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More than twenty years ago, a biological regulation of climate was proposed whereby emissions of dimethyl sulphide from oceanic phytoplankton resulted in the formation of aerosol particles that acted as cloud condensation nuclei in the marine boundary layer. In this hypothesis-referred to as CLAW-the increase in cloud condensation nuclei led to an increase in cloud albedo with the resulting changes in temperature and radiation initiating a climate feedback altering dimethyl sulphide emissions from phytoplankton. Over the past two decades, observations in the marine boundary layer, laboratory studies and modelling efforts have been conducted seeking evidence for the CLAW hypothesis. The results indicate that a dimethyl sulphide biological control over cloud condensation nuclei probably does not exist and that sources of these nuclei to the marine boundary layer and the response of clouds to changes in aerosol are much more complex than was recognized twenty years ago. These results indicate that it is time to retire the CLAW hypothesis.
Cloud condensation nuclei (CCN) can affect the amount of solar radiation reaching Earth's surface by altering cloud droplet number concentration and size and, as a result, cloud reflectivity or albedo1. CCN are atmospheric particles that are sufficiently soluble and large enough in diameter to take up water vapour and serve as nuclei for cloud droplet formation. For the range of water supersaturations typical of marine boundary layer (MBL) clouds and the chemical composition ofmarine aerosols, theCCNpopulation is dominated by particles between 40 and 300nm in diameter. Although particles larger than 300nm will activate to form cloud droplets regardless of composition, there are relatively few particles of this size. It is therefore the particles less than 300nm in diameter that determine the CCN concentration in the remote MBL and that have the potential to change cloud properties.
Marine phytoplankton produce the osmolytedimethylsulphoniumpropionate, which undergoes enzymatic cleavage to form dimethyl sulphide (DMS). Both dimethylsulphoniumpropionate andDMS scavenge reactive oxygen species and act as antioxidants under conditions of high ultraviolet radiation and oxidative stress2,3. DMS is the dominant volatile sulphur compound in ocean surface waters and is the most significant biological source of gaseous sulphur to the remotemarine troposphere4. After emission to the atmosphere, DMS is oxidized by the hydroxyl radical to form SO2 andmethane sulphonic acid (MSA)5. SO2 can...