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Downloaded 09/25/21 10:31 AM UTC 15 JANUARY 1999 SEMILETOV 287 286 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56 Aquatic Sources and Sinks of CO2 and CH4 in the Polar Regions I. P. SEMILETOV Paci®c Oceanological Institute, Vladivostok, Russia (Manuscript received 2 September 1997, in ®nal form 15 June 1998) ABSTRACT The highest concentration and greatest seasonal amplitudes of atmospheric CO 2 and CH4 occur at 608±708N, outside the 308±608N band where the main sources of anthropogenic CO2 and CH4 are located, indicating that the northern environment is a source of these gases. Based on the author's onshore and offshore arctic experimental results and literature data, an attempt was made to identify the main northern sources and sinks for atmospheric CH4 and CO2. The CH4 ef¯ux from limnic environments in the north plays a signi®cant role in the CH 4 regional budget, whereas the role of the adjacent arctic adjacent seas in regional CH 4 emission is small. This agrees with the aircraft data, which show a 10%±15% increase of CH4 over land when aircraft ¯y southward from the Arctic Basin. Offshore permafrost might add some CH4 into the atmosphere, although the preliminary data are not suf®cient to estimate the effect. Evolution of the northern lakes might be considered as an important component of the climatic system. All-season data obtained in the delta system of the Lena River and typical northern lakes show that the freshwaters are supersaturated by CO2 with a drastic increase in the CO2 value during wintertime. The arctic and antarctic CO2 data presented here may be used to develop understanding of the processes controlling CO2 ¯ux in the polar seas. It is shown that Arctic seas are a sink for atmospheric CO 2, though supersaturation by CO2 is obtained in areas in¯uenced by riverine output and in coastal sites. The pCO 2 difference between the surface of the Southern Ocean and atmosphere observed in the austral autumn shows that the area east of 78W might be considered a source of CO2 into the atmosphere, whereas the area west of 78W is a net sink of CO2. This is corroborated with literature data that indicate an overestimate of the role of antarctic waters as a sink for atmospheric CO2. 1. Introduction (Rassmussen and Khalil 1984; Steele et al. 1987; Ree- burgh and Crill 1996). Data from the present interna- Carbon dioxide is the most abundant and most im- tional network of atmospheric CO2 monitoring sites, portant greenhouse gas (other than water vapor) in the located almost exclusively in oceanic areas, cannot be atmosphere. The maximum annual average concentra- used to resolve longitudinal gradients. Thus identi®ca- tion of atmospheric CO2 is located within 608±808N tion of the important source±sink areas is dif®cult (Tans (Tucker et al. 1986; Conway et al. 1994), outside the et al. 1990; Conway et al. 1994; IGAC 1994), especially 308±608N band where the main sources of industrial in the Arctic, where only three North American stations CO2 are located (Rotty 1983). Annually about 0.1 3 for CO monitoring have been established. At present 15 2 10 g C of anthropogenic CO2 is emitted within the the global carbon cycle cannot be balanced to better 608±708N latitudinal band. The equilibrium greenhouse than about 25% of anthropogenic CO2 emissions. In warming associated with increase of CH4 and chemical order to bring the atmospheric budget CO2 and CH4 into feedback (through change of stratospheric water vapor) balance to the desired closure, improved measurements relative to warming during deglaciation is about 30% of CO must be combined with measurements of CH of the warning due to CO (Chappellaz et al. 1990). 2 4 2 and with improved oceanic and terrestrial measurements Currently, the rate of increase warming due to CH is 4 of both. Because the CO and CH interhemispheric about 38% of the CO warming effect (Craig and Chun 2 4 2 gradients and seasonal amplitudes show that the north- 1982). The maximum of atmospheric CH is also located 4 ern environment is a major contributor to the Northern over the Arctic and subarctic; the value of CH over 4 Hemisphere CO and CH maxima and seasonal vari- Greenland exceeds that over Antarctic by 8%±10% 2 4 ations (Rassmussen and Khalil 1984; Tucker et al. 1986; Cicerone and Oremland 1988; Nisbet 1989; Tans et al. 1990; Fung et al. 1991; Quay et al. 1991; Reeburgh et al. 1994), the role of Arctic seas and terrestrial ecosys- Corresponding author address: Dr. I. P.Semiletov, Arctic Regional Center, Paci®c Oceanological Institute, 43, Baltiyskaya Street, Vla- tems as sources and sinks of these greenhouse gases divistok 690041, Russia. should be evaluated. E-mail: [email protected] The interhemispheric gradient of atmospheric CO2 q 1999 American Meteorological Society Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 15 JANUARY 1999 SEMILETOV 287 was ;1 matm in the 1960s and is ;3.0 matm now (Fung sider here a northern source of atmospheric CH4 (meth- 1993). This change in the interhemispheric gradient ane hydrate, organics buried in the permafrost, and nat- might be determined by an increase of the Arctic and ural gas) as a CO2 source also. subarctic natural source of CO2 and/or an increase of a Anaerobic oxidation of organic matter in the season- sink in the Southern Ocean. The recent data (1989/90± ally thawed layer of onshore permafrost (Bartlett et al. 96), based on year-round measurements of CO2 ef¯ux 1992; Reeburgh et al. 1994) and peat bogs (Matthews from the typical northern soil in Siberia, show that only and Fung 1987; Nisbet 1989) may be an important wintertime soil respiration could provide a net source northern source of CH4. Different estimates show a of CO2 into the atmosphere up to 1 Gt C-CO2 and more large range of summertime ef¯ux of CH4 from the north- (Zimov et al. 1993; Zimov et al. 1996). This agrees with ern environment. This uncertainty in the ef¯ux of CH4 the evaluation of Ciais et al. (1995a), which shows that is related to inhomogenious time and spatial distribu- arctic ecosystems north of 658N are a net source of CO2 tions of CH4 ¯ux, and to limited experimental ground- of 1±1.2 Gt. The CO2 concentration that was measured level data (Reeburgh et al. 1994). In general, it was beneath the snow in winter in Siberia was similar to stated that CH4 might be produced in the northern eco- that measured in arctic tundra in Alaska (Kelley et al. systems only during summertime (100 days), and total 1968; Coyne and Kelley 1974) and temperate alpine evaluation of the northern source of CH4 was based on (Sommerfeld et al. 1993), suggesting also that the north- this assumption (Matthews and Fung 1987; Fung et al. ern ecosystems could exhibit large winter CO2 ef¯uxes. 1991; Reeburgh et al. 1994). However, the dramatic The winter CO2 emission from the northern soils would autumn surge in CH4 levels in the northern atmosphere increase the average CO2 concentration in air over the (Steele et al. 1987; Quay et al. 1991) requires us to Arctic by approximately 7±16 ppm in different years search for other regional sources of CH4. Early inves- (Zimov et al. 1993). Such a value is close to 50%±100% tigations of CH4 emission from a small lake in north- of the CO2 seasonal amplitude observed in this area. It western Ontario show that lake overturning might be a was found also that the range of CO2 ¯ux change is in signi®cant factor in the ultimate evasion of methane to good agreement with the varied double peak or near the atmosphere (Rudd and Hamilton 1978). double peak in atmospheric CO2 wintertime distribution Other arctic sources of CH4 include methane gas hy- (Semiletov et al. 1993) that was recorded by monitoring drates and natural gas losses, especially in Siberia. One stations such as Alert, Mould Bay, Point Barrow, and of the largest potential sources of CH4 emission to the Sable Island (Wong et al. 1984). Some measurements atmosphere is natural gas hydrates (Makagon 1982); the of summer CO2 ¯ux to the atmosphere were made in shelf and continental slope reservoir is estimated to be the 1990s in northern Siberia (Zimov et al. 1993; Zimov roughly 6 3 1018 g (or 6000 Gt), and the onshore per- et al. 1996) and in Alaska (Oechel et al. 1993). Oechel mafrost reservoir is about 16 3 1015 to 32 3 1015 g. et al. (1993) estimate that arctic tundra might contribute Usually, onshore CH4 is stable in hydrate in permafrost 0.19 Gt C-CO2 during summer, though long-term data at depths of 100 m and more, and offshore CH4 hydrate obtained in northeastern Siberia show a higher value of is potentially stable at shallow levels in the sea¯oor, in CO2 emission (Zimov et al. 1996). The conclusion of water depths in excess of about 250±300 m. At present, these works is that the large pools of carbon accumu- the Alaskan records show a warming of permafrost in lated in the north have became vulnerable to decay, and this century (Lachenbruch and Marshall 1986), and no carbon is released from soils mainly in CO2 form. It is experimental data indicate a temperature increase of on- important that the main input of CO2 into the atmo- shore permafrost in Siberia and Canada. Temperature sphere be determined by ef¯ux in winter when respi- pro®les in the offshore permafrost are episodic or lim- ration of biota adopted for cold soil environment is not ited in timescale (Fartishev 1993; Romanovskiy et al.
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