Atmos. Chem. Phys., 18, 17895–17907, 2018 https://doi.org/10.5194/acp-18-17895-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Detecting changes in Arctic methane emissions: limitations of the inter-polar difference of atmospheric mole fractions Oscar B. Dimdore-Miles1, Paul I. Palmer1, and Lori P. Bruhwiler2 1School of GeoSciences, University of Edinburgh, Edinburgh, UK 2National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, Colorado, USA Correspondence: Paul I. Palmer ([email protected]) Received: 15 November 2017 – Discussion started: 8 January 2018 Revised: 5 November 2018 – Accepted: 30 November 2018 – Published: 17 December 2018 Abstract. We consider the utility of the annual inter-polar dom noise. We find that it can take up to 16 years to detect difference (IPD) as a metric for changes in Arctic emissions the smallest prescribed trend in Arctic emissions at the 95 % of methane (CH4). The IPD has been previously defined as confidence level. Scenarios with higher, but likely unrealis- the difference between weighted annual means of CH4 mole tic, growth in Arctic emissions are detected in less than a fraction data collected at stations from the two polar regions decade. We argue that a more reliable measurement-driven (defined as latitudes poleward of 53◦ N and 53◦ S, respec- approach would require data collected from all latitudes, em- tively). This subtraction approach (IPD) implicitly assumes phasizing the importance of maintaining a global monitoring that extra-polar CH4 emissions arrive within the same cal- network to observe decadal changes in atmospheric green- endar year at both poles. We show using a continuous ver- house gases. sion of the IPD that the metric includes not only changes in Arctic emissions but also terms that represent atmospheric transport of air masses from lower latitudes to the polar re- gions. We show the importance of these atmospheric trans- 1 Introduction port terms in understanding the IPD using idealized numeri- cal experiments with the TM5 global 3-D atmospheric chem- Atmospheric methane (CH4) is the second most important istry transport model that is run from 1980 to 2010. A north- contributor to anthropogenic radiative forcing after carbon ern mid-latitude pulse in January 1990, which increases prior dioxide. Observed large-scale variations of atmospheric CH4 emission distributions, arrives at the Arctic with a higher (Nisbet et al., 2014) have evaded a definitive explanation due mole fraction and ' 12 months earlier than at the Antarc- to the sparseness of data (Kirschke et al., 2013; Rigby et al., tic. The perturbation at the poles subsequently decays with 2016; Turner et al., 2016; Schaefer et al., 2016; Saunois et al., an e-folding lifetime of ' 4 years. A similarly timed pulse 2016). Atmospheric CH4 is determined by anthropogenic emitted from the tropics arrives with a higher value at the and natural sources, as well as by loss from oxidation by the Antarctic ' 11 months earlier than at the Arctic. This pertur- hydroxyl radical (OH) with smaller loss terms from soil mi- bation decays with an e-folding lifetime of ' 7 years. These crobes and oxidation by Cl. This results in an atmospheric simulations demonstrate that the assumption of symmetric lifetime of ' 10 years. Anthropogenic CH4 sources include transport of extra-polar emissions to the poles is not realis- leakage from the production and transport of oil and gas, tic, resulting in considerable IPD variations due to variations coal mining, and biomass burning associated with agricul- in emissions and atmospheric transport. We assess how well tural practices and land use change. Microbial anthropogenic the annual IPD can detect a constant annual growth rate of sources include ruminants, landfills, and rice cultivation. The Arctic emissions for three scenarios, 0.5 %, 1 %, and 2 %, largest natural source is microbial emissions from wetlands, superimposed on signals from lower latitudes, including ran- with smaller but significant contributions from wild rumi- nants, termites, wildfires, landfills, and geologic emissions Published by Copernicus Publications on behalf of the European Geosciences Union. 17896 O. B. Dimdore-Miles et al.: Detecting Arctic CH4 emissions (Kirschke et al., 2013; Saunois et al., 2016). Here, we focus inter-polar source (mass CH4 per unit time) emitted at posi- on our ability to quantify changes in Arctic emissions using tion r and time t. The IPDC is given by polar atmospheric mole fraction data. 90 −53 Warming trends over the Arctic, approximately twice the 1 Z 1 Z IPDC.t/ D c.r;t/dr − c.r;t/dr; (1) global mean (AMAP, 2015), are eventually expected to re- 1r 1r sult in thawing of permafrost. Observational evidence shows rD53 r=−90 that permafrost coverage has begun to shrink (Christensen et al., 2004; Reagan and Moridis, 2007). Arctic soils store where 1r is the graduation in latitude in the model and c.r;t/ an estimated 1700 GtC (Tarnocai et al., 2009). As the soil denotes atmospheric CH4 mole fraction (ppb) at latitude r organic material thaws and decomposes it is expected that and time t that includes influences from all other latitudes some fraction of this carbon will be released to the atmo- and previous times. The mole fraction can then be described as sphere as CH4, depending on soil hydrology. Current under- standing is that permafrost carbon will enter the atmosphere t0Dt 90 Z Z slowly over the next century, reaching a cumulative emis- 0 0 0 0 t0−t 0 0 c.r;t/ D k.r ;t /S.r ;t /H 0 dr dt ; (2) sion of 130–160 PgC (Schuur et al., 2015). If only 2 % of r −r t0=−∞ r0=−90 this carbon is emitted as CH4, annual Arctic emissions could approximately double by the end of the century from current where S.r0;t0/ denotes the surface emission fluxes −1 0 estimates of 25 Tg CH4 year inferred from atmospheric in- −2 −1 t −t (g cm s ); Hr0−r denotes the fraction of emissions versions (AMAP, 2015). At present, using data from the cur- from location r0 at initial time t0 that contributes to the rent observing network there is no strong evidence to sug- concentration at location r and a later time t, which includes gest large-scale changes in Arctic emissions (Sweeney et al., atmospheric chemistry and transport; and k.r0;t0/ (cm3 g−1) 2016). describes the conversion between emissions and atmospheric The inter-polar difference (IPD) has been proposed as a mole fraction (parts per billion, ppb) and takes the form sensitive indicator of changes in Arctic emissions that can be Na k.r;t/ D , where Na and Mw denote Avogadro’s derived directly from network observations of atmospheric Mwρ.r;t/ constant (molecules mole−1) and the molar weight of CH CH mole fraction. The IPD, as previously defined (Dlu- 4 4 (g mole−1), respectively, and ρ.r;t/ denotes the number gokencky et al., 2003), is the difference between weighted density of air (molec cm−3). annual means of CH mole fraction data collected at polar 4 Our expression for IPDC can be reformulated as the dif- stations (those poleward of ±53◦ > latitude) such as those ference between values determined at time t and a refer- from the NOAA Earth System Research Laboratory (ESRL) ence time t . The reader is referred to AppendixA for a full network (https://www.esrl.noaa.gov/gmd/dv/site/site_table2. 0 derivation of the expressions used in this introduction. Equa- php, last access: 10 December 2018). Data from individual tion (3) describes the IPD using the assumptions previously sites are weighted inversely by the cosine of the station lat- used (e.g. Dlugokencky et al., 2003): (1) the southern polar itude and by the standard deviation of the data at a particu- region contains no local sources, and (2) emissions from the lar site. Dlugokencky et al.(2003) reported an abrupt drop northern polar region are too diffuse after transport between in IPD during the early 1990s. They suggested this magni- poles to significantly affect mole fractions at the southern po- tude of change was indicative of a 10 Tg CH year−1 reduc- 4 lar region. tion, which they attribute to the collapse of fossil fuel pro- duction in Russia following the 1991 breakup of the Soviet C C IPD .t/ − IPD .t0/ D Union (Dlugokencky et al., 2011). In more recent work, Dlu- t0Dt 90 90 gokencky et al.(2011) proposed that the IPD metric is po- Z Z Z 1 0 0 0 0 t0−t 0 tentially sensitive to changes in Arctic emissions as small k.r ;t /S.r ;t /Hr0−r dr − 1r 1 0 0 as 3 Tg CH4 year , representing a value of 10 % of north- t Dt0 rD53 r D53 ern wetland emissions. However, studies have reported little 53 Z or no increase in IPD between 1995 and 2010 (Fig.1, Dlu- 0 0 0 0 t0−t 0 C k.r ;t /S.r ;t /H 0 dr dr gokencky et al., 2011, 2003), a period during which rising r −r Arctic temperatures were expected to lead to an increase in r0=−53 −53 53 emissions (Mauritsen, 2016; McGuire et al., 2017). In this Z Z 0 0 0 0 t0−t 0 0 work, we examine how sensitive the IPD is to changing CH4 − k.r ;t /S.r ;t /Hr0−r dr dr dt (3) emissions by using model simulations guided by results from r=−90 r0=−53 an analytical approach. First, we derive the continuous version of the IPDC to in- The first integral in Eq.