Identifying main uncertainties in estimating past and present radiative forcing of peatlands

Reconstructions of past climate impact, that is, radiative forcing (RF), of peatland carbon (C) dynamics show that immediately after peatland initiation the climate warming effect of CH₄ emissions exceeds the cooling effect of CO₂ uptake, but thereafter the net effect of most peatlands will move toward cooling, when RF switches from positive to negative. Reconstructing peatland C dynamics necessarily involves uncertainties related to basic assumptions on past CO₂ flux, CH₄ emission and peatland expansion. We investigated the effect of these uncertainties on the RF of three peatlands, using either apparent C accumulation rates, net C balance (NCB) or NCB plus C loss during fires as basis for CO₂ uptake estimate; applying a plausible range for CH₄ emission; and assuming linearly interpolated expansion between basal dates or comparatively early or late expansion. When we factored that some C would only be stored temporarily (NCB and NCB+fire), the estimated past cooling effect of CO₂ uptake increased, but the present‐day RF was affected little. Altering the assumptions behind the reconstructed CO₂ flux or expansion patterns caused the RF to peak earlier and advanced the switch from positive to negative RF by several thousand years. Compared with NCB, including fires had only small additional effect on RF lasting less than 1000 year. The largest uncertainty in reconstructing peatland RF was associated with CH₄ emissions. As shown by the consistently positive RF modelled for one site, and in some cases for the other two, peatlands with high CH₄ emissions and low C accumulation rates may have remained climate warming agents since their initiation. Although uncertainties in present‐day RF were mainly due to the assumed CH₄ emission rates, the uncertainty in lateral expansion still had a significant effect on the present‐day RF, highlighting the importance to consider uncertainties in the past peatland C balance in RF reconstructions.