THe claw hypothesis: A new perspective on the role of biogenic sulphur in the regulation of global climate

In 1987, R.J. Charlson et al. (Nature 326, 655-661) proposed that biogenic sulphur, primarily dimethyl sulphide (DMS), a by-product of dimethyl sulphoniopropionate (DMSP) produced by marine organisms, could act as cloud condensation nuclei (CCN) in the atmosphere. Higher levels of CCN would lead to increased cloud cover, with consequent changes to the radiation budget on Earth, thereby establishing a climatic feedback loop reliant on marine organisms. This CLAW hypothesis inspired a large body of research intended to test key aspects of the proposed feedback mechanism. We consider here the key components of the hypothesis as outlined by the original authors: light, temperature, salinity, DMS-CCN correlation, and the palaeo record. Literature reviewed here indicates that it is time to modify the CLAW hypothesis in light of recent advances in knowledge, particularly the emerging importance of microbial activity in the control of water column DMS concentrations and increased awareness of the importance of dimethylsulphoxide (DMSO) as a source and sink of DMS.