DMS has weak climate influence once microphysics resolved

The CLAW hypothesis (Charlson et al., 1987) relates the production of dimethyl-sulphide (DMS) by phytoplankton in the surface ocean to the optical properties of clouds via the first and second aerosol indirect effects, thereby proposing a climate feedback. The CLAW feedback has been the subject of much investigation over the last two decades, but only recently have aerosol models with detailed aerosol microphysics become available.

To investigate the influence of DMS on climate, simulations with the UK Met Office HadGAM atmospheric general circulation model were carried out where the flux of DMS to the atmosphere from the oceans is perturbed. The impact of the DMS perturbation on aerosol in HadGAM was simulated by two different aerosol schemes. The first is the existing CLASSIC scheme (used in HadGEM2-ES for IPCC AR5) which carries only mass in several aerosol types, each with a prescribed size distribution, and is typical of most aerosol schemes used in recent global climate assessments. The second is the UKCA aerosol scheme (Mann et al., 2010) which uses a two-moment approach transporting number and mass in several size modes, allowing particle number to be conserved and growth to be simulated.

The CCN sensitivity to the DMS change (change in CCN per unit flux of DMS) is shown in this File:UKCA DMSCCNsensitivity.pdf for the two different aerosol schemes in HadGAM.

The UKCA aerosol scheme predicts a very weak CCN response to changes in DMS flux suggesting that the CLAW feedback is either very weak, or negligible. The simpler CLASSIC scheme predicts a larger response of CCN to the DMS changes. The greater response in the mass-only aerosol scheme is believed to be incorrect, due to the very limited representation of aerosol microphysics within the scheme. These results have significance for the representation and quantification of the CLAW feedback in current climate assessments, suggesting that the strength of the feedback may be over-estimated by simple aerosol schemes.

Charlson, R. J., Lovelock, J. E., Andreae M. O. and Warren, S. G., Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate, Nature, vol. 326, pp. 655-661, 1987.

Mann, G. W., Carlsaw, K. S., Spracklen, D. V., Ridley, D. A., Manktelow, P. T., Chipperfield, M. P., Pickering, S. J. and Johnson, C. E., Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model Geoscientific Model Development, 3, pp. 519-551, 2010.