plants: an Arctic example. Glob. Change Biol. 7, (2001). 25. Wilhelm, E., Battino, R. & Wilcock RJ. Low-pressure solubility of gases in liquid water. Chemical Reviews 77, (1977). 26. Rothfuss, F. & Conrad, R. Effect of Gas Bubbles on the Diffusive Flux of Methane in Anoxic Paddy Soil. Limnol. and Oceanogr. 43, (1998). We thank Maarten Krol for kindly making results of his MCF simulations available to us. 8

9 Supplementary Notes Notes in relation to Figure 1 (main paper). All methane flux data points in Figure 1 represent an average of daily fluxes from up to six individual chambers. These are in turn each averages, in time, of up to 24 individual flux measurements. Every individual flux measurement (initial slope of CH 4 concentration increase with time in a closed chamber) was quality-controlled; in case of any sign of chamber miss operation the data point was rejected from the average. If a chamber did not work, or provided unreliable data (due to leakage, etc.) for the whole 24 hours, the final daily flux value did not include this chamber at all. It follows that the number N in supplementary Tables 1 and 2 shows the number of individual chambers (replicates) that provided reliable data on the respective days of operation. The standard error is based on the deviation between daily fluxes for N chambers. For technical reasons we do not have flux data for the early part of the 2007 season which is why we have inserted the data from the previous, 2006, season in Figure 1. The 2006 and 2007 seasons had very different snow melt dates, so for the fluxes to be comparable at the seasonal scale we aligned the data according to the date of snow melt (5 July 2006 and 12 June 2007). The entire dataset from 2006 season is shown in the Supplementary Table 2. During the early part of the 2006 season only four chambers were working (two were damaged during winter by muskoxen and reconstructed later). For comparative reasons data from those reconstructed chambers was not included in the average fluxes in the Supplementary Table 2. The coloured area of the soil temperatures shown in Figure 1 represents the range between daily maximum and minimum at the respective depth. The temperature was logged every hour about 3 meters away from chambers # 3 and 4 (in the middle of the row of chambers). We do not have soil temperature data directly from this site before 5 July 2007 but data from the nearby climate station (about 1 km away) is complementing the early part of the seasonal temperature pattern shown in Figure 1. As this climate station stands at a dryer place, daily soil temperature variations there are higher. The climate station data is courtesy of Zackenberg Ecological Research Operations ( 9