Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Programthe direct impacts of climate change (alteredtemperature and moisture regimes, and in onecase enhanced vegetation productivity) but notindirect impacts, such as changing landscapehydrology with permafrost degradation andchanging vegetation distribution. At this time,it is not known whether direct or indirect effectswill have a stronger impact on net methaneemissions. These models all predict fairlysmooth increases in annual wetland emissions,with no abrupt shifts in flux.6.5 Conclusion About Potential forAbrupt Release of Methane FromWetlandsTropical wetlands are a stronger methane sourcethan boreal and arctic wetlands and will likelycontinue to be over the next century, duringwhich fluxes from both regions are expectedto increase. However, four factors differentiatenorthern wetlands from tropical wetlands andmake them more likely to experience a largerincrease in fluxes: (1) high-latitude amplificationof climatic warming will lead to a strongertemperature impact, (2) for regions with permafrost,warming-induced permafrost degradationcould make more organic matter available fordecomposition and substantially change the systemhydrology, (3) the sensitivity of microbialrespiration to temperature generally decreaseswith increasing temperatures (e.g., Davidsonand Janssens, 2006), and (4) most northernwetlands have substantial carbon as peat. Onthe other hand, two characteristics of northernpeatlands counter this: (1) northern peatlandsare complex, adaptive ecosystems, with internalfeedbacks and self-organizing structure (Belyeaand Baird, 2007) that allow them to persist in arelatively stable state for millennia and that mayreduce their sensitivity to hydrological change,and (2) much of the organic matter in peat iswell-decomposed (e.g., Frolking et al., 2001)and may not be good substrate for methanogens.The balance of evidence suggests that anticipatedchanges to northern wetlands inresponse to large-scale permafrost degradation,thermokarst development, a positive P–E trendin combination with substantial soil warming,enhanced vegetation productivity, and anabundant source of organic matter will likelyconspire to drive a chronic increase in CH 4emissions from the northern latitudes duringthe 21st century. Due to the strong interrelationshipsbetween temperature, moisture,permafrost, and nutrient and vegetation change,and the fact that negative feedbacks such asthe draining and drying of wetlands are alsopossible, it is difficult to establish how largethe increase will be over the coming century.Current models suggest that a doubling of CH 4emissions from northern wetlands could be realizedfairly easily. However, since these modelsdo not realistically represent all the processesthought to be relevant to future northern highlatitudeCH 4 emissions, much larger (or smaller)increases cannot be discounted.It is worth noting that our understanding of thenorthern high-latitude methane cycle continuesto evolve. For example, a recent field study suggeststhat prior estimates of methane emissionsfrom northern landscapes may be biased lowdue to an underestimation of the contributionof ebullition from thermokarst hot spots in Siberianthaw lakes (Walter et al., 2006). Anotherrecently discovered phenomenon is the cold adaptationof some methanogenic microorganismsthat have been found in permafrost depositsin the Lena River basin (Wagner et al., 2007).These microbes can produce methane even inthe very cold conditions of permafrost, oftendrawing on old soil organic matter. The activitylevels of these cold-adapted methanogens aresensitive to temperature, and even a modestsoil warming can lead to an accumulation ofmethane deposits which, under scenarios wherepermafrost degradation leads to thermokarstor coastal erosion, could be quickly released tothe atmosphere.Chapter 5200