= Solution
Identified Arctic <methane> sources include anaerobic decomposition in wetlands and saturated tundra, bubbles from lakes and <thermokarst> lakes formed by thawing <permafrost>, geological gas seeps, and leakage from fossil-fuel extraction or transport. Warming can expose frozen organic carbon to microbes, promoting <methanogenesis> where oxygen is scarce; it does not convert all thawed carbon into <methane>. https://doi.org/10.1038/nature05040[Field measurements of Siberian thaw lakes] directly documented substantial <methane> bubbling associated with lake expansion into <permafrost>.
Shelf sediments can also generate or release <methane>, including gas stored beneath frozen sediment and, where pressure and temperature permit, gas in hydrates. The proposed <Arctic methane feedback> needs several links: seabed warming must reach the relevant material, release or production must occur, and enough <methane> must reach the atmosphere rather than dissolve or be oxidized in sediment and water. Visible plumes alone do not establish an imminent large atmospheric pulse.
<Methane> is a powerful <greenhouse gas> because it absorbs planetary thermal <infrared> radiation and alters the atmospheric emission balance. It also affects atmospheric chemistry, including ozone and stratospheric water vapour. Its relatively short atmospheric lifetime makes its warming contribution particularly strong over short horizons. The <global warming potential> compares time-integrated forcing from equal-mass pulses with that of <carbon dioxide>; the horizon must always be stated. Values available before this examination were about \b[72 over 20 years and 25 over 100 years], including the specified <methane> indirect effects, as recorded in https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html[the 2007 IPCC assessment]. These are historical assessment values, not timeless material constants or instantaneous temperature multipliers.
A separate direct feedback is the <ice-albedo feedback>. When <sea ice> disappears, darker open water reflects less sunlight, stores more <ocean heat content>, and favours further melting and delayed freeze-up. Locally,
$$
\boxed{\Delta q=(\alpha_{\rm ice}-\alpha_{\rm water})S_\downarrow>0.}
$$
For illustration, replacing ice of <surface albedo> $0.6$ by water of <surface albedo> $0.1$ under $200\,\mathrm{W\,m^{-2}}$ incoming sunlight adds $100\,\mathrm{W\,m^{-2}}$ of absorption on the newly exposed area. Global forcing requires weighting by affected area, season and clouds; this is a local example. \b[Sea-ice loss can amplify warming through reduced reflection without requiring <methane> release.] Increased evaporation and altered clouds supply further feedbacks, but their short-wave and long-wave effects can compete.
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