Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5The so-called“wetland hypothesis”postulates thatabrupt warmingin Greenland isassociated withwarmer and wetterclimate in terrestrialwetland regions,which results ingreater emissionsof methane fromwetlands.IGY Photo 10.jpgservations (Frankenberg et al., 2006). Coveragein the tropics greatly increases measurementsthere, but coverage in the Arctic remains poorbecause of the adverse impact of clouds on theretrievals. Use of these satellite data in inversemodel studies will reduce uncertainties inemissions estimates, particularly in the tropics.2.2 The Ice Core RecordThe long-term record shows changes in methaneon glacial-interglacial time scales of~300–400 ppb (Fig. 5.6D), dominated by astrong ~100,000-year periodicity, with higherlevels during warm interglacial periods andlower levels during ice ages. Periodicity of~40,000 and 20,000 years is also apparent,associated with Earth’s cycles of obliquity andprecession (Delmotte et al., 2004). Methaneis believed to provide a positive feedback towarming ultimately caused by changes in theEarth’s orbital parameters on these time scales.The cyclicity is widely attributed to processesaffecting both northern high-latitude and tropicalwetlands, including growth and decay ofNorthern Hemisphere ice sheets, and variationsin the strength of the monsoon circulation andassociated rainfall patterns in Asia, Africa, andSouth America (Delmotte et al., 2004; Spahniet al., 2005; Loulergue et al., 2008).The ice core record also clearly shows anotherscale of variability, abrupt shifts in methaneon millennial time scales that are coincidentwith abrupt changes in temperature observedin Greenland ice cores (Fig. 5.6C). Theseabrupt shifts have been studied in detail inthree deep ice cores from Greenland and inseveral Antarctic ice cores (Chappellaz et al.,1993a; Brook et al., 1996; Brook et al., 2000;Severinghaus et al., 1998; Severinghaus andBrook, 1999; Huber et al., 2006; Grachev etal., 2007). Detailed work using nitrogen andargon isotope ratios as gas phase indicators ofwarming in the ice core record shows clearlythat the increase in methane associated withthe onset of abrupt warming in Greenlandis coincident with, or slightly lags (by a fewdecades at most), the warming (Severinghaus etal., 1998; Severinghaus and Brook, 1999; Huberet al., 2006; Grachev et al., 2007). Methaneclosely follows the Greenland ice isotopic record(Fig. 5.6C), and the amplitude of methanevariations associated with abrupt warming inGreenland appears to vary with time. Brooket al. (1996) suggested a long-term modulationof the atmospheric methane response to abruptclimate change related to global hydrologicchanges on orbital time scales, an issue furtherquantified by Flückiger et al. (2004).2.3 What Caused the Abrupt Changesin Methane in the Ice Core Record?Because the modern natural methane budgetis dominated by emissions from wetlands, it islogical to interpret the ice core record in thiscontext. The so-called “wetland hypothesis”postulates that abrupt warming in Greenlandis associated with warmer and wetter climatein terrestrial wetland regions, which results ingreater emissions of methane from wetlands.Probable sources include tropical wetlands(including regions now below sea level) andhigh-latitude wetlands in regions that remainedice free or were south of the major ice sheets.Cave deposits in China, as well as marine andlake sediment records, indicate that enhancedmonsoon rainfall in the Northern Hemisphere’stropics and subtropics was closely linked toabrupt warming in Greenland (e.g., Peterson etal., 2000; Wang et al., 2004; Yuan et al., 2004;Dykoski et al., 2005; Kelly et al., 2006). Thecave records in particular are important becausethey are extremely well dated using uraniumseries isotopic techniques, and high-resolutionoxygen isotope records from caves, interpretedas rainfall indicators, convincingly match largeparts of the Greenland ice core methane record.The wetland hypothesis is based on climatedrivenchanges in methane sources, but it isalso possible that changes in methane sinks,primarily the OH radical, played a role in the174