Global Dimming

Bill Stoddard bill at
Fri Dec 19 06:50:56 PST 2003


   Posted on Tue, Jul. 23, 2002

   9/11 offered rare chance to study contrail-free sky
      By Dan McKinney
      Special to the Mercury News

   Last September, out of tragic circumstances, atmospheric scientists were
   given a once-in-a-lifetime opportunity.

   For several days after the attacks on the World Trade Center, all
   commercial aviation was grounded -- an event unprecedented in the
   industry's history. The episode offered scientists a rare glimpse at North
   America without contrails -- and a chance to compare those atmospheric
   conditions to more routine ones.

   In the late afternoon of Sept. 11, the only three contrails visible from
   space, according to satellite images, were those coming from Air Force
   One and its two fighter jet escorts on their way to Washington. Scientists
   closely monitored the atmosphere during that time because it was devoid
   of all but a few aircraft.

   ``The 9/11 study helped because we were able to identify contrails
   developing without interference from others,'' said Patrick Minnis, a senior
   research scientist at the NASA Langley Research Center in Hampton, Va.

   The Sept. 11 aviation shutdown gave scientists some of their first hard
   evidence of regional climate shifts. One researcher now believes contrails
   contribute to an overall cooling during the day and warming at night -- a
   finding that may sound beneficial, but actually points to worrisome
   artificial climate change, scientists say.

   David Travis, a climatologist at the University of Wisconsin-Whitewater,
   presented a paper at a recent American Meteorological Society
   conference showing a widening of day-to-night temperature ranges in
   the days following Sept. 11. His research includes findings that challenge
   prevailing views of contrails and climate.

   Without contrails, Travis was able to measure temperatures across the
   continent and compare them to similar measurements taken before Sept.
   11. He found that the difference between the warmest temperature during
   the day and the coldest temperature at night grew 3 degrees on average,
   and as much as 5 degrees in parts of the country where contrails were
   likely to form.

   Net cooling effect

   This would mean contrails shield the Earth from the sun during the day,
   causing lower temperatures, and warm it at night, trapping the Earth's
   heat like a blanket. Because there are generally more flights and more
   trails during the day than at night, Travis speculates contrails have a net
   cooling effect on the Earth. His conclusion is a radical departure from the
   conventional theory that contrails contribute to warming.

   ``Contrails can cool or warm the Earth. The key is, there are more contrails
   at day than at night,'' Travis said. ``Because of that, the daytime effect is
   likely to dominate,'' meaning contrails do more cooling than heating.

   Travis said his background as a climatologist helped him arrive at that
   answer. Most previous knowledge of contrail effects had been gleaned
   from complex meteorological theories. But Travis compared hard data from
   the Sept. 11 shutdown to previous climate history and based his
   conclusion on what the evidence told him.

   ``Most of what my colleagues have done is theoretical modeling done with
   computers,'' he said. ``I'm an empirical researcher looking at data.''

   Travis said a major shift in how scientists think about contrails' effect on
   climate may be in the making. ``Now we see that a contrail effect can be
   measured in the climate record,'' he said. Travis' finding will be published
   in an upcoming issue of the journal Nature.

   ``The big implication is that this is the first direct evidence that the global
   warming debate is complicated by jet contrails,'' Travis said.

   So if contrails are off-setting warming, is that a good thing? Travis is
   quick to point out that contrails only affect regional climates.

   ``We don't want people to think that it's a great thing that contrails are out
   there,'' he said. ``Because any artificial temperature change isn't good for
   the environment.''

   The plane-free skies after the terrorist attacks also allowed scientists to
   study another contrail-related issue, the connections between the vapor
   trails and a type of naturally occurring high cloud cover known as cirrus clouds.

   Like cirrus clouds

   Researchers have found that contrails behave exactly like cirrus clouds.
   Cirrus clouds are excellent insulators, and they may contribute
   significantly to global warming. Some scientists believe contrails could
   encourage their formation, and the terrorist attacks gave researchers a
   chance to see how many cirrus clouds would naturally be present without

   The mysterious cloud species is to atmospheric scientists what dark
   matter is to astrophysicists -- mostly unknown.  Wispy thin cirrus clouds
   travel at 30,000 to 60,000 feet above ground, and, unlike cumulus clouds,
   their lower, puffy, popcorn-shaped cousins, cirrus clouds can be nearly

   ``The way that the climate community has been trying to think about this
   is by separating natural cloudiness from human-induced perturbations,''
   said Ronald Cohen, a specialist in atmospheric chemistry at the University
   of California-Berkeley.

   Cohen spent almost 10 years studying the effects of aviation on the
   atmosphere. The links between cirrus cloud formation and contrails are
   still elusive, he said, but theories addressing aviation's role are being
   investigated. A recent NASA study suggests that the small particles found
   in airplane emissions could serve as nuclei, attracting water vapor to
   themselves and eventually forming artificial cirrus clouds.

   A paper Minnis presented in early May at an American Meteorological
   Society meeting makes the connection between the high wispy clouds
   and contrails more clear. By watching the few, isolated contrails present
   during the Sept. 11 aviation shutdown, Minnis was able to precisely
   measure their growth as they transformed into cirrus clouds.

   ``During regular air traffic, we will see linear contrails crossing each other,
   often in the midst of thin cirrus clouds,'' he said. ``The thin cirrus clouds
   could be the old contrails formed a few hours earlier. Surely, that
   happens. We just need to learn how often it happens.''

   Other regions that are regularly covered by cirrus clouds were
   unobscured in post-Sept. 11 skies, Minnis found.  ``Large areas that
   normally have cirrus clouds were clear because there were no contrails,''
   he said.

   Contact Dan McKinney at science at

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