RESEARCH FRONT CSIRO PUBLISHING Highlight C. Leck and E. K. Bigg, Environ. Chem. 2007, 4, 400–403. doi:10.1071/EN07061 www.publish.csiro.au/journals/env A modified aerosol–cloud–climate feedback hypothesis Caroline Leck A,C and E. Keith Bigg A,B A Department of Meteorology, Stockholm University, 10691 Stockholm, Sweden. B Present address: 12 Wills Ave., Castle Hill, NSW 2154, Australia. C Corresponding author. Email: lina@misu.su.se Environmental context. Problems with the aerosol–cloud–climate feedback process known as the ‘CLAW’hypothesis are discussed and a modified scheme that poses a stronger possible link between marine biology, cloud properties and climate than is provided by dimethyl sulfide alone is proposed. Introduction Clouds have an important influence on the amount of sunlight that reaches the Earth’s surface, and the amount they reflect back to space (albedo) depends on the concentration of cloud drops, which in turn depends on the concentration of cloud condensa- tion nuclei (CCN). The latter depends on the number of airborne particles of ∼80 nm in diameter and larger that have a hygro- scopic component to aid the uptake of water. Airborne particles (aerosols) come in a variety of sizes/modes, and have widely vari- able properties that depend on their sources. There are primary aerosols that are directly emitted to the atmosphere, for example dust, sea salt particles and marine organic particulate matter, and secondary aerosols that are formed by photochemical oxidation of gases or chemical alteration of primary particles. Number size distributions expressed as the rate of change of number with the logarithm of diameter (dn/dlog D) often have a number of well-defined peaks (modes). The smallest is the ‘nucleation mode’ with a diameter of less than 10 nm, sometimes fol- lowed by a 10–25-nm ‘ultrafine mode’. Next comes the ‘Aitken mode’ usually between 25 and 80nm in diameter, the ‘accu- mulation mode’ of 80 nm to 1 µm and a ‘coarse mode’ greater than 1 µm. Shaw [1] suggested a biothermostasis mechanism that would operate by altering planetary albedo and thus climate through the creation of atmospheric particles by oxidation of biospheric organic sulfide gases. This possibility was considered in more detail by Charlson, Lovelock, Andreae and Warren [2] and has become widely known as the ‘CLAW’hypothesis on the basis of their initial letters. It proposed a possible feedback mechanism Caroline Leck holds a Professorial position at the Department of Meteorology, Stockholm University, in Sweden. She has 20 years of experience in the field of couplings between biogeochemical cycling and climate with the emphasis on regions with a minimum of influences by man-made sources. Examples are cycling of sulfur and marine biogenic matter, which provide a still poorly understood link between cloud radiative properties and marine primary production. Leck’s research has been conducted in an international and interdisciplinary context and she has played a major role as deputy coordinator of the atmospheric program of three internationalArctic Ocean Expeditions to the North Pole. Keith Bigg worked with the Radiophysics and Cloud Physics Divisions of CSIRO from 1948 to 1985, mainly on aerosol and cloud seeding topics. He set up the aerosol measuring program at Cape Grim,Tasmania in 1976–1984 and has returned several times since then for further work. After retiring from CSIRO, he joined three trips to the Antarctic with the Antarctic Division, then three to theArctic with Professor Leck’s expeditions. on the biological influence on cloud formation, radiation and cli- mate. The CLAW hypothesis stands on four main assumptions: (1) Increased phytoplankton production of dimethyl sulfide (DMS) as a result of global warming leads to (2) an increase in CCN production by its oxidation products (dimethyl sulfide oxi- dation products, DMSOP). (3) Increased CCN concentrations then lead to an increase in cloud albedo (reflectivity) and (4) the increased loss of shortwave radiation would result in sur- face cooling. The DMS link might, therefore, help climate to be self-regulating. The only step in this hypothesis that has not been challenged is that in remote marine stratus and stratocumulus clouds (responsi- ble for most of the large net global cooling by clouds [3] ) increased CCN concentrations would add to the cooling effect. The accu- racy of predictions of global warming depends, amongst other things, on a correct representation of climate feedback processes, of which the CLAW hypothesis is potentially one. This paper aims to review some of the complications that have emerged as a result of further research and to introduce another type of link between marine biota and climate. The DMS production–sea surface temperature link Climate warming is likely to lead to very complex changes in marine ecosystems. [4] The overall effect on DMS net production and sea/air emissions has been modelled [5,6] and predicts sig- nificant increases at high latitudes. Modelling a very complex system will give an answer that depends on the assumptions made. Another study [7] predicted a 3% decrease in atmospheric DMS over the 200 year period 1861–1890 to 2061–2090.A more © CSIRO 2007 400 1448-2517/07/060400