Methane in the atmosphere; direct and indirect climate effects Gunnar Myhre Cicero

Figure 8.32

GWP of Methane GWP 100 (CH 4 ) = 28 Indirect effects contribute to half of the GWP value Of the indirect effects, CH 4 lifetime enhance the value with 34%, ozone 50% and stratospheric water vapour 15%. What is the for GWP 100 (CH 4 ) for large CH 4 emissions?

There is high confidence that reductions in permafrost extent due to warming will cause thawing of some currently frozen carbon. However, there is low confidence on the magnitude of carbon losses through CO 2 and CH 4 emissions to the atmosphere, with a range from 50 to 250 PgC between 2000 and 2100 for RCP8.5. The CMIP5 Earth System Models did not include frozen carbon feedbacks. Figure SPM.8

Future CH 4 Hydrate Emissions Substantial quantities of methane are believed to be stored within submarine hydrate deposits at continental margins (see also Section 6.1, FAQ 6.2). There is concern that warming of overlying waters may melt these deposits, releasing CH4 into the ocean and atmosphere systems. Considering a potential warming of bottom waters by 1, 3 and 5 K during the next 100 years, Reagan and Moridis (2007) found that hydrates residing in a typical deep ocean setting (4°C and 1000 m depth) would be stable and in shallow low-latitude settings (6°C and 560 m) any sea- floor CH 4 fluxes would be oxidized within the sediments. Only in cold-shallow Arctic settings (0.4°C and 320 m) would CH 4 fluxes exceed rates of benthic sediment oxidation. Simulations of heat penetration through the sediment by Fyke and Weaver(2006) suggest that changes in the gas hydrate stability zone will be small on century timescales except in high-latitude regions of shallow ocean shelves. In the longer term, Archer et al. (2009a) estimated that between 35 and 940 PgC could be released over several thousand years in the future following a 3 K seafloor warming. Using multiple climate models (Lamarque, 2008), predicted an upper-estimate of the global sea-floor flux of between 560 and 2140 Tg(CH 4 ) yr –1, mostly in the high-latitudes. Hunter et al. (2013) also found 21 st century hydrate dissociation in shallow Arctic waters and comparable in magnitude to Biastoch et al. (2011), although maximum CH 4 sea floor fluxes were smaller than Lamarque (2008), with emissions from 330 to 450 Tg(CH 4 ) yr –1 for RCP 4.5 to RCP8.5. Most of the sea-floor flux of CH 4 is expected to be oxidised in the water column into dissolved CO 2. Mau et al. (2007) suggest only 1% might be released to the atmosphere but this fraction depends on the depth of water and ocean conditions. Elliott et al. (2011) demonstrated significant impacts of such sea-floor release on marine hypoxia and acidity, although atmospheric CH 4 release was small. Observations of CH4 release along the Svalbard margin seafloor (Westbrook et al., 2009) suggest observed regional warming of 1°C during the last 30 years is driving hydrate disassociation, an idea supported by modelling (Reagan and Moridis, 2009). However, these studies do not consider subsea-permafrost hydrates suggested recently to be regionally significant sources of atmospheric CH 4 (Shakhova et al., 2010). There was no positive excursion in the methane concentration recorded in ice cores from the largest known submarine landslide, the Storegga slide of Norway 8,200 years ago. Large methane hydrate release due to marine landslides is unlikely as any given landslide could only release a tiny fraction of the global inventory (Archer, 2007). There is low confidence in modelling abilities to simulate transient changes in hydrate inventories, but large CH 4 release to the atmosphere during this century is unlikely.

Changes in methane burden Global CH 4 burden as function of time for scenarios 2.5 × CH4 (green), 4 × CH 4 (blue), and 7 × CH 4 (yellow). Additional scenarios are indicated, for which Oslo CTM2 simulations where not performed, but the lifetime dependence of CH 4 on its own concentration is taken into account: 100 Tg(CH 4 ) yr −1 sustained (black), 200 Tg (CH 4 ) yr −1 sustained (red), and a 1 year 50 Pg(CH 4 ) yr −1 emission scenario (“hydrate” scenario, purple).

Radiative balance and radiative forcing

Radiative forcing The lightest colors refer to the 4 × CH 4. The medium light colors to the 7 × CH 4. The dark colors to the 13 × CH 4 case.

Radiative forcing Indirect effect 3-4 times larger than direct CH 4 and much larger than for current emissions -> much higher GWP 100 (CH 4 )

Short summary Climate effect of CH 4 is difficult to compare to CO 2 due to the difference in lifetime Indirect effects associated with CH 4 emissions are important and increases with CH 4 emissions The conclusion in IPCC is that CH 4 emissions from hydrates is unlikely to be large within this century (low confidence)