Book 2.indb - US Climate Change Science Program

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The U.S. Climate Change Science Program Chapter 5important globally, as the necessary geologicalsetting is rare.Mining terrestrial hydrates for gas productionwill necessarily destabilize them, but presumablymost of this methane will be captured, used, andthe carbon emitted to the atmosphere as CO 2 .5.3 Evidence of Past Release ofTerrestrial Hydrate MethaneNo direct evidence has been identified ofpast release of terrestrial hydrate methane insignificant quantities. Analyses related to thePETM and clathrate gun hypothesis discussedin Section 4 have focused on methane emissionsfrom the larger and more vulnerable marinehydrates. Emissions from terrestrial hydratesmay have contributed to changes in methaneobserved in the ice core record, but there are sofar no distinctive isotopic tracers of terrestrialhydrates, as is the case for marine hydrate (Sowers,2006).5.3.1. Quantity of Methane ReleasedFrom Terrestrial Hydrates in the PastWeitemeyer and Buffett (2006) modeled theaccumulation and release of biogenic methanefrom terrestrial hydrates below the Laurentideand Cordilleran ice sheets of North Americaduring the last glaciation. Methane was generatedunder the ice sheet from anaerobicdecomposition of buried, near-surface soilorganic matter, and hydrates formed if the icesheet was greater than ~250 m thick. Hydratedestabilization arose from pressure decreaseswith ice sheet melting/thinning. They simulatedtotal releases for North America of about40–100 Tg CH 4 , with most of the deglacialemissions occurring during periods of glacialretreat during a 500-year interval around 14 kyrbefore present (BP), and a 2,000-year intervalcentered on about 10 kyr BP. The highestsimulated emission rates (~15–35 Tg CH 4 yr –1 )occurred during the dominant period of icesheet melting around 11–9 kyr BP.Shakova et al. (2005) measured supersaturatedmethane concentrations in northern Siberiancoastal waters. This supersaturation is thoughtto arise from degradation of coastal shelfhydrate, hydrate that had formed in permafrostwhen the shelf was exposed during low sealevel of the last glacial maximum. Methaneconcentrations in the Laptev and East SiberianSeas were supersaturated up to 800% in 2003and 2,500% in 2004. From this and an empiricalmodel of gas flux between the atmosphere andthe ocean, they estimated summertime (i.e.,ice-free) fluxes of up to 0.4 Mg CH 4 km –2 y –1 (or0.4 g CH 4 m –2 y –1 ). They assume that the methaneflux from the sea floor is of the same orderof magnitude and may reach 1–1.5 g CH 4 m –2y –1 . These fluxes are low compared to wetlandfluxes (typically ~1–100 g CH 4 m –2 y –1 ; Bartlettand Harriss, 1993), but applied across the totalarea of shallow Arctic shelf, the total annualflux for this region may be as high as 1–5 TgCH 4 y –1 , depending on degree of oxidation inthe seawater. (See Table 5.1 above for globalmethane emissions by source.)5.3.2 Climate Impact of Past MethaneRelease From Terrestrial HydratesMost studies of climate impacts from possiblepast methane hydrate releases have consideredlarge releases from marine hydrates (see Sec. 4above). It is generally not well known whatfraction of the methane released from hydratedestabilization is either trapped in overlyingsediments or oxidized to carbon dioxide beforereaching the atmosphere (Reeburgh, 2004), andthe same considerations are relevant to releasefrom terrestrial sources.Weitemeyer and Buffett (2006) estimatedintervals of 500–2,000 years when methanehydrate destabilization from retreat of the NorthAmerican ice sheet caused increases of atmosphericmethane of 10–200 ppb, with the largestperturbation at 11–9 kyr before present. Anyeffect of methane oxidation before reaching theatmosphere was ignored; this oxidation wouldhave reduced the impact on the atmosphericmethane burden. This atmospheric perturbationis equivalent to about 2–25% of pre-industrialHolocene atmospheric methane burdens, androughly equivalent to a radiative forcing of0.002–0.1 W m –2 (using contemporary valuesfor methane radiative efficiency and indirecteffects from Ramaswamy et al., 2001).192
Abrupt Climate ChangeThermokarst erosion on the Arctic coast ofSiberia is thought to cause hydrate destabilizationand emissions of methane that are at most1% of total global methane emissions (Shakhovaet al. 2005), and so this process is very unlikelyto be having a large climatic impact.5.4 Estimates of Future TerrestrialHydrate Release and Climatic ImpactHarvey and Huang (1995) modeled terrestrialmethane hydrate release due to global warming(step function temperature increases of 5 °C,10 °C, and 15 °C, and the propagation of thisheat into hydrate-bearing permafrost). Overthe first few centuries, the methane release isvery small, and after 1,000 years, the cumulativemethane release is
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TABLE OF CONTENTSAuthor Team for th
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The U.S. Climate Change Science Pro
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PREFACEReport Motivation and Guidan
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1CHAPTERAbrupt Climate ChangeIntrod
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Abrupt Climate Change-35Arctic temp
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Abrupt Climate ChangeFigure 1.2. Po
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Abrupt Climate Changewell below sea
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Abrupt Climate Changeheterogeneous
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Abrupt Climate Changeapart from dro
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Abrupt Climate Change6. Abrupt Chan
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Abrupt Climate Changeintermediate c
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Abrupt Climate Changeper square met
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Abrupt Climate Changeis the norther
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Abrupt Climate ChangeRegardless how
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Abrupt Climate Changecentrations, a
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Abrupt Climate ChangeBox 2.1. Glaci
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Abrupt Climate Changetime. Degree-d
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Abrupt Climate Changetion, and accu
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Abrupt Climate ChangeBox 2.2. Mass
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Abrupt Climate Changeof the glacier
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Abrupt Climate ChangeFigure 2.6. Ra
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Abrupt Climate Changewater) or, mor
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Abrupt Climate ChangeFigure 2.7. Re
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Abrupt Climate Changeresults in a l
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Abrupt Climate Changeis not providi
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Abrupt Climate Changemeasurements c
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Abrupt Climate Changemore than 10,0
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Abrupt Climate Changeocean models h
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Abrupt Climate Changeto atmosphere-
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3CHAPTERHydrological Variability an
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Abrupt Climate Change• The integr
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Abrupt Climate Changethe soil (King
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Abrupt Climate Changeon (Kaplan et
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Abrupt Climate Changeorbital-time-s
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Abrupt Climate ChangeFigure 3.4. (t
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Abrupt Climate Changemodels forced
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Abrupt Climate ChangeBox 3.2. Waves
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Abrupt Climate Changebecause (1) th
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Abrupt Climate ChangeHarrison et al
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Abrupt Climate Changethat even the
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Abrupt Climate Changethat seen afte
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Abrupt Climate Changeentire period
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Abrupt Climate Changelong-lived gre
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Abrupt Climate Changeplanetary-scal
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Abrupt Climate ChangeBox 3.3. Paleo
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Abrupt Climate Changerelated to the
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Abrupt Climate Changeinto the North
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Abrupt Climate ChangeThe summertime
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Abrupt Climate Change(Anderson et a
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Abrupt Climate ChangeHere the model
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Abrupt Climate Changetropical tropo
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Abrupt Climate Changefunctions; Koc
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Abrupt Climate Changechanges than t
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Abrupt Climate Changeboundary condi
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U.S. Climate Change Science Program
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Book 2.indb - US Climate Change Science Program

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