~N_A_TU-'-R~E~V~O~L-'-. -'-33-'-6~ 1 =D~E=C=E_M_B_ER~ l-'-98"-'8----- --- --ARTICLES 441 Are global cloud albedo and climate controlled by marine phytoplankton? Stephen E. Schwartz Environmental Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, USA The recent suggestion that dimethylsulphide emissions by marine phytoplankton control global albedo and mean temperature would also imply a strong climatic influence of man-made S02• Anthropogenic S02 emissions exceed marine emissions of dimethylsulphide globally and are confined largely to the Northern Hemisphere. But no such influence of S02 emissions is found either in the present cloud component of planetary albedo or in JOO-year temperature records. RECENTLY, Charlson, Lovelock, Andreae and Warren1 gaseous sulphur emissions between the Northern Hemisphere (hereinafter CLAW) proposed that production of dimethylsul­ (NH) and Southern Hemisphere (SH) constitute a global experi­ phide (DMS) by marine microorganisms is the major source of ment that tests the hypothesis of ref. 1, that planetary albedo aerosol sulphate in the remote marine atmosphere and, in turn, and mean temperature are regulated by marine DMS emissions. of cloud condensation nuclei (CCN). They suggest that this It will be seen that a comparison of the present NH and SH process, by influencing planetary albedo, comprises a com­ cloud component of planetary albedo and of 100-year tem­ ponent of a biological mechanism for climate regulation. perature records yields no indication of any influence of Specifically, it is hypothesized that an increase in marine DMS anthropogenic SO2 emissions that is either qualitatively or quan­ emissions would increase the number density of aerosol sulphate titatively consistent with the expectations in this respect based particles and, in turn, the number density of cloud droplets, on the CLAW hypothesis. thus enhancing cloud albedo. This enhancement would reduce global temperature generally, including ocean temperatures, and Emission rates would consequently reduce marine productivity and DMS Emissions of reduced sulphur compounds and of sulphur 5 7 emission; that is to say, the process is a negative feedback system dioxide have been critically reviewed recently - and are sum­ for control of the Earth's temperature. Such a feedback system marized in Table 1. Andreae6 estimates total global biogenic 2 was subsequently and independently suggested by Meszaros • emissions of sulphur gases to be 95 ± 30 Tg of sulphur per year, Here I examine the hypothesized link between gaseous precur­ DMS and H2S each accounting for about half of this total 1 6 sors of aerosol sulphate, and global albedo and mean tem­ ( 43 ± 20 and 32 ± 27 Tg S yr- , respectively). The estimate for 1 perature. Because marine DMS emissions are substantially lower marine DMS emissions, 40 ± 20 Tg S yr- , is based on concentra­ than present emissions of other gaseous precursors of atmos­ tions of OMS in ocean surface water at a variety of locations pheric sulphate (principally SO2 associated with fossil fuel com­ together with estimates of sea-air exchange rates. A lower OMS bustion) globally and especially in the Northern Hemisphere, emission rate of 16 Tg S yr- 1 (uncertain to a factor of two) is 8 then consequently, if, as CLAW suggest, marine DMS emissions given by Bates et a/. • Natural emission rates in Table 1 are exert a major influence on global mean albedo and temperature, consistent with budget estimates based on considerations of such an influence should also result from anthropogenic SO2 turnover times and of concentrations of sulphur compounds in 9 emissions. The potential for such anthropogenic influence on the atmosphere • cloud albedo and global mean temperature has been noted On a global basis, SO 2 emissions are roughly equal to biogenic 4 previously3· • But CLAW explicitly "ignore the perturbations of emissions and are at least twice as great as marine OMS the atmospheric sulphur cycle by man-made fluxes of em1ss1ons. A prima facie case exists, therefore, that SO 2 .•• and ... consider only the natural fluxes, which currently anthropogenic sulphur emissions cannot be neglected in examin­ represent about 50% of the total flux of gaseous sulphur to the ing the possible influence of biogenic gaseous sulphur com­ atmosphere, and which still dominate the atmospheric sulphur pounds on global cloud albedo and climate. Also, as cycle in the Southern Hemisphere". I argue here that the increase anthropogenic SO 2 emissions have increased to their present in SO 2 emissions over the past 100 years and the contrast in level almost entirely within the last 100 years (ref. 5 and H. Rodhe, private communication cited in ref. 7), any climatic influence of these emissions should be discernible in records Gaseous sulphur emission rates 10 Table 1 over this period . Furthermore, the asymmetric distribution of anthropogenic SO emissions over the globe suggests that the Northern Southern 2 of gaseous sulphur emissions on climate may also be Emission source Hemisphere Hemisphere Global influence tested by interhemispheric comparisons, significant interhemis­ Marine DMS 17 23 40 pheric transport (on a timescale of one to two years) being precluded by the short atmospheric residence times of these Total marine biogenic 22 28 50 9 11 substances • • Terrestrial biogenic 32 16 48 Distribution of aerosol sulphate Total biogenic 54 44 98 For sulphate derived from anthropogenic SO to serve as a Anthropogenic SO2 98 6 104 2 surrogate to test the CLAW hypothesis, this material must be Total 150 50 200 widespread throughout the NH. The geographical distribution Data are from Cullis and Hirschler5, except for marine DMS of sulphate attributable to anthropogenic emissions of SO2 is 6 (Andreae ), apportioned according to ocean surface area: 42% NH and examined in Table 2, which presents concentrations of aerosol 58 % SH. Emission rates are in units of Tg S yr- '. sulphate in boundary-layer air at locations remote from © 1988 Nature Publishing Group _44_2 ______________________ARTICLES------------N_A_T_U_R_E_V_O_L_. _33_6_1_D_E_C_E_M_B_E_R_I_9_88 Table 2 Measured concentration of sulphate (or sulphur) aerosol at remote locations in the Northern and Southern Hemisphere Sulphate concentration* 3 Location (ng S m- ) Comments Ref. Northern Hemisphere High-latitude sites Alert, Mould Bay, Igloolik; NSS so~- '; I-week samples; 3- to 4-year data record 15 Canadian Arctic (66-83° N) (200-1,000) Winter-Spring (20-70) Summer Faeroe Islands (62° N) NSS so;;- 13 1,100 (700-1,400) British trajectory (-1,000 km); 4 I-day samples 140 (30-230) Atlantic trajectory; 4 1-day samples Velen, Sweden (58° N) Sub-µ.m S; 1-day samples 18 960 [ 1.4] British trajectory (-1,000 km); 12 samples 60 [2.2] North Sea trajectory (Northwest air); 14 samples Atlantic and Caribbean Western North Atlantic 800±500 Sub-µ.m S; 26 8- to 68-hour samples 18 (33-38° N, 65-70° W) Bermuda (32° N) 530 [3.0] Sub-µ.m S; 10 2- to 4-day samples 18 Bermuda Sub-2.5-µ.m NSS so:;-; 39 I-day samples 14 1,300 Northeast US trajectory ( -1,200 km) 630 Southeast US trajectory (-1,100-1,500 km) 390 Caribbean trajectory 330 Southeast trajectory 650±230 All Barbados ( 13° N, 60° W) NSS SO;;- ; 1-day samples 17 300± 190 'High dust'-Europe or Africa trajectory 120±65 'Low dust' 250± 180 All Pacific Eastern Pacific Ocean off Aircraft sampling, boundary layer; 3 flights; Pacific trajectories; 20 Washington State (47-48° N) May 140±60 Total NSS so:;­ 80±30 Sub-1.5-µ.m NSS So;;- Midway Island (28° N) NSS so~- ; I-week samples; onshore flow only 16 260±90 'Dusty'; 29 samples 100±30 "Clean'; 27 samples 190±260 All; 56 samples Oahu (21°N) NSS sot; I-week samples; onshore flow only 16 160± 130 'Dusty'; 24 samples 90±80 'Clean'; 32 samples 120± llO All ; 56 samples Guam (l7°N) 150±240 NSS so~- ; I-week samples; onshore flow only; 49 samples 16 Belau (7° N) 210± 190 NSS so~- ; I-week samples; onshore flow only; 40 samples 16 Fanning Island ( 4° N) 210±50 NSS so~- ; I-week samples; onshore flow only; 48 samples 16 Southern Hemisphere Samoa (14° S) 60 [2.4] Sub-µ.m S; 17 3- to 5-day samples 18 Samoa 130± 50 NSS SO;;-; 42 I-week samples 19 New Caledonia (22° S) 170± 130 NSS SO~- ; 46 I-week samples 19 Norfolk Island (29° S) 110±50 NSS SO~-; 41 I-week samples 19 Cape Grim, Tasmania (41 ° S) 90±20 NSS SO;;- ; multi-year data record under 'baseline' conditions 43 Tasmania, off West Coast (22-70) NSS so~-; aircraft sampling, boundary layer; 8 flights; ocean 21 trajectories; Austral summer Punta Arenas, Chile (54° S) 52 [0.3] Austral summer; sub-µ.m S; 9 3- to 5-day samples 18 83 Austral summer; NSS S; 140 4-hour samples 11 Austral winter; sub-µ.m S; 1 5-day sample South Pole 76±24 Austral summer; total S; sea salt small; 40 several-day samples 44 29± 10 Austral winter; total S; sea salt 4-10%; 49 several-day samples * Mean ± standard deviation or geometric mean [geometric standard deviation] or (range). t NSS so;;- : Non-seasalt sulphate. industrial activity. Such locations are pertinent because aerosol of only sub-micrometre (sub-µ,m) sulphate (that fraction arising sulphate concentrations are obviously high in industrialized from gas-to-particle conversion, which is of concern here). The regions and at locations immediately influenced by transport data in Table 2 represent measurements by numerous inves­ from these regions. By comparison, median 24-hour boundary­ tigators using a variety of techniques and protocols for widely layer sulphate concentrations at non-urban locations of the differing sampling periods, so that any comparison of data from north-eastern United States are -2,000 ng S m-3 (ref.