A novel metric for estimating the climatic effect of natural mire ecosystems

Mire ecosystems, i.e. peat forming wetlands, have sequestered carbon (C) from the atmosphere for millennia, storing it in peat and thus creating a negative perturbation on the atmospheric carbon dioxide CO₂ content. At the same time the mires emit methane (CH₄) into the atmosphere, creating a positive perturbation on the atmospheric content of this powerful greenhouse gas. Thus, the exchange of these greenhouse gases (GHG) by mires creates opposing radiative forcing (RF) components, with CO₂ perturbation cooling and CH₄ perturbation warming the climate.

The current methods for commensuration of climatic effects of these greenhouse gases, such as global warming potential (GWP) or sustained GWP-based approaches, are not applicable for quantification of the current climatic effect of natural mires, as they fail to consider the effect of accumulated carbon. We have developed a novel approach to quantify the current climate impact of these systems, based on accumulated carbon and methane emission (ACME). With certain assumptions it can be shown, both by simulations with an RF model and by analytical solutions of the governing equations, that the current RF of a mire ecosystem can be closely approximated by its carbon storage and average methane emission during the last 50 years.

The ACME metric is applicable for natural mires which have accumulated a significant part of their C storage more than 1000 years ago and have not experienced major disturbances within the last 500 years. The minimum requirement for input data for the ACME metric are the estimate of total C storage density and annual methane emission. The former can be obtained from one or more peat cores taken from peat surface to the mineral soil below. The annual methane emission can be obtained by eddy-covariance measurements or by chamber measurements. The ACME metric can be further elaborated by including the data on current annual CO₂ exchange and N₂O emission.

We demonstrate the feasibility of the ACME metric by applying it to a set of northern mire systems. The ACME based RF indicates the mires to have a cooling effect on climate, with C storage dominating their climatic effect. This is an opposite to the warming effect estimated by GWP-based approaches. Furthermore, we applied the ACME approach globally to the mires north of 45°N, using their estimated C storage and CH₄ emission leading to their current RF to be between –0.45 and –0.23 W m^-2.