Volume 11, Number 44: 29 October 2008
In reporting the findings of their test of Charlson et al.'s (1987) CLAW hypothesis, Gunson et al. (2006) begin by reminding us that the hypothesis suggests that an increase in oceanic dimethylsulfide or DMS emissions -- such as has been shown to occur in warming waters on both a diurnal and seasonal basis (Sciare et al., 2000; Baboukas et al., 2002; Kouvarakis and Mihalopoulos, 2002) -- "gives rise to longer-lived clouds with increased droplet density," and that "the resulting increase in global albedo in turn leads to less solar radiation reaching the sea surface, thereby mitigating the effects of climate change" and leading to a reduction in the degree of global warming.
To further probe the robustness of this concept, the six UK researchers performed a number of climate simulations using a coupled ocean-atmosphere general circulation model that included an atmospheric sulfur cycle and a marine ecosystem model. In doing so, they determined, as they describe it, that "the modeled global climate is sensitive to ocean DMS production in the manner hypothesized by CLAW," and that "perturbations to ocean DMS production cause significant impacts on global climate."
For a halving of oceanic DMS emissions, for example, Gunson et al. found that the modeled net cloud radiative forcing increased by 3 W/m2 and, through a readjustment of the global radiative energy balance, that the surface air temperature rose by 1.6°C. On the other hand, for a doubling of oceanic DMS emissions, they found that net cloud radiative forcing declined by just under 2 W/m2 and that the surface air temperature decreased by just under 1°C.
These are encouraging findings, suggestive of the fact that earth's climate system is capable of successfully buffering itself against the propensity for warming created by rising atmospheric CO2 concentrations. In addition, we note that mesocosm experiments conducted in a Norwegian fjord have demonstrated that DMS concentrations were 26% and 18% higher in mesocosms maintained in equilibrium with air of twice and three times the ambient CO2 concentration, respectively, while the amount of the iodocarbon chloroiodomethane -- which also leads to an enhancement of cloud condensation nuclei in the marine atmosphere (O'Dowd et al., 2002) -- was 46% higher in the double-CO2 mesocosms and 131% higher in the triple-CO2 mesocosms (Wingenter et al., 2007).
Consequently, both the ongoing increase in the air's CO2 content, in and of itself, plus any warming it might cause via its greenhouse properties, both tend to mitigate against a dangerous net increase in global temperature.
Sherwood, Keith and Craig Idso
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Kouvarakis, G. and Mihalopoulos, N. 2002. Seasonal variation of dimethylsulfide in the gas phase and of methanesulfonate and non-sea-salt sulfate in the aerosols phase in the Eastern Mediterranean atmosphere. Atmospheric Environment 36: 929-938.
O'Dowd, C.D., Jimenez, J.L., Bahreini, R., Flagan, R.C., Seinfeld, J.H., Hameri, K., Pirjola, L., Kulmala, M., Jennings, S.G. and Hoffmann, T. 2002. Marine aerosol formation from biogenic iodine emissions. Nature 417: 632-636.
Sciare, J., Mihalopoulos, N. and Dentener, F.J. 2000. Interannual variability of atmospheric dimethylsulfide in the southern Indian Ocean. Journal of Geophysical Research 105: 26,369-26,377.
Wingenter, O.W., Haase, K.B., Zeigler, M., Blake, D.R., Rowland, F.S., Sive, B.C., Paulino, A., Thyrhaug, R., Larsen A., Schulz, K., Meyerhofer, M. and Riebesell, U. 2007. Unexpected consequences of increasing CO2 and ocean acidity on marine production of DMS and CH2CII: Potential climate impacts. Geophysical Research Letters 34: 10.1029/2006GL028139.