Janne Rinne, Juha-Pekka Tuovinen, Annalea Lohila

Commensuration of the radiative effects of different greenhouse gases (GHGs) is crucial for understanding the effects of land cover and ecosystem changes on the global climate. However, none of the current commensuration approaches are suitable for addressing the current climatic effect of mire ecosystems as compared to the situation in which such mires would not exist. The mire ecosystems have accumulated carbon for millennia, creating a negative perturbation to the atmospheric carbon dioxide content, but at the same time they emit methane into the atmosphere. Thus, the functioning of mires involves GHG fluxes with opposing effects on Earth’s radiative balance. Here, based on a simple radiative forcing (RF) model, we propose a new metric for commensuration of the effects of accumulated carbon and methane emission (ACME) on Earth’s energy balance. This ACME approach is applicable to natural mires with a significant part of their carbon accumulated more than 1000 years ago and requires relatively few input data. We demonstrate the feasibility of the ACME approach by applying it to a set of northern mires. The ACME-based RF estimate indicates that these mires have a cooling effect on the current climate, contrary to what a global warming potential-based calculation suggests, since the climatic effect is dominated by the sustained carbon accumulation. By applying the new metric with varying estimates of the total carbon storage and methane emission of northern mires, we estimate the current RF of these mires to range from –0.45 to –0.23 W m-2.