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models at present. However, the newly calculated global annual dimethylsulphide emissions are 17% higher than the last estimate11, totalling 28.1 Tg S yr−1. Occupying only 6% of the ocean’s total surface, the Southern Ocean contributes 62% of the global dimethylsulphide flux, according to the updated estimate. The disproportionately high level of dimethylsulphide production in the Southern Ocean is attributed to recurring blooms of efficient dimethylsulphide producers — Phaeocystis species — in the austral summer12.

A complementary study by Cameron-Smith and co-workers7 suggests that oceanic dimethylsulphide emissions are set to increase in large regions of the Southern Hemisphere. Using climate simulations coupled with a global ocean biogeochemical model, they examined the impact of late-twentieth-century (355 ppm) and projected late-twenty-first-century (970 ppm) concentrations of carbon dioxide on dimethylsulphide levels and emissions in the Southern Hemisphere. Their model incorporates five phytoplankton groups, including diatoms, which possess low levels of the dimethylsulphide precursor, and Phaeocystis species, which contain high levels of the precursor. The simulation of late-twentieth-century conditions broadly agrees with the updated emissions maps6. For example, high emissions are apparent near Antarctica, where Phaeocystis species dominate, and low emissions prevail between 40° S and 60° S, where DMSP producers are scarce, and diatoms containing low levels of the dimethylsulphide precursor reside; both features are present in the updated climatology6.

In the high-carbon-dioxide simulation, dimethylsulphide emissions increase by

around 30% between 30° S and 50° S, and by up to 170% south of 60° S. In both cases, the modelled increase in dimethylsulphide production reflects a poleward shift in phytoplankton communities. In a warmer ocean, diatoms possessing low levels of the dimethylsulphide precursor — which were previously present south of 40° S — shift towards the south, allowing a flagellate-dominated community, which contains high levels of the precursor, to thrive in the 30–50° S band. Farther south, the loss of sea ice favours the growth of Phaeocystis species, and therefore dimethylsulphide production. The findings agree with previous model simulations, which project a comparable, albeit weaker poleward increase in dimethylsulphide emissions of up to 30% in a high-carbon-dioxide world7. Together, the models emphasize the sensitivity of dimethylsulphide production in the Southern Ocean to climate warming.

The studies of Lana et al. and Cameron-Smith et al. may suggest that we have a solid grasp on the global distribution of dimethylsulphide and its dynamics, and that a CLAW-type negative feedback in the Southern Ocean could help to mitigate climate warming. However, climatologies are notoriously sensitive to the availability of data13, and the current dimethylsulphide database, even with its 47,000 data points, is by no means rich. Expeditions in under-sampled and sensitive regions, along with the use of recently developed semi-automated dimethylsulphide analysis systems14, should help to alleviate this shortcoming.

Another problem is that the oceanic dimethylsulphide cycle is not fully understood, and hence is only partly represented in present models15. For

instance, bacteria that can both produce and consume dimethylsulphide are still not parameterized. Plus, additional stressors, such as ocean acidification16, could alter plankton community structure, and therefore dimethylsulphide production.

The studies of Lana et al.6 and Cameron-Smith et al.7 point to the climatic importance of dimethylsulphide emissions in the Southern Ocean. However, in the light of the caveats it would be premature to conclude that the projected carbon dioxide emissions will generate a Gaia-type negative feedback in the Southern Ocean. But if Gaia could talk, chances are that she would tell us to keep looking south. ❐

Maurice Levasseur is in the Department of Biology (Québec-Océan), Laval University, Quebec City, Quebec, Canada G1V 2R3. e-mail: maurice.levasseur@bio.ulaval.ca

References1. Charlson, R. J., Lovelock, J. E., Andreae, M. O. & Warren, S. G.

Nature 326, 655–661 (1987).2. Lovelock, J. E. & Margulis, L. Icarus 21, 471–489 (1974).3. Krüger, O. & Graβl, H. J. Geophys. Res. 38, L08809 (2011).4. Valina, S. M. & Simó, R. Science 315, 506–508 (2007).5. Gondwe, M., Krol, M., Gieskes, W., Klaassen, W. & de Baar, H.

Glob. Biogeochem. Cycles 17, 1056 (2003).6. Lana, A. et al. Glob. Biogeochem. Cycles 25, GB1004 (2011).7. Cameron, P., Elliott, S., Malrud, M., Erickson, D. & Wingenter, O.

Geophys. Res. Lett. 38, L07704 (2011).8. Stefels, J., Steinke, M., Turner, S., Malin, G. & Belviso, S.

Biogeochemistry 83, 245–275 (2007).9. Kettle, A. J. et al. Glob. Biogeochem. Cycles 13, 399–444 (1999).10. Simó, R. & Pedrós-Alió, C. Nature 402, 396–399 (1999).11. Kettle, A. J. & Andreae, M. O. J. Geophys. Res.-Atmos.

105, 26793–26808 (2000).12. DiTullio, G. & Smith, W. O. Jr Deep-Sea Res. I 42, 873–892 (1995).13. Belviso, S. et al. Glob. Biogechem. Cycles 18, GB3013 (2004).14. Tortell, P. D. et al. Deep-Sea Res. I 58, 241–259 (2011).15. Le Clainche, Y. et al. Glob. Biogeochem. Cycles 24, GB3021 (2010).16. Doney, S. C., Fabry, V. J.,  Feely, R. A. & Kleypas, J. A.

Annu. Rev. Mar. Sci. 1, 169–192 (2009).

A few weeks ago, flying from Bangkok to Kathmandu, I was greeted by the familiar sheen of brown haze

obscuring the view of the landscape. The haze was visible from the aeroplane window nearly continuously throughout the flight, except for a few short reprieves when the

waters of the Bay of Bengal could be seen through pockets of cleaner air. The haze, sometimes termed brown cloud, is so thick that it can even be seen from space, extending over thousands of kilometres in latitude and longitude. The visible part of the brown haze comprises particles such as soot, dust and

sulphate, and it is accompanied by numerous invisible pollutant gases such as ozone, carbon monoxide and nitrogen oxides. The bulk of the haze remains near its source just above the Earth’s surface, but some can be transported upwards. Writing in Geophysical Research Letters, Vernier and colleagues1

ATMOSPHERIC SCIENCE

Asia under a high-level brown cloudGaseous pollutants such as ozone and carbon monoxide from Asia are lifted to altitudes of more than 10 km during the summer monsoon season. Satellite observations show that aerosol particles, too, can rise high and spread across thousands of kilometres.

Mark G. Lawrence

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1,000

750

500

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Pres

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Figure 1 | Plume of southern Asian pollution. Vernier and colleagues1 report satellite observations of a plume of aerosols that appeared over Asia in the summer months of 2006 to 2009, linked to the monsoons. The image shows a numerical simulation of the three-dimensional structure of the southern Asian plume of carbon monoxide, an insoluble pollutant gas that is transported by similar mechanisms and exhibits similar spread as the pollutant aerosols. Figure adapted with permission from ref. 8, © EGU 2003.

report that these aerosols form an extensive layer of aerosol haze at altitudes of 13–18 km.

The transport of Asian pollution over large distances during all seasons of the year has been amply documented2. Most of the pollutants remain near the surface, at altitudes up to about 3 km. However, during the summer monsoon season, convective cumulus clouds rise high in the sky over southern Asia, and the rapidly rising air masses within these clouds can carry massive amounts of insoluble pollutant gases such as ozone and carbon monoxide from the surface to altitudes of 8–12 km — the upper troposphere. The polluted air then spreads out, mostly towards the west, forming a giant umbrella of pollution over southern Asia, northern Africa, the Middle East and the eastern Mediterranean (Fig. 1). In situ measurements, satellite observations and numerical model simulations have documented this spread of pollutants, but have focussed almost entirely on the insoluble, gas-phase pollutants2.

Whether significant amounts of tiny aerosol particles could also make it to the upper troposphere in this region has been unclear: aerosol particles, as well as their mostly water-soluble gaseous precursors, tend to be very effectively scavenged and removed by precipitation. Until now, only a few tentative indications from localized observations and numerical modelling have pointed to the existence of an Asian aerosol pollution layer in the upper troposphere3–5. Yet clear observational evidence of the existence of a widespread Asian aerosol pollution layer in the upper troposphere has been missing.

Vernier and colleagues1 fill this gap. They make use of the unprecedented ability of the CALIOP instrument on the CALIPSO satellite to examine the upper troposphere with a vertical resolution of about 60 m, and show that the Asian aerosol pollution does indeed rise up into the upper troposphere in vast amounts during the summer monsoon. Using data for 2006 to 2009, Vernier and colleagues demonstrate that the aerosol plume recurs annually from June to August, and extends from the eastern Mediterranean and northern Africa to western China and Thailand. They are also able to identify two distinct episodes of injections of volcanic aerosols, one of which largely obscured the southern Asian plume during spring and summer 2009. Vernier and colleagues go on to demonstrate that the aerosol layer is very unlikely to be a result of satellite data retrieval artefacts, such as statistical contamination by cloudy pixels or a misclassification of ice crystals, and conclude that the transport is so substantial that it is “likely to be a significant source

of non-volcanic aerosols for the global upper troposphere”.

The implications of such a widespread, annually recurrent aerosol pollution layer in the upper troposphere are manifold. Like aerosol particles near the surface, the high-altitude aerosol layer will affect the atmospheric energy budget — and hence climate — through the scattering and absorption of solar and terrestrial radiation. Furthermore, the particles could influence the formation and properties of cirrus clouds, which in turn also affect the climate. Finally, the observations record substantial concentrations of aerosols very close to the tropopause, the boundary between the troposphere and the next layer up in the atmosphere, the stratosphere. The observation of gaseous pollutants at these altitudes during the summer monsoon a decade ago led to the hypothesis6 that substantial transport of gas-phase pollution into the stratosphere might occur here, which has found recent further support7. However, to what extent this applies to the Asian aerosol particles, with their extra gravitational sedimentation and potential for being taken up in cirrus clouds, is still unclear.

Given the rapid population growth in Asia, which is accompanied by increasing emissions of aerosols and their precursors near the surface, these general modifications of the make-up and energy budget of the upper troposphere and lower stratosphere are likely to intensify in the future.

As ever, the identification of the summertime high-altitude aerosol layer over Asia raises a plethora of follow-up questions. For instance, the satellite observations do not allow a distinction between aerosol types, such as soot, dust, organics and

sulphate; they also cannot resolve the particle size distribution. These need to be determined through in situ measurements. Nor do we know which factors contribute to the formation of the layer and at what levels of relative importance: regional and sector-based origins of aerosol emissions, as well as the details of the aerosol and precursor transport to and within the upper troposphere, are still poorly understood. Finally, we have only a qualitative, theoretical understanding of the effects of aerosols in the upper atmosphere on energy budgets and cloud formation, and thus we are far from being able to quantify the present and future climate impacts of the upper tropospheric Asian monsoon aerosol layer.

Vernier and colleagues1 provide compelling evidence for the existence of an annually recurring aerosol pollution layer in the upper troposphere above Asia. It will be the task of future studies to investigate how exactly the Asian haze forms and how it affects the present and future climate of the Earth. ❐

Mark G. Lawrence is at the Max Planck Institute for Chemistry, Joh.-J.-Becher-Weg 27, 55128 Mainz, Germany. e-mail: mark.lawrence@mpic.de

References1. Vernier, J-P., Thomason, L. W. & Kar, J. Geophys. Res. Lett.

38, L07804 (2011).2. Lawrence, M. G.& Lelieveld, J. Atmos. Chem. Phys.

10, 11017–11096 (2010).3. Kim, Y. S. et al. Proc. SPIE 4893, 496–503 (2003).4. Tobo, Y. et al. Atmos. Res. 84, 233–241 (2007).5. Li, Q. et al. Geophys. Res. Lett. 32, L14826 (2005).6. Lelieveld, J. et al. Science 298, 794–799, (2002).7. Randel, W. J. et al. Science 328, 611–613 (2010).8. Lawrence, M. G. et al. Atmos. Chem. Phys. 3, 267–289 (2003).

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