Are natural mires warming or cooling Earth’s climate? - A perspective by the new ACME metric

Introduction Mire ecosystems, i.e. peat forming wetlands, have sequestered carbon dioxide (CO 2 ) from the atmosphere for millenia accumulating its carbon (C) into thick peat deposits. This has created a negative perturbation to the atmospheric CO 2 content with a consequent climate cooling effect. At the same time, the mires emit methane (CH 4 ) into the atmosphere, which results in a climate warming effect as CH 4 is a powerful greenhouse gas (GHG). Thus, the functioning of mire ecosystems involves GHG fluxes with opposing effects on Earth’s radiative balance (e.g. Frolking et al. (2006)). Commensuration of the radiative forcing (RF) of different GHGs is crucial for understanding the effects of land cover and ecosystem changes on the global climate. However, none of the current commensuration approaches, such as those based on the Global Warming Potential (GWP) or Sustained GWP values, are suitable for addressing the current climatic effect of natural mire ecosystems. Here, our aim has been to develop a practical method to correctly quantify the climatic effect of natural mire ecosystems as compared to a situation in which such a mire ecosystem would not exist. Materials and Methods The radiative forcing due to a perturbation to the atmospheric mixing ratio of a well-mixed GHG depends on the magnitude of the perturbation and the radiative efficiency of the GHG in question. The temporal dynamics of the atmospheric GHG content can be modelled by integrating an atmospheric impulse-response function (Enting 2003). For CH 4 and nitrous oxide (N 2 O), we adopted first-order decay functions (Myhre et al. 2013). For CO 2 , the dynamics are more complex, and the processes acting in widely differing time scales were modelled by dividing the total CO 2 mass into several compartments. One of these has a very long perturbation time scale, thus resulting in a permanent change in the atmospheric CO 2 content, while in the others an atmospheric mass pulse decays with a characteristic, finite time scale (Joos et al. 2013). The radiative efficiencies were derived from the RF parameterization of Etminan et al. (2016). As discussed earlier by e.g. Frolking et al. (2006), and also shown by the calculations with impulse-response RF model, the current RF of a mire ecosystem depends mostly on its total C storage and its recent CH 4 emission. Thus the RF can be quantified by multiplying these two input variables by the corresponding RF coefficients, with further refinement by addition of data on recent Carbon Accumulation Rate (CAR) and N 2 O emission. 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. It provides an easy-to-use tool that requires few input data. Results and Discussion We demonstrate the feasibility of the ACME approach by applying it to a set of northern mires. The ACME-based RF estimates indicate that these mires have a cooling effect on the current climate, contrary to what a traditional GWP-based calculation suggests (Fig. 1). This exhibits a clear qualitative difference in the climatic effect of mire systems as suggested by these two approaches. The climatic effect as quantified by the ACME metric is dominated by the C accumulated into the mires over the millennia, which is ignored when using the GWPs and present-day fluxes. Furthermore, by applying the new metric with estimates of the global C storage and CH 4 emission of mires north of 45°N, we can demonstrate their global cooling effect and estimate their current RF to range from –0.45 to –0.23 W m -2 .