Sulfur pollution suppression of the wetland methane source in the 20th and 21st centuries

Abstract

Natural wetlands form the largest source of methane (CH(4)) to the atmosphere. Emission of this powerful greenhouse gas from wetlands is known to depend on climate, with increasing temperature and rainfall both expected to increase methane emissions. This study, combining our field and controlled environment manipulation studies in Europe and North America, reveals an additional control: an emergent pattern of increasing suppression of methane (CH(4)) emission from peatlands with increasing sulfate (SO(4)(2-)-S) deposition, within the range of global acid deposition. We apply a model of this relationship to demonstrate the potential effect of changes in global sulfate deposition from 1960 to 2080 on both northern peatland and global wetland CH(4) emissions. We estimate that sulfur pollution may currently counteract climate-induced growth in the wetland source, reducing CH(4) emissions by approximately 15 Tg or 8% smaller than it would be in the absence of global acid deposition. Our findings suggest that by 2030 sulfur pollution may be sufficient to reduce CH(4) emissions by 26 Tg or 15% of the total wetland source, a proportion as large as other components of the CH(4) budget that have until now received far greater attention. We conclude that documented increases in atmospheric CH(4) concentration since the late 19th century are likely due to factors other than the global warming of wetlands.

Fig. 1.

Percentage change in suppression of CH₄ flux and change in sulfate-reduction rates with SDEP. Percentage of CH₄ flux suppression and sulfate reduction rates (SRR) as a function of SDEP. •, changes in suppression of CH₄ emission with manipulated input; ○, changes in sulfate reduction rates with SDEP (ref. and references therein). DS (Se), Degerö Stormyr, Västerbotten, Sweden, 1996 (9); MM (UK), Moidach More, Morayshire, Scotland (23); BLP (USA), Bog Lake Peatland, MN (22); CONV, laboratory peat monolith study under controlled conditions (24). Calculations of annual SDEP for CH₄ studies are based on multiplying weekly addition rates by 52.

Fig. 2.

Effect of SDEP on northern wetland CH₄ emission with time. (a) Change in northern wetland CH₄ source (>40° North) with time under three different climate/sulfate scenarios. Gray line indicates the size of the estimated preindustrial northern wetland methane source (Tg) (38). (b) Relative impact of the interaction on northern wetland CH₄ emissions for two climate scenarios (GHG and GHG+AERO).

Fig. 3.

Effect of SDEP on the global wetland CH₄ source with time. (a) Change in wetland CH₄ source with time under three different climate/sulfate scenarios. Gray line indicates the size of the estimated preindustrial wetland methane source (Tg) (38). (b) Relative impact of the interaction on the wetland CH₄ source for two climate scenarios (GHG) and (GHG+AERO).