Constraining global methane emissions and uptake by ecosystems

Abstract

Natural methane (CH<sub>4</sub>) emissions from wet ecosystems are an important part of today's global CH<sub>4</sub> budget. Climate affects the exchange of CH<sub>4</sub> between ecosystems and the atmosphere by influencing CH<sub>4</sub> production, oxidation, and transport in the soil. The net CH<sub>4</sub> exchange depends on ecosystem hydrology, soil and vegetation characteristics. Here, the LPJ-WHyMe global dynamical vegetation model is used to simulate global net CH<sub>4</sub> emissions for different ecosystems: northern peatlands (45°–90° N), naturally inundated wetlands (60° S–45° N), rice agriculture and wet mineral soils. Mineral soils are a potential CH<sub>4</sub> sink, but can also be a source with the direction of the net exchange depending on soil moisture content. The geographical and seasonal distributions are evaluated against multi-dimensional atmospheric inversions for 2003–2005, using two independent four-dimensional variational assimilation systems. The atmospheric inversions are constrained by the atmospheric CH<sub>4</sub> observations of the SCIAMACHY satellite instrument and global surface networks. Compared to LPJ-WHyMe the inversions result in a significant reduction in the emissions from northern peatlands and suggest that LPJ-WHyMe maximum annual emissions peak about one month late. The inversions do not put strong constraints on the division of sources between inundated wetlands and wet mineral soils in the tropics. Based on the inversion results we adapt model parameters in LPJ-WHyMe and simulate the surface exchange of CH<sub>4</sub> over the period 1990–2008. Over the whole period we infer an increase of global ecosystem CH<sub>4</sub> emissions of +1.11 Tg CH<sub>4</sub> yr<sup>−1</sup>, not considering potential additional changes in wetland extent. The increase in simulated CH<sub>4</sub> emissions is attributed to enhanced soil respiration resulting from the observed rise in land temperature and in atmospheric carbon dioxide that were used as input. The long-term decline of the atmospheric CH<sub>4</sub> growth rate from 1990 to 2006 cannot be fully explained with the simulated ecosystem emissions. However, these emissions show an increasing trend of +3.62 Tg CH<sub>4</sub> yr<sup>−1</sup> over 2005–2008 which can partly explain the renewed increase in atmospheric CH<sub>4</sub> concentration during recent years

Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase

“…Some bottom-up studies indicate upward decadal-scale trends in global wetland emissions (29), but top-down estimates using ground-based observations and satellite-based products combined with atmospheric models tend to favor constant or declining wetland emissions over the past few decades (3), consistent with this work. Global CH 4 emissions from solid waste are generally considered to have increased over the past few decades from increases in landfilled solid waste, even as CH 4 recovery increased (8,30).…”

Microbial minorities modulate methane consumption through niche partitioning

“…Methane emission estimates from wetlands are characterized by high uncertainty (Bloom et al, 2010). Recent variation in atmospheric methane concentrations, including the resumed increase from 2007 onwards, has been linked to changes in wetlands (e.g., in surface area, fertilizer use in rice cultivation and microbial processes) (Bousquet et al, 2006;Kai et al, 2011;Spahni et al, 2011). Especially, wet mineral soils (e.g., riparian floodplains), which can serve as a sink as well as a source of methane depending on the hydrological conditions, are proposed to contribute to the 2007 increase (Spahni et al, 2011).…”

“…Recent variation in atmospheric methane concentrations, including the resumed increase from 2007 onwards, has been linked to changes in wetlands (e.g., in surface area, fertilizer use in rice cultivation and microbial processes) (Bousquet et al, 2006;Kai et al, 2011;Spahni et al, 2011). Especially, wet mineral soils (e.g., riparian floodplains), which can serve as a sink as well as a source of methane depending on the hydrological conditions, are proposed to contribute to the 2007 increase (Spahni et al, 2011). The question is to what extent the diversity of MOB communities and the traits of the species therein modulate methane consumption, thereby contributing to variability in methane emission from wetland habitats.…”

Multiple greenhouse-gas feedbacks from the land biosphere under future climate change scenarios

“…LPX-Bern [13][14][15][16][17][18] is applied here to simulate the coupled cycling of carbon and nitrogen and the emissions of GHGs from agricultural and natural land and from peat. Sitescale evaluations of this model have been presented earlier 7,[15][16][17][18][19][20] . For this test, we force LPX-Bern with observational data for climate 21 , cCO 2 (ref.…”

“…Simulated changes in eCH 4 from boreal peatlands, inundated areas (natural and anthropogenic) and wet mineral soils remain comparably small over the industrial period. The magnitudes of these fluxes were scaled individually to match results of an atmospheric CH 4 transport inversion 19 4 . Note also that even in the control set-up (ctrl), land use and Nr affect terrestrial GHG emissions, and eEXT is added and T ctrl is not zero.…”