METHANE AS A GREENHOUSE GAS

Atmospheric methane (CH4) is a significant greenhouse gas that is emitted to the atmosphere by natural and anthropogenic sources (Dlugokencky et al. 2011). Radiative forcing (RF) is defined as the difference of insolation (sunlight) absorbed by the Earth and energy radiated back to space, and it is used to assess the natural and anthropogenic drivers of climate change. CH4 has the second-largest radiative forcing (approx. 0.5 Wm-²) of all long-lived greenhouse gases (CH4, CO2 and N2O), which is about 20 % of the total direct radiative forcing of the atmosphere (Myhre et al. 2013). The global warming potential (GWP) of CH4 is estimated to be 28 times larger in comparison to CO2 for a 100-year timescale (Myhre et al. 2013).

Atmospheric CH4 concentration has significantly increased from 700 ppb in the pre-industrial era to 1850 ppb in 2016 (Dlugokencky 2017). This increase has been explained by the dynamics between CH4 sources and sinks (Kirschke et al. 2013). Most of the sources and sinks of CH4 have been identified, but their relative effects on CH4 levels in the atmosphere are still poorly understood (Dlugokencky et al. 2011; Kirschke et al. 2013). There are uncertainties in the physical and chemical processes, emissions and meteorology that complicate the estimation of the global CH4 budget (Dalsøren et al 2016).

CH4 has a short lifetime (approx. 9 years) in the atmosphere compared to CO2 (100-300 years) (Dlugokencky et al. 2011), which is why a reduction of CH4 emissions would decrease CH4 radiative forcing fast and have notable benefits for climate in a short period of time (Saunois et al. 2016).

Therefore, the atmospheric CH4 could have a significant role mitigating climate change (Dlugokencky et al. 2011; Saunois et al. 2016).

2.1.2 Sources of methane

CH4 is emitted to the atmosphere from various sources, which can be classified in different ways. The share of natural versus anthropogenic sources is a common way of classification, but the CH4 sources can also be grouped into three categories by the emitting process: thermogenic, biogenic and pyrogenic processes (Kirschke et al. 2013). Global estimates of CH4 emissions from natural and anthropogenic sources by Saunois et al. (2016) are summarized in table 1. Recent bottom-up

approaches (process-based models) estimate natural sources to be slightly over half of the total emissions, while in the top-down approaches (atmospheric inversion models) anthropogenic sources dominate CH4 emissions (Bousquet et al. 2006; Kirschke et al. 2013; Saunois et al. 2016).

Table 1. Global emissions of CH4 annually in 2003-2012 (Saunois et al. 2016). Estimates are based on the bottom-up approach that includes process-based models of CH4 emissions.

Source Annual emissions Tg CH4 yr⁻¹ Emission process

CH4 from biogenic sources is the final product of the decomposition of organic matter in anaerobic conditions (Saunois et al. 2016). Biogenic CH4 is mainly produced by methanogenic archaea (methanogens) (Kirschke et al. 2013), but also aerobic bacterial degradation of dissolved organic matter (DOM) phosphonates has been recognized in marine waters (Repeta et al. 2016). Thermogenic CH4 is produced through geological processes by the buried OM with the presence of heat and pressure in the Earth’s crust (Saunois et al. 2016), and it is emitted to the atmosphere by the exploitation and distribution of fossil fuels (Kirschke et al. 2013). CH4 from pyrogenic sources is formed by the incomplete combustion of biomass and soil carbon (Kirschke et al. 2013). The most significant sources of pyrogenic CH4 are wildfires, biomass burning in deforested and degraded areas, and fossil or biofuel usage (Saunois et al. 2016).

The largest contribution of natural emissions comes from wetlands with approximately 150-200 Tg CH4 yr⁻¹ (Dlugokencky et al. 2011). Wetlands include peatlands, such as bogs and fens, mineral wetlands, such as swamps and marshes, and floodplains. However, factors like temperature, oxygen levels, substrate availability, hydrology and precipitation cause significant variations to annual CH4

emissions of wetlands (Dlugokencky et al. 2011; Saunois et al. 2016). Thereby the variations of wetland emissions are the major drivers for year-to-year changes of CH4 concentration in the atmosphere (Dalsøren et al 2016). Other natural sources of CH4 include lakes, ponds, rivers, estuaries, oceans, land geological sources, permafrost areas, termites, wild fires and wild ruminants (Dlugokencky et al. 2011; Saunois et al. 2016). CH4 emissions from oceanic sources also include methane hydrates (or chlathrates) that have a crystal structure similar to ice. Hydrates are produced under specific temperature conditions in the marine sediments (Saunois et al. 2016). In addition, besides the emissions from wetlands and thermokarst lakes in permafrost areas, the thawing of permafrost is expected to increase CH4 emissions in the longer term (Kirschke et al. 2013).

The increase in the atmospheric CH4 since the pre-industrial times is caused mainly by human activ-ities such as fossil fuel burning, agriculture, waste management, and biomass and biofuel burning (Dlugokencky et al. 2011). The CH4 emissions from fossil fuel burning include exploitation, trans-portation and usage of coal, oil and natural gas (Saunois et al. 2016). Total emissions of fossil fuel section are estimated to be over 100 Tg CH4 yr^-1 (Dlugokencky et al. 2011; Saunois et al. 2016).

Agricultural sources of CH4 include emissions from livestock (enteric fermentation in ruminants, and manure) and rice cultivation, of which livestock is the largest source of emissions (about 80–90 Tg CH4 yr⁻¹) (Dlugokencky et al. 2011; Saunois et al. 2016). Significant amount of CH4 is produced by anaerobical microbial activity in the digestive systems of domestic livestock, such as cattle, sheep and goats (Johnson et al. 2002).

The emissions of waste management (about 50-60 Tg CH4 yr⁻¹)are caused especially by decomposi-tion of biodegradable solid waste (landfills) and wastewater treatment (Dlugokencky et al. 2011).

Environmental conditions such as pH, moisture and temperature cause variation to CH4 production of waste management. In 2000, the emissions of waste management were globally about 11 % of the total anthropogenic CH4 emissions (Saunois et al. 2016).

Biomass and biofuel burning are emitting CH4 under incomplete combustion conditions. Anthropo-genic biomass burning occurs particularly in the tropical and subtropical areas, where burning activ-ities are usually related to agricultural land clearing (Saunois et al. 2016). However, meteorological

variables, such as El Niño, cause annual variation to emissions of biomass burning (Dlugokencky et al. 2011). CH4 emissions of biofuel burning originate from domestic cooking and heating in stoves, boilers and fireplaces. The burned material is mostly wood, charcoal, agricultural residues or animal dung (Saunois et al. 2016).

2.1.3 Sinks of methane

CH4 is the most abundant reactive trace gas in the troposphere (Saunois et al. 2016). The most important removal process of CH4 is the reaction with hydroxyl radicals (OH) in the troposphere (reaction 1), which contributes about 90 % of the total CH4 sink (Dlugokencky et al. 2011). The reaction with OH reduces the oxidizing capacity of the atmosphere and generates ozone (O3) in the troposphere (Kirschke et al. 2013). Other sinks are the reactions with electronically charged oxygen atoms in the stratosphere, the reactions with atomic chlorine in the marine boundary layer and oxidation by methanotrophic microbes in soils, wetlands, lakes and oceans (Dlugokencky et al. 2011;

Saunois et al. 2016).

CH

+ OH·→ CH

· + H

O

Hydroxyl radicals are produced by the photolysis of the O3 and destroyed by the reactions with CO, CH4 and volatile organic compounds (VOC) (Saunois et al. 2016). OH occurs in photochemical equilibrium with hydroperoxyl (HO2), which is why the net effect of CH4 oxidation on the HOx budget is also affected by NOx and other competitive oxidants. Although CH4 and CO are effective OH sinks, OH is not necessarily depleted, because for example NOx can recycle part of the radicals (Lelieveld et al. 2002).

2.2 METHANE DYNAMICS IN BOREAL LAKES