Page 27 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

between solar irradiance absorbed by the Earth and energy radiated back to space. The increase in atmospheric concentrations since pre-industrial times has resulted in a direct radiative forcing of CH4 of about 0.5 W m-2. In comparison, this equals about one third of the radiative forcing of CO2 (Myhre et al. 2013) (Fig. 2). This emphasizes the significant contribution of CH4 to the greenhouse effect. However, estimates of the climate impacts of CH4 are still evolving. Recent work suggested that the radiative forcing of CH4 from the year 1750 to 2011 may have been 25% higher than estimated by the Intergovernmental Panel on Climate Change (IPCC) in 2013 (Etminan et al. 2016). When including atmospheric feedbacks, the total change in radiative forcing of CH4 since 1750 is about 1 W m-2 (Myhre et al. 2013) (Fig. 3). This equals roughly 60% of the radiative forcing of CO2.
Figure 3. Radiative forcing bar chart for the period 1750–2011 based on the GHGs CO2 and CH4. The colors indicate the different GHGs that are affected by CH4 dynamics in the atmosphere. The net impacts of the individual contributions are shown by a diamond symbol and its uncertainty (5 to 95% confidence range) is given by horizontal error bars. The vertical error bars indicate the relative uncertainty of the radiative forcing induced by each component. Their length is proportional to the thickness of the bar, with full bar thickness representing ± 50% uncertainty. O3: ozone. H2O (strat.) represents stratospheric water vapor. Adapted from Myhre et al. (2013).
The fraction of the produced CH4 that enters the atmosphere, where it acts as a powerful GHG, depends conceptually on three factors: the production rate, the rate of transport from the region of production/storage to the atmosphere, and the rate of consumption along this transport pathway. Methane is produced by both biotic and abiotic processes. The biotic production rate is mainly dependent on the activity of methanogenic archaea, which are in turn controlled by biotic and abiotic environmental factors. Abiotic CH4 production is mainly related to mantel- derived and magmatic processes (Etiope and Sherwood Lollar 2013). The rate of transport depends largely on the physical properties of the environment, including sediment type, density, and local hydrological conditions. The rate of consumption is dependent on the activity of CH4-
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