Book 2.indb - US Climate Change Science Program

Abrupt Climate Change1. Background: Why AreAbrupt Changes inMethane of PotentialConcern?1.1 IntroductionMethane (CH 4 ) is the second most importantgreenhouse gas that humans directly influence,carbon dioxide (CO 2 ) being first. Concernsabout methane’s role in abrupt climate changestem primarily from (1) the large quantitiesof methane stored as solid methane hydrateon the sea floor and to a lesser degree in terrestrialsediments, and the possibility that thesereservoirs could become unstable in the face offuture global warming, and (2) the possibilityof large-scale conversion of frozen soil in thehigh-latitude Northern Hemisphere to methaneproducingwetland, due to accelerated warmingat high latitudes. This chapter summarizesthe current state of knowledge about thesereservoirs and their potential for forcing abruptclimate change.1.2 Methane and ClimateA spectral window exists between ~7 and12 micrometers (μm) where the atmosphereis somewhat transparent to terrestrial infrared(IR) radiation. Increases in the atmosphericabundance of molecules that absorb IR radiationin this spectral region contribute to the greenhouseeffect. Methane is a potent greenhousegas because it strongly absorbs terrestrial IRradiation near 7.66 μm, and its atmosphericabundance has more than doubled since the startof the Industrial Revolution. Radiative forcing(RF) is used to assess the contribution of aperturbation (in this case, the increase in CH 4since 1750 A.D.) to the net irradiance at the topof the tropopause (that area of the atmospherebetween the troposphere and the stratosphere)after allowing the stratosphere to adjust toradiative equilibrium. The direct radiativeforcing of atmospheric methane determinedfrom an increase in its abundance from its preindustrialvalue of 700 parts per billion (ppb)(Etheridge et al., 1998; MacFarling Meure etal., 2006) to its globally averaged abundance of1,775 ppb in 2006 is 0.49±0.05 watts per squaremeter (W m –2 ) (Hofmann et al., 2006). Methaneoxidation products, stratospheric water (H 2 O)vapor and tropospheric ozone (O 3 ), contributeindirectly to radiative forcing, increasingmethane’s total contribution to ~0.7 W m –2 (e.g.,Hansen and Sato, 2001), nearly half of that forcarbon dioxide (CO 2 ). Increases in methaneemissions can also increase the methane lifetimeand the lifetimes of other gases oxidizedby the hydroxyl radical (OH). Assuming theabundances of all other parameters that affectOH stay the same, the lifetime for an additionalpulse of CH 4 (e.g., 1 teragram, Tg; 1 Tg = 10 12 g= 0.001 Gt, gigaton) added to the atmospherewould be ~40% larger than the current value.Additionally, CH 4 is oxidized to CO 2 ; CO 2produced by CH 4 oxidation is equivalent to ~6%of CO 2 emissions from fossil fuel combustion.Over a 100-year time horizon, the direct andindirect effects on RF of emission of 1 kilogram(kg) CH 4 are 25 times greater than for emissionof 1 kg CO 2 (Forster et al., 2007).The atmospheric abundance of CH 4 increasedwith human population because of increaseddemand for energy and food. Beginning in the1970s, as CH 4 emissions from natural gas ventingand flaring at oil production sites declinedand rice agriculture stabilized, the growth rateof atmospheric CH 4 decoupled from populationgrowth. Since 1999, the global atmosphericCH 4 abundance has been nearly stable; globallyaveraged CH 4 in 1999 was only 3 ppb less thanthe 2006 global average of 1,775 ppb. Potentialcontributors to this stability are decreasedemissions from the Former Soviet Union aftertheir economy collapsed in 1992 (Dlugokenckyet al., 2003), decreased emissions from naturalwetlands because of widespread drought (Bousquetet al., 2006), decreased emissions from ricepaddies due to changes in water management(Li et al., 2002), and an increase in the chemicalsink (removal terms in the methane budget arereferred to as “sinks”) because of changingclimate (Fiore et al., 2006). Despite attempts toexplain the plateau in methane levels, the exactcauses remain unknown, making predictions offuture methane levels difficult. Hansen et al.(2000) have suggested that, because methanehas a relatively short atmospheric lifetime (seebelow) and reductions in emissions are oftencost effective, it is an excellent gas to target tocounter increasing RF of CO 2 in the short term.Methane (CH 4 )is the secondmost importantgreenhouse gas thathumans directlyinfluence, carbondioxide (CO 2 ) beingfirst.165