Book 2.indb - US Climate Change Science Program

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The U.S. Climate Change Science Program Chapter 5Relatively fewmeasurementsare reported forthe Arctic, andsites are typicallyfar from potentialpermafrost, hydrate,and wetland sources.1.4 Observational Network and ItsCurrent Limitations, ParticularlyRelative to the Hydrate, Permafrost,and Arctic Wetland SourcesThe network of air sampling sites where atmosphericmethane mixing ratios are measuredcan be viewed on the World MeteorologicalOrganization (WMO) World Data Centre forGreenhouse Gases (WDCGG) Web site (http://gaw.kishou.go.jp/wdcgg/) and is reproduced inFigure 5.2. Methane data have been reported tothe WDCGG for ~130 sites. Relatively few measurementsare reported for the Arctic, and sitesare typically far from potential permafrost, hydrate,and wetland sources. Existing Arctic siteshave been used to infer decreased emissionsfrom the fossil-fuel sector of the Former SovietUnion (Dlugokencky et al., 2003) and provideboundary conditions for model studies ofemissions, but they are too remote from sourceregions to accurately quantify emissions, souncertainties on northern emissions will remainlarge until more continuous measurement sitesare added close to sources. The optimal strategywould include continuous measurements fromtall towers and vertical profiles collected fromaircraft. Measurements from tall towers areinfluenced by emissions from much larger areasthan those derived from eddy-correlation fluxtechniques, which have footprints on the orderof 1 square kilometer (km 2 ). When combinedwith global- or regional-scale models, thesemeasurements can be used to quantify fluxes;the vertical profiles would be used to assess thequality of the model results through the troposphere.To properly constrain CH 4 emissions inthe tropics, retrievals of CH 4 column-averagedmixing ratios must be continued to complementsurface observations.1.5 Abrupt Changes in AtmosphericMethane?Concern about abrupt changes in atmosphericmethane stems largely from the large amountsof methane present as solid methane hydratein ocean sediments and terrestrial sediments,which may become unstable in the face offuture warming. Methane hydrate is a solid substancethat forms at low temperatures and highpressures in the presence of sufficient methane,and is found primarily in marine continentalmargin sediments and some arctic terrestrialsedimentary deposits (see Box 5.1). Warmingor release of pressure can destabilize methanehydrate, forming free gas that may ultimately bereleased to the atmosphere. The processes controllinghydrate stability and gas transport arecomplex and only partly understood. Estimatesof the total amount of methane hydrate varywidely, from 500 to 10,000 gigatons of carbon(GtC) stored as methane in hydrates in marinesediments, and 7.5 to 400 GtC in permafrost(both figures are uncertain, see Sec. 4 ). The totalamount of carbon in the modern atmosphere90 N60 N30 N0 30 S60 S90 S0 30 E 60 E 90 E 120 E 150 E 180 150 W 120 W 90 W 60 W 30 WFigure 5.2. World Meteorological Organization global network of monitoring sites (red and blue dots) for long-termobservation of atmospheric methane as of this date. Source: World Data Centre for Greenhouse Gases (WDCGG;http://gaw.kishou.go.jp/wdcgg/).168
Abrupt Climate ChangeBox 5.1. Chemistry, Physics, and Occurrence of Methane HydrateA clathrate is a substance in which a chemical lattice or cage of one type of molecule traps another type ofmolecule. Gas hydrates are substances in which gas molecules are trapped in a lattice of water molecules(Fig. 5.3). The potential importance of methane hydrate to abrupt climate change results from the fact that largeamounts of methane can be stored in a relatively small volume of solid hydrate. For example, 1 cubic meter(m 3 ) of methane hydrate is equivalent to 164 m 3 of free gas (and 0.8 m 3 of water) at standard temperature andpressure (Kvenvolden, 1993). Naturally occurring gas hydrate on Earth is primarily methane hydrate and formsunder high pressure–low temperature conditions in the presence of sufficient methane. These conditions aremost often found in relatively shallow marine sediments on continental margins, but also in some high-latitudeterrestrial sediments (Fig. 5.4). Although the amount of methane stored as hydrate in geological reservoirs isnot well quantified, it is very likely that very large amounts are sequestered in comparison to the present totalatmospheric methane burden.The right combination of pressure and temperature conditions forms what is known as the hydrate stability zone,shown schematically in Figure 5.5. In marine sediments, pressure and temperature both increase with depth,creating a relatively narrow region where methane hydrate is stable. Whether or not methane hydrate formsdepends not only on temperature and pressure but also on the amount of methane present. The latter constraintlimits methane hydrate formation to locations of significant biogenic or thermogenic methane (Kvenvolden, 1993).When ocean bottom water temperatures are near 0 ºC, hydrates can form at shallow depths, below ~200 mwater depth, if sufficient methane is present. The upper limit of the hydrate stability zone can therefore be atthe sediment surface, or deeper in the sediment, depending on pressure and temperature. The thickness ofthe stability zone increases with water depth in typical ocean sediments. It is important to note, however, thatmost marine methane hydrates are found in shallow water near continental margins, in areas where the organiccarbon content of the sediment is sufficient to fuel methanogenesis. In terrestrial sediments, hydrate can format depths of ~200 m and deeper, in regions where surface temperatures are cold enough that temperatures at200 m are within the hydrate stability zone.Figure 5.3. Photographs of methane hydrate as nodules, veins, and laminae in sediment.Courtesy of USGS (http://geology.usgs.gov/connections/mms/joint_projects/methane.htm).169
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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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