Mumbai: White clouds over oceans can stop
global warming and bring more rains to the land, according to
a recent study.
"Clouds reflect sunlight and cool the planet. Clouds with
bigger droplets tend to be darker and those with tiny droplets
for the same amount of water are more white.
"If the droplet number in the clouds is increased, the
reflectivity increases and it will increase rainfall on land
and also slow down global warming," Prof Govindasamy Bala, the
lead author of the study from Indian Institute of Science
(IISc) Bangalore, said today.
"Reducing the droplet size and thus increasing the
reflectivity is the basic mechanism behind the proposal for
whitening the marine clouds to counteract global warming,"
Bala said.
The new climate modelling study by the IISc in
collaboration with the Carnegie Institution, Stanford
University, and NASA-AMES, California suggested that altered
atmospheric circulation under the scheme could create a
monsoonal circulation and cause the continents to become
wetter, not drier, on average.
The study is reported in the latest issue of the journal
`Climate Dynamics` titled `Albedo enhancement of marine clouds
to counteract global warming: impacts on the hydrological
cycle.`
Bala said the number of marine cloud droplets could be
increased by spraying tiny droplets of sea water into the
marine atmosphere. Salt particles in sea water serve as seeds
for many tiny cloud droplets.
"For uniform reductions in sunlight over land and oceans,
our earlier modelling work showed that the planet could become
drier.
"However, when you selectively reduce the sunlight over
only the ocean surface by whitening only the marine clouds
(which we have done in the current model), a monsoonal
circulation is triggered which increases the rainfall. We find
an increase of 7.5 per cent in the overall water budget over
land," Bala said.
To test the climate consequences, Bala and his co-authors
used a computer simulation of the global climate system in
which atmospheric carbon dioxide concentrations were set at
approximately twice of that of the present day.
Cloud droplets over the oceans in the model were reduced
to make the clouds more reflective. Clouds over land were
unaltered. "As expected, the whitened clouds reflected more
solar radiation and offset the warming effect of the high
carbon dioxide levels," Bala, attached with Divecha Center for
Climate Change and Center for Atmospheric and Oceanic Sciences
at the IISc, said.
The scientists claimed that the effect of this marine
cloud seeding is very different on the planetary water cycle
when compared with other sunlight reduction methods such as
injecting sulphate aerosols into the stratosphere.
In reality, persistent marine clouds are found off the
west coasts of the continents and only these clouds can be
made whiter by making their droplets smaller.
"The local effects of such patchier schemes are likely to
be different from what we find in our study, where we made all
the marine clouds uniformly more reflective," Bala said.
"However, we do believe that any mechanism that increases
the albedo of the ocean will lead to more rainfall over land.
Further investigation using other climate models is required
to test the robustness of our results," he added.
This kind of seeding the cloud and making them whiter is
relatively safer and cheaper, Bala said, adding "this is just
one of many proposed emergency fixes to halt global warming
which requires macro-engineering and also an international
consensus."
Other authors of the study are Ken Caldeira, Long Cao
George Ban-Weiss and Ho-Jeong Shin from Department of Global
Ecology, Carnegie Institution and Rama Nemani from NASA Ames
Research Center, Moffett Field, US.
The scientists claimed that the effect of this marine
cloud seeding is very different on the planetary water cycle
when compared with other sunlight reduction methods such as
injecting sulphate aerosols into the stratosphere.
In the real world, persistent marine clouds are found off
the west coasts of the continents and only these clouds could
be made whiter by making their droplets smaller.
"The local effects of such patchier schemes are likely to
be different from what we find in our study where we made all
the marine clouds uniformly more reflective," Bala said.
"However, we do believe that any mechanism that increases
the albedo of the ocean will lead to more rainfall over land.
Further investigation using other climate models is required
to test the robustness of our results," he added.
"This kind of seeding the cloud and making them whiter is
relatively safe and cheap," Bala said adding "this is just one
of many proposed emergency fixes to halt global warming which
requires macro-engineering and also an international
consensus."
Other authors of the study include Ken Caldeira, Long
Cao, George Ban-Weiss, and Ho-Jeong Shin, Department of Global
Ecology, Carnegie Institution and Rama Nemani from NASA Ames
Research Center, Moffett Field, USA.
PTI
