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

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

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

Methane in the atmosphere; direct and indirect climate effects

Gunnar MyhreCicero

Figure 8.32

GWP of Methane

• GWP100 (CH4) = 28• Indirect effects contribute to half of the GWP

value• Of the indirect effects, CH4 lifetime enhance

the value with 34%, ozone 50% and stratospheric water vapour 15%.

• What is the for GWP100 (CH4) for large CH4 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 CO2 and CH4 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 CH4 Hydrate EmissionsSubstantial 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 CH4 fluxes would be oxidized within the sediments. Only in cold-shallow Arctic settings (0.4°C and 320 m) would CH4 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 21st century hydrate dissociation in shallow Arctic waters and comparable in magnitude to Biastoch et al. (2011), although maximum CH4 sea floor fluxes were smaller than Lamarque (2008), with emissions from 330 to 450 Tg(CH4) yr–1 for RCP 4.5 to RCP8.5. Most of the sea-floor flux of CH4 is expected to be oxidised in the water column into dissolved CO2. 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 CH4 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 CH4 (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 CH4 release to the atmosphere during this century is unlikely.

Changes in methane burdenGlobal CH4 burden as function of time for scenarios2.5 × CH4 (green), 4 × CH4 (blue), and 7 × CH4 (yellow).Additional scenarios are indicated, for which OsloCTM2 simulations where not performed, but the lifetimedependence of CH4 on its own concentration is taken intoaccount: 100 Tg(CH4) yr−1 sustained (black), 200 Tg(CH4) yr−1 sustained (red), and a 1 year 50 Pg(CH4) yr−1

emission scenario (“hydrate” scenario, purple).

Radiative balance and radiative forcing

Radiative forcing

• The lightest colors refer to the 4 × CH4.

• The medium light colors to the 7 × CH4.

• The dark colors to the 13 × CH4 case.

Radiative forcing

• Indirect effect 3-4 times larger than direct CH4 and much larger than for current emissions -> much higher GWP100 (CH4)

Short summary

• Climate effect of CH4 is difficult to compare to CO2 due to the difference in lifetime

• Indirect effects associated with CH4

emissions are important and increases with CH4 emissions

• The conclusion in IPCC is that CH4 emissions from hydrates is unlikely to be large within this century (low confidence)