Holocene radiative forcing impact of northern peatland carbon accumulation and methane emissions

Throughout the Holocene, northern peatlands have both accumulated carbon and emitted methane. Their impact on climate radiative forcing has been the net of cooling (persistent CO(2) uptake) and warming (persistent CH(4) emission). We evaluated this by developing very simple Holocene peatland carbon flux trajectories, and using these as inputs to a simple atmospheric perturbation model. Flux trajectories are based on estimates of contemporary CH(4) flux (15-50 Tg CH(4) yr(-1)), total accumulated peat C (250-450 Pg C), and peatland initiation dates. The contemporary perturbations to the atmosphere due to northern peatlands are an increase of similar to 100 ppbv CH(4) and a decrease of similar to 35 ppmv CO(2). The net radiative forcing impact northern peatlands is currently about -0.2 to -0.5 W m(-2) (a cooling). It is likely that peatlands initially caused a net warming of up to +0.1 W m(-2), but have been causing an increasing net cooling for the past 8000-11 000 years. A series of sensitivity simulations indicate that the current radiative forcing impact is determined primarily by the magnitude of the contemporary methane flux and the magnitude of the total C accumulated as peat, and that radiative forcing dynamics during the Holocene depended on flux trajectory, but the overall pattern was similar in all cases.