3. As surface permafrost melts it produces a landscape which is largely waterlogged since lower, intact permafrost inhibits drainage resulting in formation of thermokast lakes. This creates anoxic conditions essential for methanogens to produce CH4 from biota but inhibit oxidation by methanotrophs. Release of carbon from waterlogged land surfaces is likely to be in the form of CH4 rather than >95% CO2 predicted by some.
Fig. 2. Comparison between atmospheric concentration of CH4 and CO2 over the last 400,000 years and their effects on average global temperature to 2013. While most CH4 emissions result from human activity, mining and animal husbandry, emissions associated with permafrost degradation are increasing. Source: R.Morrison, Wikipedia
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Analysis of foraminifera and Antarctic ice cores show that globally, CH4 concentration in the atmosphere is now at its highest level in over 800,000 years. What makes present levels so dangerous is that studies by Reisinger et al (2011) show that over a 20 year period, CH4 now has radiative forcing properties 84 times greater than CO2, a finding accepted by 5AR. Yet the 5AR dismisses the notion that CH4 emissions from Arctic continental shelves and adjacent onshore areas contribute to Arctic amplification or that this contribution can, let alone will, rapidly accelerate permafrost degradation and larger CH4 releases this century.
The result of underestimating CH4 emissions in the Arctic and their GWP makes it likely that the 5AR significantly underestimates Arctic amplification, loss of ice mass from the GIS, consequent sea level rise this century and the speed with which sea ice covering the Arctic Ocean reduces.
Albedo Loss
The 5AR defines an ice-free Arctic Ocean as sea ice extent less than 1 × 106km2for at least 5 consecutive years (5AR Ch. 11.3.4.1)and asserts that these conditions could pertain by summer 2050 (5AR Ch. 2, Fig. 2.1.b) and that consequential loss of albedo will have an important (though unquantified) effect on ocean warming, accelerating ice loss, coastal erosion and permafrost degradation. Most scientists would agree, though many are of the view that the Arctic Ocean will be ice free in summer long before 2050, possibly as soon as 2030. The 5AR is seen by many as underestimating Arctic Ocean warming, loss of sea ice and albedo.
Relatively rapid loss of sea ice, combined with ingress of warmer water from the Pacific and Atlantic will reduce formation of multi-year sea ice, making the Arctic coastline vulnerable to erosion, exposing carbon bearing ice and biota to a warming atmosphere. This is likely to result in greater release of CH4 as sub-surface ice containing biota are exposed to methanogens.
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Fig. 3 Soot and aerosols produced by human activity and from natural sources contribute to atmospheric warming and when they settle on ice in the Arctic, absorb solar energy which accelerates melting. Source: Arctic News/Dark Snow Project.
Aerosol soot in the Arctic originates in human activity and, as the climate warms, a rising incidence of forest burning. It falls over a vast area of the Arctic covering large areas of sea ice and the GIS. This reduces albedo and increases absorption of solar radiation, thereby accelerating seasonal melting, increasing mass ice loss
since 2000. In the case of the GIS, surface melting produces lakes which ultimately flow to the ice sheet base, lubricating the land surface on which its rests before flowing to the ocean, enhancing ice sheet mobility.
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