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
“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
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.”
##
Contact:
John
Toon
Georgia Institute of Technology
404-894-6986
This text derived from: