limiting factor on cloud formation was
how fast winds could lift aerosols high
into the atmosphere. The study underscores how the complex relationship
between aerosols and clouds can differ
over pristine and polluted areas.
Aerosols serve as tiny seeds on which
water vapor can condense. So more
aerosols mean more “cloud condensation nuclei” for clouds to form around;
long thin clouds known as ship tracks,
for example, trace a ship’s airborne emission like a jet contrail. More seed particles mean that clouds become thicker,
whiter and more reflective, cooling
things down even more.
Hot plates in the sky
Many scientists consider this indirect
effect to be the largest single uncertainty
in understanding the inter-
actions between aerosols
and climate. Over the last
decade and more, research-
ers have launched a flotilla
of observing equipment
in more than a dozen field
campaigns to measure the
distribution and effects of
aerosols in regions from
One kind of aerosol is particularly cap-
turing researchers’ attention these
days— black carbon, better known as
soot. These particles come from fos-
the atmosphere over time. In one ongoing
project, researchers fly repeatedly from
the Arctic to the Antarctic, traveling high
into the atmosphere and dipping down
nearly to touch the ocean. The National
Science Foundation project, called
HIAPER Pole-to-Pole Observations or
simply HIPPO, collects air samples to
test for many atmospheric gases, notably
carbon dioxide, but it also collects black
carbon.
Once thought
to be a
minor player
in global
warming, black
carbon is now
sil fuel–burning power
plants, fires started to
clear land for agriculture
and, to a lesser extent,
from coal- and woodburning cookstoves in
developing nations.
Asia to North America.
of interest.
Typically, airplanes will fly a gridlike
pattern gathering air samples, while
researchers on the ground use vehicles
or ships to measure patterns of aerosol
emission. (NASA’s Glory mission, scheduled for launch in February, will continue
this full-out push.)
Once thought to be a
minor player in global
warming, black carbon is
now a major focus of inter-
est. Unlike other aerosols that have net
cooling effects, black carbon particles
absorb solar radiation and warm the
atmosphere, the way wearing a black
During its first flights, in January 2009,
the HIPPO plane measured unexpected
concentrations of black carbon along its
path. That could be because computer
models don’t accurately estimate how
well rain would remove the particles
from the air, says Joshua Schwarz, an
atmospheric scientist at the NOAA Earth
System Research Laboratory in Boulder,
Colo. Different air masses also contained
surprisingly similar sizes of black carbon
particles, no matter where that air came
from. “We don’t have a reasonable expla-
nation for that yet,” Schwarz says.
Such detailed work is now paying off.
Antony Clarke and Vladimir Kapustin of
the University of Hawaii at Manoa analyzed more than 1,000 vertical slices of
the atmosphere, taken mostly around the
Pacific in various campaigns since 1995.
The researchers report in the Sept. 17
Science that there were more cloud condensation nuclei, along with other measures of aerosols, above regions with a lot
of human activity compared with more
pristine areas. The work “provides a more
revealing picture of combustion influences over global scales,” says Clarke.
T-shirt on a sunny day warms the body.
Studies have fingered these tiny hot
plates in the sky as a major contributor
to Arctic warming seen in recent decades
( SN: 11/21/09, p. 5), and some researchers
think soot may be an important factor in
glaciers melting in the Himalayas.
Still, scientists don’t fully understand
how much black carbon there is, where it
comes from and how it gets removed from
Spurred by prodding from scientists,
policy makers are increasingly looking at
ways to cut black carbon emissions. The
concept is politically palatable, since it
involves cleaning up pollution in a way
that helps people’s health. Black car-
bon is also an easy target because soot
falls out of the atmosphere in a matter
of weeks, yielding measurable results
in a short period of time. Strict pollu-
tion measures introduced in Beijing
before the 2008 Olympics, for instance,
Another paper in the same issue of
Science scrutinized how atmospheric
chemistry over the Amazon naturally
produces aerosols and clouds there.
A team led by Ulrich Pöschl of the
Max Planck Institute for Chemistry in
Mainz, Germany, found that the num-
ber of clouds above the rainforest was
limited by the natural supply of aero-
sols. In regions with more pollution, the
Brown clouds hovering over India (such as one captured in December 2008, left)
contain smoke from agricultural burning, industrial pollution and inefficient cook-
stoves (right). Such clouds can magnify the effects of climate change.
