Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeis ~810 GtC, but the total methane content ofthe atmosphere is only ~4 GtC (Dlugokencky etal., 1998). Therefore, even a release of a smallportion of the methane hydrate reservoir to theatmosphere could have a substantial impact onradiative forcing.Massive releases of methane from marine orterrestrial hydrates have not been observed.Evidence from the ice core record indicatesthat abrupt shifts in methane concentrationhave occurred in the past 110,000 years (Chappellazet al., 1993a; Brook et al., 1996, 2000),although the concentration changes duringthese events were relatively small. Fartherback in geologic time, an abrupt warming atthe Paleocene-Eocene boundary (about 55 millionyears ago) has been attributed to a largerelease of methane to the atmosphere, althoughalternate carbon sources such as oxidation ofsedimentary organic carbon or peats have alsobeen proposed (see discussion in Sec. 4). Thesepast abrupt changes are discussed in detailbelow, and their existence provides furthermotivation for considering the potential forfuture abrupt changes in methane.The large impact of a substantial release ofmethane hydrates to the atmosphere, if itwere to occur, coupled with the potential fora more steady increase in methane productionfrom melting hydrates and from wetlands in awarming climate, motivates several questionsthis chapter attempts to address:1. What is the volume of methane in terrestrialand marine sources and how much of itis likely to be released if the climate warms inthe near future?2. What is the impact on the climate systemof the release of varying quantities of methaneover varying intervals of time?3. What is the evidence in the past forabrupt climate change caused by massivemethane release?4. What conditions (in terms of sea-levelrise and warming of bottom waters) wouldallow methane release from hydrates locked upin sea-floor sediments?5. How much methane is likely to bereleased by warming of northern high-latitudesoils, sea-level rise, and other climate-drivenchanges in wetlands?6. What are the observational and modelingrequirements necessary to understandmethane storage and its release under variousfuture scenarios of abrupt climate change?2. History of AtmosphericMethaneOver the last ~300 years the atmospheric methanemixing ratio increased from ~700–750 ppbin 1700 A.D. to a global average of ~1,775 ppb in2006. Direct atmospheric monitoring has beenconducted in a systematic way only since thelate 1970s, and data for previous times comeprimarily from ice cores (Fig. 5.6). Currentlevels of methane are anomalous with respectto the long-term ice core record, which nowextends back to 800,000 years (Spahni et al.,2005; Loulergue et al., 2008). New internationalplans to drill at a site of very low accumulationrate in Antarctica may in the future extend therecord to 1.5 million years (Brook and Wolff,2005).2.1 Direct ObservationsEarly systematic measurements of the globaldistribution of atmospheric CH 4 established arate of increase of ~16 ppb yr –1 in the late 1970sand early 1980s and a strong gradient betweenhigh northern and high southern latitudes of~150 ppb (Blake and Rowland, 1988). By theearly 1990s it was clear that the CH 4 growthrate was decreasing (Steele et al., 1992) andthat, if the CH 4 lifetime were constant, atmosphericCH 4 was approaching steady statewhere emissions were approximately constant(Dlugokencky et al., 1998). Significant variationsare superimposed on this declining growthrate and have been attributed to climate-inducedvariations in emissions from biomass burning(van der Werf et al., 2004) and wetlands (Walteret al., 2001), and changes in the chemical sinkafter the eruption of Mount Pinatubo (Dlugokenckyet al., 1996). Recent measurementsshow that the global atmospheric CH 4 burdenhas been nearly constant since 1999 (Fig. 5.7).This observation is not well understood, underscoringour lack of understanding of howindividual methane sources are changing.Recently published column-averaged CH 4mixing ratios determined from a satellite sensorgreatly enhance the spatial coverage of CH 4 ob-Current levelsof methane areanomalous withrespect to thelong-term ice corerecord, which nowextends back to800,000 years.171