School of Earth and Environment

Is the CLAW hypothesis dead?

The CLAW hypothesis takes its name from Charlson, Lovelock, Andreae and Warren, whose 1987 paper suggested that phytoplankton could help regulate Earth’s climate. Phytoplankton – single-celled algae – emit a gas called dimethylsulphide (DMS) and the authors suggested that DMS forms tiny new particles (or aerosol) in the atmosphere which controls climate by affecting the amount of sunlight reflected by clouds. New aerosol particles from DMS have the potential to increase cloud reflectivity because they are effective cloud condensation nuclei and can increase the number of cloud drops.

However, new results from GLOMAP suggest that CLAW may be very weak.

The key aerosol quantity is the number concentration of cloud condensation nuclei (CCN), but until recently global models did not include the necessary aerosol physics to quantify CCN. We used GLOMAP to calculate the sensitivity of CCN to changes in DMS emission using multiple present-day and future sea-surface DMS climatologies.

The DMS flux from a future globally warmed climatology was 0.2 Tg (sulphur) per year higher than present day. The largest CCN response to this extra DMS was seen in the Southern Ocean, contributing to a Southern Hemisphere mean annual increase of less than 0.2%.

We show that the changes in DMS flux and CCN concentration between the present day and global warming scenario are similar to interannual differences due to variability in windspeed. So although DMS makes a significant contribution to global marine CCN concentrations, the sensitivity of CCN to potential future changes in DMS flux is very low. This finding, together with the predicted small changes in future seawater DMS concentrations, suggests that the role of DMS in climate regulation is very weak.

Read the article
The story in NERC‘s Planet Earth magazine
Article in Der Spiegel following Matt Woodhouse’s presentation at EGU 2011. The essence of this article is that new data suggest that DMS concentrations are higher than previously thought in the southern hemisphere, but that the CLAW effect is probably too small to be important.

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