Global Warming Twilight Zone Detected Around Clouds
Pastor Dale Morgan
Global Warming Twilight Zone Detected Around Clouds*
Greenbelt MD (SPX) May 07, 2007
There seems to be something new under the sun -- in the sky,
specifically -- that could complicate scientists' efforts to get a fix
on how much the world will warm in the future. Greenhouse gases are not
the only things in the air that influence the temperature of our
atmosphere. Clouds and small airborne particles called aerosols also
play an important and complicated role.
Now a new ingredient has been discovered: an extensive and previously
unseen "twilight zone" of particles that represents a gradual transition
from cloud droplets to dry particles.
In a study published last month, scientists from the Weizmann Institute,
Rehovot, Israel, and NASA's Goddard Space Flight Center, Greenbelt, Md.,
document for the first time that air around clouds that was previously
considered clear is actually filled with particles that are neither
cloud droplets nor typical dry aerosols such as dust and air pollution.
Worldwide, up to 60 percent of the atmosphere labeled as cloud-free in
satellite observations is actually filled with this twilight zone of
in-between particles, according to the study.
"With the highly sensitive Earth-observing instruments NASA has used
since 2000, we can distinguish aerosols and clouds in greater detail
than ever before," said Goddard's Lorraine Remer, a co-author on the
study. "But the area around clouds has given us trouble. The instruments
detected something there, but it didn't match our understanding of what
a cloud or an aerosol looked like. What we think we're seeing is a
transitional zone where clouds are beginning to form or are dying away,
and where humidity causes dry particles to absorb water and get bigger."
Precisely accounting for everything in the atmosphere that can influence
changes in global temperatures is critical to scientists' quest to
accurately predict what Earth's climate will be in the future. The
latest report from the Intergovernmental Panel on Climate Change, which
assessed the potential risks of human-induced climate change, notes that
the overall effect of clouds and aerosols on the amount of heat held in
the atmosphere is still uncertain. Finding a previously unknown
ingredient in the mix further complicates an already complex picture,
but it also holds out the promise of resolving some nagging problems in
climate change science.
"The effects of this zone are not included in most computer models that
estimate the impact of aerosols on climate," said lead author Ilan Koren
of the Weizmann Institute "This could be one of the reasons why current
measurements of this effect don't match our model estimates." The study
was published April 18 in the American Geophysical Union's Geophysical
Research Letters. Atmospheric scientists have been aware of an
indistinct "halo" of particles immediately surrounding individual
clouds, which are sometimes visible to the naked eye. These are thought
to be aerosols accumulating moisture and growing in size, or a cloud
droplet shrinking as it evaporates. But the newly detected twilight zone
extends far beyond single clouds to fill an entire cloud field.
The research team first came across evidence for this transitional zone
in satellite measurements of aerosols that looked "suspicious,"
according to Remer. "After working with several years of data from the
Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra
and Aqua spacecraft, we consistently saw what appeared to be elevated
levels of aerosols near clouds. But we couldn't be sure that the
instrument wasn't actually detecting stray light bouncing off of the
clouds."
The region around clouds is difficult to accurately observe with
instruments like MODIS because they operate like our eyes, collecting
light reflected by objects below. Scientists interpret the different
characteristics of the light received, matching them to known light
patterns from different objects such as clouds. But clouds are notorious
light scatterers, and the "glare" from the edge of clouds makes it hard
to clearly detect what is around them. To be on the safe side,
scientists mapping worldwide aerosols with MODIS avoid a 1-kilometer
border around clouds.
To find out whether the apparent aerosol detection around clouds in the
satellite data was real, Koren and his colleagues, including the late
Yoram Kaufman of NASA Goddard, turned to an independent observing system
on the ground: the NASA-sponsored Aerosol Robotic Network. The automated
instruments in this global network minimize scattered light effects as
they track the sun and take readings of the amount and size of aerosols
in a narrow column of atmosphere between the instrument and the sun.
When the sun is blocked by a cloud, the instrument doesn't make one of
its regularly scheduled readings, which provides an indirect measure of
the presence of a cloud.
Combining thousands of observations from 15 sites around the world, the
researchers found that the amount of aerosol systematically increased as
clouds got closer, as did the size of the particles. This held true
regardless of whether the site was in a relatively clean setting or one
where aerosols from air pollution or biomass burning were common.
"We found that the region affected by this cloud field 'twilight zone'
extends to tens of kilometers beyond the identified cloud edge," said
Koren. "This suggests that 30 to 60 percent of the atmosphere previously
labeled as 'cloud-free' is actually affected by cloud-aerosol processes
that reflect solar energy back into space."
Introducing this new factor could lead climate scientists to recalculate
their best estimates of how Earth's atmosphere holds and reflects solar
energy -- the key to accurately predicting the future of global warming.
"Current estimates of the effect of aerosols on global temperatures,
which is primarily cooling, may be too small because the large
contribution from this transition zone has been overlooked," Remer said.
"If aerosols are offsetting warming more than we thought, it's possible
that warming could increase more than expected in the future if aerosols
continue to decline, as has been reported recently."
This summer the scientists hope to get a closer look at the "twilight
zone" and the hard-to-detect particles inside it with new measurements
by the Aerosol Robotic Network and NASA aircraft.