- "Many of the key processes that take place in clouds
occur on scales so small that large climate models can't see them."
-
- During the summer of 1976, a dozen years before climate
change burst on the scene as a hot-button issue, climate scientist Bruce
Wielicki saw the future, and it was in clouds.
-
- A grad student at the time, Dr. Wielicki was summering
at the Woods Hole Oceanographic Institution on Cape Cod, taking part in
a 12-week brainstorming session on climate.
-
- At one point, he recalls, he teamed up with a climate
modeler studying how the Earth and atmosphere balance the energy they receive
from the sun. Wielicki had been working on a simple computer model to simulate
the effect changing sea-surface temperatures have on climate, including
cloudiness. The two decided to see what would happen if they linked their
models.
-
- "I added clouds to his model, and of course, they
blew his results right up," Wielicki says, laughing. "He was
sure I'd messed up something in my calculations. So we spent the summer
figuring out that, no, my calculations were right and clouds were that
powerful."
-
- "That was 25 years ago," he says, "and
we still haven't figured clouds out."
-
- Indeed, Joni Mitchell's '60s-era lyrics still resonate
with climate scientists today as they probe the mysteries of clouds and
their impact on Earth's climate system.
-
- "We've made a lot of progress" in understanding
and modeling clouds and their impact on climate, acknowledges Jeffrey Kiehl,
who heads the climate modeling section at the National Center for Atmospheric
Research in Boulder. Colo. "But in the last five years, progress has
flattened out in trying to sort out cloud processes." Clouds are still
a weak point, he says, in reducing the uncertainties in climate forecasts.
-
- Small clouds hard to gauge
-
- Some of the challenges, he notes, lie in the nature of
modeling itself. Many of the key processes that take place in clouds occur
on scales so small that large climate models can't see them. As knowledge
about these small-scale processes grows, the potential to include them
in models exists, researchers say, but to do so would require an enormous
increase in computing power. As an alternative, researchers are using smaller-scale
models that can accurately capture features, important to events such as
thunderhead formation, and then using those results as reality checks for
how larger-scale models treat such phenomena.
-
- Challenges also lie in understanding basic cloud processes,
such as factors that influence where a cloud's base forms or what determines
how high clouds get, Dr. Kiehl says. "These are critical to understanding
whether clouds present a positive or negative feedback to the system"
- essentially whether they act to heat or cool the planet.
-
- A thick layer of stratus clouds, for example, which can
range from fog at ground level to cloud masses a few hundred feet above
the surface, typically moderates daytime temperatures by reflecting most
of the sunlight striking them back into space.
-
- High-flying cirrus clouds, on the other hand, can be
thin enough to let sunlight through while trapping heat that rises from
Earth's surface. Researchers note that when the sun is low on the horizon,
sunlight can strike the underside of high-latitude cirrus layers, which
reflect that radiation back toward Earth.
-
- By some estimates, a 50 percent increase in cirrus-cloud
cover could warm the climate much more than a 50 percent increase in atmospheric
carbon dioxide.
-
- In other cases, the challenge lies in more subtle interactions
between clouds and other constituents of the atmosphere.
-
- Enough progress has been made on basic cloud processes,
that to continue to focus on them "may be looking under the streetlight
for the lost keys," says James Hansen, a climate researcher at the
National Aeronautics and Space Administration's Goddard Institute for Space
Science in New York. He holds that the "real uncertainty lies in the
effect aerosols have on clouds."
-
- Aerosols are the wild card
-
- Aerosols are tiny particles of material such as sulfates,
dust, sea salt, and soot, and often they are tied to industrial pollution.
During the late 1980s and early '90s, an appreciation for their direct
effect on climate grew as scientists came to understand that aerosols can
reflect incoming solar radiation back into space. Moreover, scientists
had long known that aerosols act as seeds around which cloud droplets can
form. By increasing droplet size and reducing precipitation, aerosols can
help clouds retain their moisture, boosting a cloud's ability to reflect
sunlight.
-
- But recent studies have shown that aerosols and soot
can have the opposite effect as well. An ambitious international experiment
in 1998 and 1999 combined data from aircraft, satellites, ships, and ground
stations to track the effect of aerosols and soot on cloud formation over
the Indian Ocean.
-
- Dubbed INDOEX, the experiment showed how soot particles
can absorb sunlight and re-radiate it as heat. That added heat can raise
air temperatures sufficiently to burn off nascent clouds before they fully
form.
-
- In addition, says Wielicki, an atmospheric scientist
at NASA's Langley Research Center in Hampton, Va., the study also showed
that when a cloud moves into a thick layer of soot, the soot within the
cloud joins forces with the cloud's water droplets to trap heat, changing
the distribution of heat in the atmosphere's boundary layer, which in turn
affects cloud formation.
-
- Beyond the impact of aerosols lies a need to get a better
handle on tropical clouds, particularly tropical cirrus clouds, researchers
say. Indeed, this class of cloud represents a priority for cloud research,
notes Bruce Albrecht, a professor of meteorology and physical oceanography
at the University of Miami's Rosenstiel School of Marine and Atmospheric
Science in Coral Gables, Fla.
-
- Tropical cirrus clouds typically form as thunderheads,
transferring heat from the ocean to the atmosphere, and grow to heights
of 12 kilometers (7 miles) or more. At those heights, high-altitude winds
shear their tops into an anvil shape, then extend them until they become
cirrus clouds. Their water droplets will have turned to ice crystals.
-
- "These layers are relatively thin, sometimes so
thin you can't see them from the ground. But they are still important"
to the Earth's radiation budget, Dr. Albrecht says.
-
- Natural thermostat?
-
- How they respond to changing climate, however, has been
the subject of debate for years. Some researchers have proposed that processes
in the tropics may regulate cirrus formation in a way that serves as a
natural thermostat.
-
- A paper published earlier this year in the Bulletin of
the American Meteorological Society, for example, proposes that when temperatures
rise in cloudy regions of the tropical Pacific, cirrus formation drops
off, allowing more heat to escape into space. The paper's lead author,
Massachusetts Institute of Technology atmospheric scientist Richard Lindzen,
and his colleagues suggest this as a plausible explanation for trends in
cloud formation and temperatures they saw in data from satellites and buoys.
They have dubbed it "the iris effect."
-
- Others, however, suggest that newer data may indicate
that the region does not exhibit a self-regulating mechanism.
-
- Researchers hope to get a better handle on this and other
issues through NASA-funded experiments scheduled for the next few years.
In 2002, researchers are slated to study tropical cirrus formation from
thunderheads over southern Florida, in a project dubbed CRYSTAL-FACE. That
effort also is designed to serve as a warm-up to another study that will
focus on the Western Pacific.
-
- I've looked at clouds from both sides now, From up and
down and still somehow It's cloud's illusions I recall, I really don't
know clouds at all.
-
- - Joni Mitchell, singer-songwriter
-
- (c) Copyright 2001 The Christian Science Monitor. All
rights reserved. http://www.csmonitor.com/durable/2001/07/12/fp11s1-csm.shtml
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