Feature A silver lining to Arctic clouds? The relentless increase in summer sea-ice melt is likely to amplify Arctic warming. But could the same conditions also spur the activity of marine microbiota, increase cloudiness and counteract the melting? Paty Matrai and Caroline Leck explore.

evin Arrigo and colleagues visible manifestations of Arctic into the subsequent winter. If Kreported recently in . This summer, ice such a process were to recur over Science that in July last year, cover melted to its lowest extent several years, it could delay or had bloomed in the satellite record, breaking even prevent from melting strongly beneath Arctic pack ice the previous record low observed completely during the Arctic of the Chukchi Sea. So lush was in 2007. If this trend continues, the summer. In other words, it would the bloom that Paula Bontempi, region is likely to witness ice-free constitute a negative feedback. NASA’s Ocean Biology and summers in the near future. What are the odds of a negative Biogeochemistry Program Sea ice reflects incoming solar feedback loop developing? To Manager, likened it to finding radiation, but the open ocean answer this question we need to the “Amazon rainforest in the absorbs and stores solar radiation know, among other things, what middle of the Mojave Desert.” during the summer. Later, controls the optical properties of Like any plant, the unicellular during the autumn, this heat is Arctic low-level clouds and how marine phytoplankton need released and further warms the they would change in a warming light to thrive. Old, thick sea ice, atmosphere. As more ocean is climate. The Arctic’s inaccessibility especially when covered with exposed, a positive feedback loop ensures that data remain sparse, snow, is opaque but thinner develops accelerating summer and our understanding of the ice underlying melt ponds sea-ice melt – attested to by complex relationship between is more transparent, and it is observations in the past decade. the clouds, sea ice, ocean and under such ice that the blooms But low-level clouds, which atmosphere is still evolving. But reported by Arrigo et al. occurred. also control the Arctic surface the situation is improving. In We know such blooms have radiation balance, could potentially particular, we are beginning to occurred before: but, as the Arctic slow down or even reverse the get a handle on the sources of continues to warm faster than warming. For most of the summer, the small atmospheric particles – any other region on Earth, we such clouds tend to warm the cloud condensation nuclei (CCN; can expect them more frequently. surface. But during the peak-melt see Box) – that eventually spawn In fact, phytoplankton and season at the end of the summer, clouds. We now know that the other marine micro-organisms the right type of low-level clouds greatest number comes from could ultimately help counter (see Box) could cool the surface marine micro-organisms (Leck and the rapid warming. To find out We will need to and thereby influence the timing Bigg 2005a; Orellana et al. 2011). how, we will need to stick our stick our heads of the autumn freeze-up. Earlier How such organisms respond to heads into the Arctic clouds. into the Arctic freeze-up will cause thicker ice the melting sea ice, whether in The ever-shrinking area of that might melt less during the ways reported by Arrigo et al. or summer sea ice is one of the most clouds. following summer, surviving in other ways, will thus strongly

8 ❚ Global Change ❚ Issue 79 ❚ October 2012 Marine microbiota in the icy hold the key to cloud formation. © iStockphoto.com/Brainmaster

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influence cloud formation and Getting the clouds right their optical properties, and perhaps the rate of future melting. Clouds, which come in all shapes and sizes, form when Seeding clouds with water vapour condenses. But vapour needs something microgels to condense on – tiny airborne aerosol particles known as cloud condensation nuclei (CCN). Typically, CCN are Marine microbial food webs about 100 nanometres in diameter. Depending on their produce a gas called dimethyl properties and heights, clouds can either warm the surface sulphide (DMS), which is released by triggering a localised greenhouse effect or cool it by to the atmosphere from the reflecting solar radiation. uppermost ocean. There, it oxidises to form various intermediate If CCN are scarce, the resulting clouds will contain fewer products and ultimately sulphate and larger droplets. Such clouds will reflect little sunlight particles. In 1987, Robert J Charlson to space while blocking the escape of heat from Earth’s and colleagues reviewed existing surface, causing it to warm. However, if CCN are plentiful, evidence to implicate DMS in the many fine droplets form and the resultant clouds are better production of oceanic CCN. Thus reflectors, which can cool the surface below. was born the CLAW hypothesis, Anthropogenic particles are virtually absent in the summer named so informally after the over the central Arctic, north of latitude 80°N. This “clean” paper’s authors (Charlson, air, with few CCN, makes the summer low-level clouds Lovelock, Andreae and Warren). optically thin, with fewer but larger droplets: a heat trap. This provocative hypothesis But if Arctic warming spurs the activity of microbiota, posits that in the marine realm, organic sources of CCN might become more prominent and DMS emissions would trigger lead to more reflecting clouds. cloud formation, which would cool the ocean surface. This would in turn affect further emissions of DMS by changing (polymers) resembling viruses could also provide sites for the speciation/abundance or microcolloids. Subsequently, condensation of the oxidation of marine phytoplankton, researchers detected large products of DMS. In 2005, Leck leading to a feedback loop. numbers of similar particles and Bigg tested predominantly Observations in the early 1990s within the thin surface film at the airborne sulphate particles for from the Arctic did indeed show water-air interface between ice the presence of microgels. They that the intermediate oxidation floes. These are often aggregated detected water-insoluble marine products provided most of the into <100-nanometre-diameter microgel material in half or more mass for the CCN-sized particles compact balls, assembled as of their samples. The co-occurrence observed over pack ice (Leck and microgels bound by calcium of atmospheric organic material Persson 1996). The source of most atoms. The microgels are networks and biologically active marine of the DMS, though, was at the of polymer filaments, only a few waters has been confirmed for the fringe of the central Arctic Ocean, nanometres in size, made up of high Arctic waters, and has also just around the hospitable edges polysaccharides or sugars. In 2011, been documented for temperate of the pack ice. At that time, this researchers confirmed that the waters (Faccini et al. 2008, Russell suggested that winds carried particles found in the atmosphere et al. 2010). But the universality of DMS-rich air towards the North behave as microgels and such microgels, both in the coastal Pole, and oxidation of the DMS originate in the water (Orellana and open-water regions of the led to extremely small sulphuric et al. 2011) from the activity of Arctic Ocean and at lower latitude acid particles. Theoretically, sea-ice , phytoplankton oceans, has not yet been confirmed. these particles would then grow and, perhaps, bacteria. slowly by further condensation Across the central Arctic Ocean, Beyond the CLAW of the acids until they were large the ubiquitous diatoms Melosira Observations from the Arctic enough to serve as CCN. arctica and Fragilaryopsis cylindrus question the key role attributed Surprisingly, it turns out, Microgels are known for surrounding their to DMS in the CLAW hypothesis sulphuric acid had nothing to do cells with polymeric substances, (Leck and Bigg 2007). In the with the small precursors of CCN. have the right suggesting an important role emerging picture of the Arctic Observations from the Arctic in properties to for them in the production of atmosphere, DMS concentration the mid-1990s instead showed that act as nuclei for polymers. Microgels have the will determine the mass of the these small precursors are mostly right properties to act as nuclei particles by producing material five or six-sided insoluble solids clouds. for clouds. Furthermore, they for their growth. But it is the

10 ❚ Global Change ❚ Issue 79 ❚ October 2012 number of airborne microgels Clearly, there are too many that will primarily influence the unknowns at this stage to number of CCN and the resulting fully assess the likelihood of a optical properties of the cloud negative feedback involving droplets. Indeed, research during micro-organisms and clouds. the past two decades – reviewed But given how sensitive the last year in Nature (Quinn and Arctic is to climate change and Bates 2011) – does not corroborate how important it is for the the CLAW hypothesis for other regional and global radiation regions as well. But this does not balance, there is a strong rule out a link between marine rationale for continued research micro-organisms and climate, to test this hypothesis. ❚ especially on a regional scale. From that perspective, the Arctic Patricia A Matrai is a Senior observations discussed here Research Scientist at the Bigelow could provide a more nuanced Laboratory for Ocean Sciences, link between marine biology, PO Box 380, 60 Bigelow Drive, cloud properties and climate. East Boothbay, ME 04544, USA. The Arctic low-level clouds E-mail: [email protected]. have a pronounced influence on the surface energy budget. Caroline Leck is Professor at In summer, a scarcity of CCN the Department of Meteorology, leads to optically thin clouds. The Stockholm University, S-106 sources of these CCN are mostly Figure 1. Schematic sketch (not to scale) depicting the negative feedback. 91 Stockholm, Sweden. located along the marginal ice Melting sea ice spurs the activity of marine microbiota, thereby increasing the E-mail: [email protected]. zone and north thereof towards availability of the polymeric sugar precursors (grey dots) of CCN. The low-level clouds thus formed reflect some of the incoming solar radiation and cause the pole. Marine micro-organisms surface cooling. This process can hasten the autumn freeze-up. References are the primary contributors to Arrigo K R et al. (2012) Science CCN, and hence an important 336 (6087): 1408, doi: 10.1126/ control on the optical properties. habitat of ice algae. And it might science.1215065. Their response to the changing have indirect effects. The presence Arctic climate is thus key to a of sea ice has prevented, or Charlson R J et al. (1987) Nature 326: possible negative feedback that significantly controlled, wind- 655–661. would slow down the melting of driven mixing of the surface layer Facchini M C et al. (2008) summer sea ice. We know that the of the Arctic Ocean. This has Geophysical Research Letters 35, immobile ice algae as well as the kept the floating phytoplankton doi:10.1029/2008GL034210. floating phytoplankton are likely mostly at the surface. Thinner Leck C and Bigg E K (2007) to be strongly affected by changing ice or more open ocean areas Environment Chemistry 4: 400-403. sea-ice conditions (Wassman would allow the wind to stir the and Reigstad 2011). But whereas surface ocean, deepen or break Leck C and Bigg E K (2005a) Tellus 57B: both generate dissolved organic the mixed layer, thereby reducing 305-316. matter and are hence a potential algal growth. If organic matter Leck C and Bigg E K (2005b) source of airborne microgels, derived from ice algae was Geophysical Research Letters 32, their relative importance is confirmed to be a major source doi:10.1029/2005GL023651. not fully understood. of the microgels throughout the Leck C and Persson C (1996) Tellus 48B: The recent report of a sub-ice Arctic, future warming might 272-299. phytoplankton bloom by Arrigo imply reduced supply of CCN and colleagues, in conjunction with and thus very optically thin Quinn P K and Bates T S (2011) Nature previous and current observations, clouds with enhanced surface 480: 51-56, doi:10.1038/nature10580. strongly suggest increased activity warming. On the other hand, Orellana M V et al. (2011) Proceedings as the Arctic warms. If they are ice formation during freeze-up But this does of the National Academy of Sciences found to be a strong contributor of excludes salt brine and other not rule out a 108: 13612-13617. microgels, phytoplankton might substances, including dissolved Russell L M et al. (2009) Proceedings facilitate an enhanced reflectivity organic matter likely assembled link between of the National Academy of Sciences, of the low-level clouds that help as gels. These gels can end marine micro- doi:10.1073/pnas.0908905107. counteract increased ice melt. up in both the surrounding organisms and The melting of sea ice might water and the atmosphere Wassmann P and Reigstad M (2011) reduce or even eliminate the during this crucial period. climate. Oceanography 24: 220-231.

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