A container ship
leaves a trail
of white clouds
in its wake that
can linger in the
air for hours.
This is exhaust
from the engines,
and a change in the
clouds caused by
small airborne
particles of pollution.
New research led by the
University of Washington
is the first to measure
this phenomenon's
effect over years
and at a regional scale.
Satellite data
over a shipping lane
in the south Atlantic
show that the ships
modify clouds to block
an additional 2 Watts
of solar energy,
on average,
from reaching
each square meter
of ocean surface
near the shipping lane.
Cloud changes that
block solar energy
mask some of the
global warming from
greenhouse gases.
The open-access study
was published March 24
in AGU Advances,
a journal of the American
Geophysical Union.
"In climate models,
if you simulate the world
with sulfur emissions f
rom shipping, and you
simulate the world
without these emissions,
there is a pretty sizable
cooling effect from
changes in the
model clouds
due to shipping,"
said first author
Michael Diamond,
a UW doctoral student
in atmospheric sciences.
"But because there's
so much natural variability
it's been hard to see
this effect in observations
of the real world."
The new study uses
observations from
2003 to 2015 in spring,
the cloudiest season,
over the shipping route
between Europe
and South Africa, and
Europe and Asia.
Small particles in exhaust
from burning fossil fuels
creates "seeds" on which
water vapor in the air
can condense into
cloud droplets.
This study focused
on a place where
the wind blows
along the
shipping lane,
keeping pollution
concentrated in
that small area.
The study analyzed
cloud properties
detected over
12 years by the
MODIS instrument
on NASA satellites
and the amount
of reflected sunlight
at the top of the
atmosphere from
the CERES group
of satellite instruments.
The authors compared
cloud properties inside
the shipping route
with nearby, unpolluted
areas.
"The difference inside
the shipping lane
is small enough
that we need about
six years of data to
confirm that it is real,"
said co-author
Hannah Director,
a UW doctoral student
in statistics.
"However,
if this small change
occurred worldwide,
it would be enough
to affect global
temperatures."
Averaged globally,
they found changes
in low clouds due to
pollution from
all sources
block 1 Watt
per square meter
of solar energy --
compared to the roughly
3 Watts per square meter
trapped today by the
greenhouse gases
also emitted by
industrial activities.
Without the cooling effect
of pollution-seeded clouds,
Earth might have already
warmed by 1.5 degrees
Celsius (2.7 F), a change
that the Intergovernmental
Panel on Climate Change
projects would have
significant societal impacts.
Tday the Earth
is estimated
to have warmed
by approximately
+1 C (1.8 F) since
the late 1800s.
"I think the
biggest
contribution
of this study
is our ability
to generalize,
to calculate
a global assessment
of the overall impact
of sulfate pollution
on low clouds,"
said co-author Rob Wood,
a UW professor of
atmospheric sciences.