So we all know what the CLAW hypothesis is about, as there has been quite a lot
of interest in posts on how dimethyl sulphide (DMS) contributes to this phenomenon. However,
over the past two decades, observations in the marine boundary layer (MBL),
laboratory studies and modelling efforts have been conducted seeking evidence
for the CLAW hypothesis. The results indicate that a DMS biological control
over cloud condensation nuclei probably does not exist and that sources of
these nuclei to the marine boundary layer and the response of clouds to changes
in aerosol are much more complex than was recognised twenty years ago. These
results indicate that it is time to retire the CLAW hypothesis, which has been nicely reviewed and put forward by Quinn & Bates (2011), which I highly recommend reading for some background knowledge.
What the CLAW hypothesis assumed
Particles that are less than 300 nm in diameter determine the cloud
condensation nuclei (CCN) concentration in the remote MBL and have the
potential to change cloud properties. The CLAW hypothesis was based on data
available at the time that indicated that (1) non-sea-salt sulphate was
ubiquitous in sub-micrometre marine aerosols and (2) concentrations of sodium
containing particles at cloud height were negligible (thus ruling out sea salt
particles as a source of CCN). Organic species were not considered because
little was known about their concentration and composition in the marine
atmosphere. The original proposed climate feedback loop (Fig. 1) requires (1) that DMS is a
significant source of CCN to the MBL, (2) a change in DMS-derived CCN yields a
change in cloud albedo, and (3) a change in cloud albedo, surface temperature,
and/or incident solar radiation leads to a change in DMS production. If any one
step in the feedback loop shown in Fig. 1 has a small response, the proposed
bio-regulation of the climate with DMS will be minimal.
 |
| Fig. 1 The climate feedback loop proposed by Charlson et al. 1987 'The CLAW hypothesis' |
The post- CLAW view of MBL CCN (there are other sources of CCN!)
Bubble bursting at the ocean surface is a major source of aerosol mass and
aerosol number to the MBL. This process introduces both inorganic and organic
components of sea-water to the atmosphere. Inorganic components are comprised
of sea salt while the organic components are derived from phytoplankton and the
large pool of organics in the ocean surface- including TEP which contains
microbes (see post ‘The importance of the sea surface microlayer: influences on
the atmosphere’). Hence, the concentration of CCN in the remote MBL is a result
of emissions of sea salt and organics in sea spray (dependent upon biological
activity and wind speed), subsidence of DMS-derived and continentally derived
particulates from the free troposphere (dependent upon oxidation and
entrainment rates), and particle growth (dependent upon condensation,
coagulation and cloud processing). This updated view of the multiple sources of
CCN to the MBL is shown in Fig. 2.
The evidence gained over the past 20 years of the significance of non-
DMS sources of MBL CCN, the lack of observational evidence for a DMS-controlled
marine biota–climate feedback, and the modelled low sensitivity between change
and response in each step of the CLAW hypothesis feedback loop all indicate
that it is time to retire the CLAW hypothesis. Please remember that retiring
CLAW does not rule out a link between ocean-derived CCN and climate- it may just be on a smaller scale that climate regulation happens. It is only
that now we have a much better appreciation of the complexity of biogeochemistry
and climate physics than when the CLAW hypothesis was first put forward. The
interdisciplinary research that it motivated is now needed to address the
complexity of multiple sources of CCN to the MBL and potential impacts on
climate.
 |
| Fig. 2 The new view- Major sources and production mechanisms for CCN in the remote MBL. DMS contributes to the MBL CCN population primarily via particle nucleation in the free troposphere in cloud outflow regions with subsequent subsidence. Sea salt and organics are emitted as a result of wind-driven bubble bursting. |
References:
Quinn, P. K., & Bates, T. S. (2011). The case
against climate regulation via oceanic phytoplankton sulphur emissions. Nature, 480(7375),
51-56.
This paper introduced the CLAW hypothesis proposing the link between marine biota and climate.
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ReplyDeleteHi Elyssa, Really interesting post! It is incredible how our knowledge of something changes just because the technology advances. I wonder what kind of model will come out 10 years later. I was wondering: in the first model in step 3, the change of clouds changes temperature and therefore the DMS production, I can`t see any "feedback mechanism" into the ocean in the new model. Is this because there isn`t a significant one?
ReplyDeleteThanks :)
Hi Tabea, yes luckily we have this piece of research really, as a lot can be assumed from the previous model that may not be true- such as the regulation of the climate by DMS. Good question- they really just said in this paper that we must not rule out the potential regulation of climate by ocean-derived CNN. I think this is really an area left for study, would be quite a task though as would have to be carried out at the right time and place over large time scales and distance! :)
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