Atmospheric scientists have reported a new and potentially important
mechanism by which chemical emissions from ocean phytoplankton may
influence the formation of clouds that reflect sunlight away from our
planet.
Discovery of the new link between clouds and the
biosphere grew out of efforts to explain increased cloud cover observed
over an area of the Southern Ocean where a large bloom of phytoplankton
was occurring. Based on satellite data, the researchers hypothesized
that airborne particles produced by oxidation of the chemical isoprene
– which is emitted by the phytoplankton – may have contributed to a
doubling of cloud droplet concentrations seen over a large area of
ocean off the eastern coast of South America.
Using complex numerical models, they estimated that the resulting
increase in cloudiness reduced the absorption of sunlight by an amount
comparable to what has been measured in highly polluted areas of the
globe. If confirmed by field studies, this connection between clouds
and biological activity could add a critical new component to global
climate models. Many environmental scientists believe that increased
cloud cover may be partially countering the effects of global warming
by reducing the amount of energy the planet absorbs from the sun.
Researchers
Athanasios Nenes of the Georgia Institute of Technology and Nicholas
Meskhidze – formerly at Georgia Tech but now at North Carolina State
University – reported their findings Nov. 2 in Science Express, the
online advance publication of the journal Science. The research was
sponsored by NASA, the National Science Foundation and a
Blanchard-Milliken Young Faculty Fellowship.
"Studies like this
one may help reshape the way we think about how the biosphere interacts
with clouds and climate," said Nenes, an assistant professor in Georgia
Tech's School of Earth and Atmospheric Sciences. "One of the largest
uncertainties right now in climate models is the ability to predict how
clouds would respond to changing particle levels – whether they
originate from humans with air pollution or from biological activity.
We can now see very strongly the influence of marine biology on oceanic
clouds."
Researchers had previously theorized that dimethyl
sulfide (DMS) – which is also emitted by phytoplankton – affects the
formation of clouds by increasing the number of sulfate particles,
which can absorb moisture and form cloud droplets. When oxidized,
isoprene may enhance the effect of DMS by increasing the number and
size of the particles while helping them to chemically attract more
moisture. The impact of isoprene on atmospheric particulate matter was
previously thought to be important only for terrestrial plants, Nenes
said.
The researchers stumbled upon the phytoplankton-cloud
connection quite accidentally. "While looking at the satellite
pictures, I noticed that cloud properties over large phytoplankton
blooms were significantly different from those that occurred away from
the blooms," recalled Meskhidze, now an assistant professor in NC
State's College of Physical and Mathematical Sciences.
The
Southern Ocean normally has relatively few particles around which cloud
droplets can form. The isoprene mechanism could therefore have a
significant effect on the development of clouds there – and may account
for most of variation in the area's cloud cover.
"If a lot of
particles form because of isoprene oxidation, you suddenly have a lot
more droplets in clouds, which tends to make them brighter," Nenes
explained. "In addition to becoming brighter, the clouds can also have
less frequent precipitation, so you might have a build-up of clouds.
Overall, this makes the atmosphere cloudier and reflects more sunlight
back into space."
In their paper, the researchers estimated
that the isoprene emissions reduced energy absorption in the area by
about 15 watts per square meter. "This is a huge signal," said Nenes.
"You would normally expect to see a change of a couple of watts."
The
Southern Ocean is ideal for study because it is largely untouched by
pollution and has relatively steady temperature and meteorological
conditions during the seasons in which phytoplankton blooms appear.
"This seems to be one of those rare regions in the globe where the
biology really takes over," Nenes explained. "That allows us to see
strongly the impact of biology on the clouds."
As a next step,
Nenes would like to examine other areas of the globe for similar
activity. "There are a lot of areas that have intense biological
activity, so with time we are going to explore more regions to see if
this is a widespread phenomenon. Chances are that we will see this in
other places," he added.
Nenes and Meskhidze used data from
satellite observations to estimate the amount of chlorophyll in the
ocean, the emission of isoprene and its connection to cloud formation.
Before this new mechanism can be incorporated into global climate
models, however, it will have to be confirmed by field experiments.
Atmospheric
scientists believe that by blocking sunlight, increased cloudiness has
up until now partially mitigated the effects of global warming. The
role of oceanic biology on cloud formation could therefore be a major
factor in controlling global climate, and the new mechanism identified
by Nenes and Meskhidze may make it even more important. This effect
needs to be better understood, Nenes noted, because anything that can
change global clouds can dramatically alter the impact of greenhouse
gases on our changing climate.
"It shows that there is still
a lot we need to explore to better understand the delicate balance in
nature," said Meskhidze. "It will require the cooperative efforts of
researchers from many different fields to identify the chemical
components in these aerosols, to estimate the amounts of this and other
potentially important gases emitted from the ocean, and to better
characterize the effort of organics on cloud droplet formation."
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Technical Contacts: Athanasios Nenes, Georgia Tech (404-894-9225); e-mail: (
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Source: http://eurekalert.org/pub_releases/2006-11/giot-rlo110606.php
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