Holding a hand near the projection screen casts a well-defined shadow. Moving the hand back toward the projector makes the shadow indistinct: the edges are too fuzzy. Were thumb and forefinger creating a silhouette with a hole, it would become difficult to discern the hole.
There is a similar difference between dimming the sun
from millions of miles away and doing it in the stratosphere close to the
target.
Uniformity of aerosol application is difficult to
accomplish—and maintain—via sun-scattering aerosols in the upper atmosphere. This
means that the cooling will tend to be as patchy. What happens with patchy
cooling from this aerosol silhouette?
Temperature differences create density differences—and
thus winds are generated between the air column of the cooling silhouette and uncooled
neighboring regions. At many altitudes, more dense air will flow sideways out
of the silhouette column. A patchy silhouette could rearrange the generally eastward
track of storm systems, though simulations are needed to estimate how sensitive
the storm tracks are to this.
Weather disruption might be avoided by doing the uneven scattering
at some distance toward the sun, though that does nothing for the shadowed nighttime side of the
globe with its warmer nights. And even a uniform application, pole to pole,
will create an ever-moving stirring along the terminator that is not the
natural one.
There is, however, a circumstance where patchy cooling
from stratospheric aerosols might be of use—say, for cooling a region with
out-of-control forest fires. There is now a need to break up the blocking highs
that stall eastbound weather systems to cause three-day downpours, prolonged
heat waves, and facilitate the prolonged arctic outbreaks into the subtropics.
In the same manner that a long barrier
island can be cut in half by waves, one can imagine an aerosol application scenario
whose changing cooling silhouette temporarily creates a channel for the stalled
storm systems to pass through the ridge of high pressure.
William H. Calvin is a professor emeritus at the University of Washington’s medical school in Seattle and the author of Global Fever: How to Treat Climate Change (University of Chicago Press, 2008). The latest version of the CO2 cleanup was a finalist in MIT's 2013 geoengineering climate contest.
November 2014 WCalvin@UW.edu
William H. Calvin is a professor emeritus at the University of Washington’s medical school in Seattle and the author of Global Fever: How to Treat Climate Change (University of Chicago Press, 2008). The latest version of the CO2 cleanup was a finalist in MIT's 2013 geoengineering climate contest.
November 2014 WCalvin@UW.edu
